Denim and Tweed

Visit Blog Website

161 posts · 169,166 views

I'm a doctoral student in evolutionary ecology; D & T is my personal 'blog, and my top topics are science, religion, and politics, with particular interest in the interface between science and religion.

Jeremy Yoder
161 posts

Sort by: Latest Post, Most Popular

View by: Condensed, Full

  • June 26, 2010
  • 08:03 PM
  • 308 views

#evol2010 day 1: In which chromosomes invert and sources sink

by Jeremy Yoder in Denim and Tweed

The first day at Evolution 2010 has been a great one. The location in Portland is proving to be great in stereotypical ways: great beer from Rogue Ales, conference t-shirts by American Apparel. There's pretty good chatter on Twitter this year under the hashtag #evol2010, and in a first for Evolution meeting coverage, there will be daily wrap-up audiocasts (in which I'll be participating) at the blog Evolution, Development, and Genomics.

Amusingly, we're sharing the Oregon Convention Center with a "Christian" homeschooling conference, but so far this has led to neither disruptions nor learning experiences.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Mimulus cardinalis. Photo by randomtruth.Some highlights of the talks I've attended so far:Jeffrey Feder proposed a new means by which chromosomal inversions might evolve, via a period of allopatric population isolation that allows a locally adaptive inversion to spread, followed by secondary contact during which gene flow creates selective pressure to reduce recombination that could break up the inversion.
Simone Des Roches presented new evidence that three lizard species, which have colonized a region of white sand in the New Mexican desert and subsequently evolved "blanched" coloration [PDF], are experiencing ecological release and density compensation. (Simone and her labmate Kayla Hardwick recently discussed their work in blog format.)
Chelsea Berns demonstrated that, alone among North American temperate hummingbirds, Ruby-throated hummingbird males have differently-shaped bills from Ruby-throated females.
Joel Sachs described the natural frequency and origins of rhizobial bacteria that "cheat" on their host plants.
Sheina Sim described host shifts in the apple maggot fly, Rhagoletis pomonella: the fly originally shifted from its native host, hawthorn, to domestic apples [PDF] when they were introduced to North America—now the fly has been introduced to the Pacific Northwest via transport of apples, and some populations have shifted back to hawthorns.
The American Society of Naturalists Vice Presidential symposium presented a large volume of work towards discovering the reasons for species' range boundaries, including great syntheses of population genetic and experimental data for the wildflowers Mimulus cardinalis and Clarkia xantiana—one emerging theme is the importance of the balance of gene flow from healthy populations in the center of ranges to poorly-adapted populations at the edges.
Primary literature referenced

Feder, J., Chilcote, C., & Bush, G. (1988). Genetic differentiation between sympatric host races of the apple maggot fly Rhagoletis pomonella. Nature, 336 (6194), 61-64 DOI: 10.1038/336061a0

Rosenblum, E. (2006). Convergent evolution and divergent selection: lizards at the White Sands ecotone. The American Naturalist, 167 (1), 1-15 DOI: 10.1086/498397

... Read more »

Feder, J., Chilcote, C., & Bush, G. (1988) Genetic differentiation between sympatric host races of the apple maggot fly Rhagoletis pomonella. Nature, 336(6194), 61-64. DOI: 10.1038/336061a0  

Rosenblum, E. (2006) Convergent evolution and divergent selection: lizards at the White Sands ecotone. The American Naturalist, 167(1), 1-15. DOI: 10.1086/498397  

  • June 23, 2010
  • 10:05 AM
  • 1,104 views

With conspecifics like these, who needs predators?

by Jeremy Yoder in Denim and Tweed

There's something special about islands. After moving to islands, plants adapted to rocky outcrops evolve to grow in rainforests and alpine meadows, and finches evolve to behave like woodpeckers. But why? Islands contain new food sources and habitats, they often lack predators, and they can provide more geographic barriers to generate reproductive isolation—to name just a few possibilities. A newly published ecological experiment now provides evidence that one group of island lizards diversfied because islands are crowded [$a].

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } There's something about islands. Photo by Storm Crypt.Diversification on islands may be related to density compensation, the frequently-observed principle that islands often support fewer species than mainland sites of the same area, but contain more individuals of each species—that is, island populations are usually at higher density than their mainland counterparts [$a]. Density compensation seems to arise both from lack of predators on islands, and because island populations have fewer competitor species. This may mean that, compared to mainland populations, island populations are under weaker natural selection from other species, and stronger selection from competition with other members of their own species.

A somewhat strained analogy

How could that difference in selective regimes spur diversification? Imagine two towns, one surrounded by other settlements, the other on its own in the middle of the wilderness. The town in densely-populated country is probably best off doing one thing well—to have, say, most of its inhabitants working at a factory making (to pick a product at random) sausages for trade with other towns. People living in this first town might want to start up a factory making a different product, but odds are good there's strong competition from another town nearby, so it's hard to get the new business off the ground—it's really just better to invest in the existing factory.

On the other hand, the inhabitants of the town in a lightly-populated district might need more products made locally because it costs too much to import. A businesswoman in the isolated town is probably better off starting a factory that makes a product no-one is making locally—if sausages are already accounted for, there might be a market for (to pick another product at random) pharmeceuticals.

In this scenario, competition from outside exerts economic pressure to do one thing well; competition from within exerts pressure to do many different things. Both kinds of competition are present in each town, but outside competition is stronger in the town surrounded by other towns, and competition from within is stronger in the isolated town.

Anole vs. anole

The density compensation hypothesis proposes that something similar happens on islands. With fewer predators or competitor species, island populations are able to maintain higher densities of individuals. That increased density means that competition within the species becomse stronger, creating natural selection that favors individuals who can use new food resources or live in new habitats.

Density compensation seems likely to be responsible for the diversification of anole lizards on the islands of the Caribbean. In the course of colonizing Caribbean islands, anoles have repeatedly evolved into a handful of different niche specialists [PDF] called "ecomorphs," ranging from "giant" species that live high in the forest canopy, to small species that can navigate and perch on fine twigs, and intermediate species that live on and around tree trunks. Anoles on the mainland of Central America are no less diverse than their Caribbean congeners, but they haven't evolved mini-radiations of replicated ecomorphs—and their population densities are much lower than those of the island species.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Anolis sagrei, the brown anole. Photo from WikiMedia Commons.If release from predators, and the ensuing increase in population density, drove the diversification of island anoles, then we might expect that natural selection from predators has less effect on the traits that differentiate the anole ecomorphs than natural selection from other anoles. Testing that hypothesis experimentally is ambitious to say the least, but that's what the new study attempts to do.

The authors, Calsbeek and Cox, identified six very small, similar islands off the coast of the Bahamian island Greater Exuma, and introduced varying numbers of brown anoles (Anolis sagrei) onto them at the beginning of the summer. The islands were small enough that Calsbeek and Cox could selectively exclude birds by enclosing the islands in netting; by introducing predatory snakes onto some islands, they could then generate three selective regimes: no predators, birds only, and birds plus snakes. Before introducing them into these experimental setups, the authors measured each anole's body size, hind-leg length, and running stamina, and marked each lizard so they could estimate selection acting on the three traits based on which lizards survived to be recaptured at the end of the season. (The experiments were carried out over two years, with both years' results compiled at the end.)

The results suggest that competition makes a bigger difference for the experimental populations than predation—while the strength of natural selection acting on all three traits increased with the anoles' population density, it didn't change when predators were allowed access to the islands. If the levels of predation simulated on the micro-islands accurately reflect what anoles experience throughout the Caribbean, then the result is, I'd say, pretty good evidence that competition is the most important evolutionary force acting on island anoles.

I should note that, although Calsbeek and Cox's raw result is suggestive, it's not clear that their sample size is big enough to support all the statistical analyses they perform on the data. On balance, I think they deserve a lot of credit just for tackling this question experimentally.

References

Calsbeek, R., & Cox, R. (2010). Experimentally assessing the relative importance of predation and competition as agents of selection. Nature, 465 (7298), 613-6 DOI: 10.1038/nature09020

... Read more »

Calsbeek, R., & Cox, R. (2010) Experimentally assessing the relative importance of predation and competition as agents of selection. Nature, 465(7298), 613-6. DOI: 10.1038/nature09020  

Givnish, T., Millam, K., Mast, A., Paterson, T., Theim, T., Hipp, A., Henss, J., Smith, J., Wood, K., & Sytsma, K. (2009) Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae). Proceedings of the Royal Society B: Biological Sciences, 276(1656), 407-16. DOI: 10.1098/rspb.2008.1204  

Losos, J. (1990) Ecomorphology, performance capability, and scaling of West Indian Anolis lizards: an evolutionary analysis. Ecological Monographs, 60(3), 369-88. DOI: 10.2307/1943062  

MacArthur, R., Diamond, J., & Karr, J. (1972) Density compensation in island faunas. Ecology, 53(2), 330. DOI: 10.2307/1934090  

Pinto, G., Mahler, D., Harmon, L., & Losos, J. (2008) Testing the island effect in adaptive radiation: rates and patterns of morphological diversification in Caribbean and mainland Anolis lizards. Proceedings of the Royal Society B: Biological Sciences, 275(1652), 2749-57. DOI: 10.1098/rspb.2008.0686  

  • June 16, 2010
  • 10:05 AM
  • 446 views

Not just babble: Cooperating birds talk it through

by Jeremy Yoder in Denim and Tweed

For cooperation to work, everyone involved needs to know what the others are willing to contribute in order to decide what she will contribute. You might think that only humans can achieve that kind of back-and-forth negotiation, but a paper recently published online by Proceedings of the Royal Society suggests otherwise. In it, ornithologists decode the negotiations [$a] that allow sociable birds to share the task of watching for predators.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } The southern pied babbler, Turdoides bicolor. Photo by Blake Matheson.Pied babblers (Turdoides bicolor) are sociable South African songbirds, which live and forage for food in groups. During foraging, some adult babblers act as sentinels, perching above the ground to scan for predators, and alerting the rest of the foragers if any danger shows up. Sentinel behavior is cooperative—sentinels free the rest of the group to concentrate on feeding, but sentinels themselves cannot forage. The study's authors, Bell et al. hypothesized that sentinels might communicate how long they're willing to stand watch to the rest of the group, so as to prompt new birds to take up watch and given the current sentinels a break to feed.

The authors first established that how hungry a babbler is determines how long he or she is willing to stand watch, which they did by feeding the birds with meal worms immediately after they concluded a period of sentinel duty. Babblers receiving ten worms returned to duty faster than those receiving just one, and stayed on duty longer. Further feeding experiments and observations established that both foragers and sentinels called to each other less frequently when they were well fed, that sentinels called more frequently the longer they stayed on watch, and that sentinels who ultimately stayed on watch the longest also called the least frequently in their first minute on watch.

So call frequency is the babblers' signal for how badly they want to forage—foragers hearing higher-frequency calls from sentinels should take them as a call for relief; and sentinels hearing lower-frequency calls from foragers should take them as permission to leave the watch and start foraging. To test this hypothesis, the authors played recorded calls to foragers and sentinels, and found that the birds responded as I've just described. The apparent babble of the babblers, then, is actually a perpetual negotiation about who should be on sentinel duty—sentinels complaining when they get hungry, and foragers telling the sentinels, "Not yet! I just need to catch a few more worms."

Reference

Bell, M., Radford, A., Smith, R., Thompson, A., & Ridley, A. (2010). Bargaining babblers: vocal negotiation of cooperative behaviour in a social bird. Proc. Royal Soc. B DOI: 10.1098/rspb.2010.0643

... Read more »

Bell, M., Radford, A., Smith, R., Thompson, A., & Ridley, A. (2010) Bargaining babblers: vocal negotiation of cooperative behaviour in a social bird. Proc. Royal Soc. B. DOI: 10.1098/rspb.2010.0643  

  • June 9, 2010
  • 10:05 AM
  • 684 views

The "Big Four," part IV: Migration

by Jeremy Yoder in Denim and Tweed

This post is the last in a special series about four fundamental forces in evolution: natural selection, mutation, genetic drift, and migration.

It's the little differences. I mean, they got the same shit over there that we got here, but it's just—it's just there it's a little different.
— Vincent, Pulp FictionDifferent places are different from each other. This is a truism bordering on tautology, but it also has real implications for the ways in which life evolves and diversifies. The specific differences between one environment and another shape how well living things can move between those environments, and what happens when they do—the evolutionary consequences of migration.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Canada geese on the wing. Photo by Brian Guest (giant rebus).Individuals moving between populations can alter the evolution of those populations in a number of ways [$a]. Migration introduces traits from one population into another, making it a source of variation not unlike mutation. Migration can swamp out the effects of local natural selection, if enough migrants come from a population experiencing a different selective regime. Migration from a larger population to a smaller one can offset the loss of variation to genetic drift; but a small group of individuals migrating to an empty habitat can be strongly affected by drift. Depending on what species concept you prefer, migration between two populations is either what prevents them from becoming separate species, or what conclusively proves that they are the same species.

Migration mixes things up

Strictly speaking, I'm not talking simply about the movement of living things from place to place, but about gene flow, which also requires interbreeding between migrants and the populations to which they migrate. Individuals might be able to travel to another environment, and even do so frequently, but fail to survive when they get there [PDF], or fail to find a mate in the local population, or have offspring that are themselves less fit than the offspring of the locals. Because tracking each of these steps directly is daunting at best, most population biology studies estimate the rate of successful migration by proxy, using some measure of the genetic similarity of populations—the more migrants successfully move between populations, the more similar the traits and gene frequencies of those populations will be.

The rate of gene flow between two populations is essentially a measure of how much those populations evolve as a single unit—if there's no gene flow, selection or drift can eventually make the two populations into completely different things. An effective migration rate of one migrant per generation is generally understood to be enough to prevent drift from causing populations to diverge [$a]. Populations linked by migration along several intervening populations may be isolated by distance [PDF] if the line of connecting populations is long enough.

If natural selection is operating in different directions in the two populations, more migration is necessary to prevent them evolving different traits. If individual genes experience different selection in each population, then those genes may still evolve differently even as migration continues to mix in neutral genes [PDF]. This movement of genes across recognized population and species boundaries is called introgression.

Gene flow versus selection

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } The bird-pollinated Iris fulva (left) can sometimes hybridize with bee-pollinated I. brevicaulis, but pollinators favor hybrids that look more like the parent species. Photos by Matt N Charlotte and Jim Petranka.One well-documented case of gene flow between apparently "good" species is that of the Louisiana irises Iris fulva and I. brevicaulis. These two species fill fairly distinct ecological niches: red-orange I. fulva is mainly pollinated by hummingbirds, and grows in very wet conditions; blue I. brevicaulis is pollinated by bees, and grows best in drier habitats.

Yet when the two species co-occur, they sometimes do cross-pollinate. What prevents the two species from merging into a single iris? (Perhaps it would have purple flowers.) Natural selection, in this case, overwhelms migration. Experimentally created fulva-brevicaulis hybrids grown in wet conditions are more likely to survive if they carry specific genes from wet-tolerant fulva; and pollinators tend to favor hybrids that look more like their preferred parent species. I. fulva and I. brevicaulis share some genes that don't affect wet tolerance or pollinator attraction, but generally remain separate evolutionary entities.

This kind of partial reproductive isolation is most widely documented in plants, but it has been found in all sorts of organisms—even the chipmunks Tamias ruficaudus and T. amoenus [PDF]. In an evolving world, this shouldn't be surprising—it makes sense to find many cases of partial reproductive isolation as populations evolve toward the point of being separate species. Sometimes, of course, divergent selection weakens, or previous barriers to migration are removed, and populations re-merge into single evolutionary entities. But sometimes, the balance of the selection, mutation, drift, and migration is just right for just long enough to create a new, independently evolving form of life. And thus, as Darwin wrote, "endless forms most beautiful have been, and are being, evolved."

References

Arnold, M., Hamrick, J., & Bennett, B. (1990). Allozyme variation in Louisiana irises: a test for introgression and hybrid speciation. Heredity, 65 (3), 297-306 DOI: 10.1038/hdy.1990.99

... Read more »

Arnold, M., Hamrick, J., & Bennett, B. (1990) Allozyme variation in Louisiana irises: a test for introgression and hybrid speciation. Heredity, 65(3), 297-306. DOI: 10.1038/hdy.1990.99  

Good J.M., Hird S., Reid N., Demboski J.R., Steppan S.J., Martin-Nims T.R., & Sullivan J. (2008) Ancient hybridization and mitochondrial capture between two species of chipmunks. Molecular ecology, 17(5), 1313-27. PMID: 18302691  

Martin, N.H., Bouck, A.C., & Arnold, M.L. (2005) Detecting adaptive trait introgression between Iris fulva and I. brevicaulis in highly selective field conditions. Genetics, 172(4), 2481-9. DOI: 10.1534/genetics.105.053538  

Martin, N., Sapir, Y., & Arnold, M. (2008) The genetic architecture of reproductive isolation in Louisiana irises: Pollination syndromes and pollinator preferences. Evolution, 62(4), 740-52. DOI: 10.1111/j.1558-5646.2008.00342.x  

Nosil, P., Egan, S., & Funk, D. (2008) Heterogeneous genomic differentiation between walking-stick ecotypes: "Isolation by adaptation" and multiple roles for divergent selection. Evolution, 62(2), 316-36. DOI: 10.1111/j.1558-5646.2007.00299.x  

Nosil, P., Vines, T., & Funk, D. (2005) Reproductive isolation caused by natural selection against immigrants from divergent habitats. Evolution, 59(4), 705-19. DOI: 10.1554/04-428  

Slatkin, M. (1987) Gene flow and the geographic structure of natural populations. Science, 236(4803), 787-92. DOI: 10.1126/science.3576198  

Wang, J. (2004) Application of the one-migrant-per-generation rule to conservation and management. Conservation Biology, 18(2), 332-43. DOI: 10.1111/j.1523-1739.2004.00440.x  

Wright, S.J. (1943) Isolation by distance. Genetics, 139-56. info:other/PMC1209196

  • June 2, 2010
  • 10:05 AM
  • 952 views

Freeloading cuckoos force their hosts to diversify

by Jeremy Yoder in Denim and Tweed

Ashy-throated parrotbills have a problem every time breeding season rolls around: how do they know whether the eggs in their nests are their own, or those of the common cuckoo? A study recently released in PLoS ONE suggests that one population of parrotbills fights this brood parasitism by laying eggs of different colors.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Common cuckoos lay eggs that mimic those of the host birds they trick into raising cuckoo chicks. Photo by Sergey Yeliseev.Brood parasitism, in which one bird species lays its eggs in another bird's nest, has long been considered a likely cause of coevolution [$a] between brood parasites and their hosts, because the interaction exerts strong natural selection on both species. Hosts suffer major fitness consequences if they take on the raising of another bird's chick—and brood parasite chicks are often bigger, and more aggressive, than their adoptive "siblings," sometimes pushing them right out of the nest. On the other hand, brood parasites run the risk of losing their offspring to hosts who can recognize a strange egg and eject it from the nest.

One way to avoid raising a cuckoo chick is to lay eggs that look different from cuckoo eggs. Cuckoos counteract this defense by evolving eggs that match their most common hosts—a selective regime proposed to explain rapid rates of species formation in parasitic cuckoo lineages. In the new study, Yang et al. show that this pattern plays out within a single population of ashy-throated parrotbills and the cuckoos that parasitize them. At a forested nature reserve in southwestern China, the team found that parrotbills lay eggs of three different colors: white, blue, or (rarely) pale blue. Common cuckoos in the same area also laid eggs of those three colors, in about the same proportions as the parrotbills—and cuckoo eggs were usually found in host nests with eggs of the same color. Experimental introduction of eggs into parrotbill nests confirmed that parrotbills were more likely to reject eggs colored differently from their own.

That result captures many of the necessary conditions for coevolution between ashy-throated parrotbills and the local cuckoo population; the frequency with which parrotbills reject eggs unlike their own should exert strong selection on the cuckoos, and (conversely) the frequency with which parrotbills fail to reject cuckoo eggs that look like their own should exert selection on the hosts. This isn't the first case in which brood parasites have apparently forced their hosts to diversify, however—notably, African village weaverbirds evolved less varied egg patterning after being introduced into parasite-free habitats on Mauritius and Hispaniola.

References

Krüger, O., Sorenson, M., & Davies, N. (2009). Does coevolution promote species richness in parasitic cuckoos? Proc. Royal Soc. B, 276 (1674), 3871-9 DOI: 10.1098/rspb.2009.1142

Lahti, D. (2005). Evolution of bird eggs in the absence of cuckoo parasitism. Proc. Nat. Acad. Sci. USA, 102 (50), 18057-62 DOI: 10.1073/pnas.0508930102

Rothstein, S. (1990). A model system for coevolution: Avian brood parasitism. Ann. Rev. Ecology and Systematics, 21 (1), 481-508 DOI: 10.1146/annurev.es.21.110190.002405

Yang, C., Liang, W., Cai, Y., Shi, S., Takasu, F., Møller, A., Antonov, A., Fossøy, F., Moksnes, A., Røskaft, E., & Stokke, B. (2010). Coevolution in action: Disruptive selection on egg colour in an avian brood parasite and its host. PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010816

... Read more »

Krüger, O., Sorenson, M., & Davies, N. (2009) Does coevolution promote species richness in parasitic cuckoos?. Proc. Royal Soc. B, 276(1674), 3871-9. DOI: 10.1098/rspb.2009.1142  

Lahti, D. (2005) Evolution of bird eggs in the absence of cuckoo parasitism. Proceedings of the National Academy of Sciences, 102(50), 18057-62. DOI: 10.1073/pnas.0508930102  

Rothstein, S. (1990) A model system for coevolution: Avian brood parasitism. Annual Review of Ecology and Systematics, 21(1), 481-508. DOI: 10.1146/annurev.es.21.110190.002405  

Yang, C., Liang, W., Cai, Y., Shi, S., Takasu, F., Møller, A., Antonov, A., Fossøy, F., Moksnes, A., Røskaft, E.... (2010) Coevolution in action: Disruptive selection on egg colour in an avian brood parasite and its host. PLoS ONE, 5(5). DOI: 10.1371/journal.pone.0010816  

  • May 27, 2010
  • 10:05 AM
  • 622 views

The "Big Four," part III: Genetic drift

by Jeremy Yoder in Denim and Tweed

This post is the third in a special series about four fundamental forces in evolution: natural selection, mutation, genetic drift, and migration.

Have a coin handy? Flip it.

If your coin is fair, I can guess that it's come up heads and have a fifty percent chance, or probability equal to 0.5, that I've guessed correctly. Now, flip the coin ten times in a row. How many times did heads come up? Again, the best guess is that it came up five times—but it's not all that unlikely that it came up six times, or four, or even as many as eight.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } God may not play dice, but evolution does. Photo by jcotherals.Now, if you flipped the coin an infinite number of times, then exactly fifty percent of the total flips would be heads. But who has time for that? Similarly, populations of living organisms are not infinite—often far from it—and this means that the frequency of genes in those finite populations can change as a result of the same phenomenon at work on your coin. Biologists call this genetic drift.

Evolution at random

The basic principle behind genetic drift is that each generation is a process of sampling from the parental population to create a population of offspring. As most folks know from discussions of opinion polls, smaller samples tend to be less representative of the pool from which they are drawn. Say a population of annual plants begins with equal numbers of plants bearing blue flowers or white flowers. If only ten seeds survive from that population to form the next generation, you would expect them to be five blue-flowered and five white-flowered seeds. However, it's just like flipping a coin ten times: the probability of drawing six blue seeds is actually a little less than 21%, or one in five. The probability of drawing nine blue seeds is almost one in one hundred—small, but hardly impossible.

Consider, too, that once you draw six blue seeds, it becomes slightly more likely that you'll draw seven in the next generation, which makes it slightly more likely you'll draw eight in the next. Repeated selection of small samples means that traits can drift to fixation (or loss, depending on your perspective), so that everyone in the population has the same trait. Rare traits are more likely to be lost to drift, and large populations are less prone to its effects. This is nicely illustrated in this online simulation from the University of Connecticut—over time, a focal gene fixes or disappears from the population as a function of the population size and the initial frequency of the gene.

In general, drift interferes with the efficient operation of natural selection. Even in relatively large populations, the probability that a new beneficial mutation will become fixed is approximately twice the selective benefit of that mutation—typically very small. (This is from a 1927 paper by J.B.S. Haldane that doesn't seem to be online in any form, but which is discussed by Otto and Whitlock in a 1997 paper extending the classic result.) In a small enough population, a trait can become fixed even if it reduces its carriers' fitness [PDF].

Evolving differences without selection

As I've discussed above, the effect of drift in a single population is to reduce variation as rare traits are lost to chance. This means that, when more than one independently-evolving population is considered, drift actually increases variation among them [$a], as different traits fix or are lost in each. That is, drift can make isolated populations evolve into different species even if they experience identical regimes of natural selection.

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 0px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Woodland salamanders (genus Plethodon: left, P. vehiculum; right, P. yonahlossee) have diversified not by adapting to different environments, but by being homebodies. Photos by squamatologist.A flagship example of this sort of non-adaptive diversification are the woodland salamanders of eastern North America, genus Plethodon. Woodland salamanders are quite diverse, having accumulated more than 40 species in the last 27 million years, but all of these species live in more or less the same habitat, under the leaf litter in moist Appalachian forests, and many are "cryptic" species distinguishable only by DNA analysis. How, then, did Plethodon become so diverse?

The answer is simply that woodland salamanders don't travel very well. Salamanders need moist environments–they breathe through their skin, which doesn't work well if it dries out—and so have difficulty moving from one stream drainage to another. This means that it doesn't take much distance to isolate one Plethodon population from another, allowing drift to take them in different directions. Salamanders form new species, in other words, by staying at home.

This effect of drift means that biologists must adjust their "null" expectation when they observe differences in natural populations—the mere fact that some Joshua trees look different from other Joshua trees does not necessarily mean that natural selection has created those differences. Furthermore, the degree to which drift or selection can generate differences among populations depends strongly on the fourth force in the Big Four, which I'll discuss next week: migration.

References

Godsoe, W., Yoder, J., Smith, C., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism. The American Naturalist, 171 (6), 816-23 DOI: 10.1086/587757

... Read more »

Godsoe, W., Yoder, J., Smith, C., & Pellmyr, O. (2008) Coevolution and divergence in the Joshua tree/yucca moth mutualism. The American Naturalist, 171(6), 816-23. DOI: 10.1086/587757  

Kozak, K., Weisrock, D., & Larson, A. (2006) Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon). Proc. Royal Soc. B, 273(1586), 539-46. DOI: 10.1098/rspb.2005.3326  

Lande, R. (1992) Neutral theory of quantitative genetic variance in an island model with local extinction and colonization. Evolution, 46(2), 381-9. DOI: 10.2307/2409859  

Otto S.P., & Whitlock M.C. (1997) The probability of fixation in populations of changing size. Genetics, 146(2), 723-33. PMID: 9178020  

Wright S. (1931) Evolution in Mendelian populations. Genetics, 16(2), 97-159. PMID: 17246615  

  • May 19, 2010
  • 10:05 AM
  • 863 views

The "Big Four," part II: Mutation

by Jeremy Yoder in Denim and Tweed

This post is the second in a special series about four fundamental forces in evolution: natural selection, mutation, genetic drift, and migration.

In order for populations to change over time, to descend with modification, as Darwin originally put it, something has to create the modifications. That something is mutation.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A mutation of large effect? Photo by Cayusa.A mutation is any change to an individual's genetic code, whether caused by an external factor like radiation, or an error in the DNA copying that takes place every time an individual cell divides. However, not all mutations are created equal.

First and foremost, for a mutation to have any future existence beyond the individual in which it occurs, it must be in a cell that will go towards forming the next generation, a germline cell. In sexually reproducing species, this means sperm or egg cells, or the progenitor cells in the testes or ovaries that form them. Second, for a mutation to be "visible" to natural selection, it must have some effect on fitness, the number of offspring an individual carrying the mutation is likely to have. The genetic code determines how and when individual cells make proteins, and proteins determine phenotypes, the visible characteristics of living things. However, many changes to the genetic code don't affect their carriers' fitness. Mutations might be more or less neutral because They occur in regions of the genome that don't code for proteins or control protein production—so-called "junk" DNA.
They are synonymous substitutions, which occur in a protein-coding region of DNA, but don't alter the protein produced.
They alter a protein, but not in a way that changes its function [PDF].
They alter a protein's function and an individual's phenotype, but in a way that doesn't affect how many offspring that individual has.
Neutral mutations are actually quite important for biological studies—DNA fingerprinting and population genetics studies rely on them. Their frequencies evolve at random, reflecting the history of the populations that carry them rather than the effects of natural selection.

Favorable new mutations sweep the population

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
TOP: DNA sequences from a population are variable (blue) before a new mutation (red dot) arises; after it "sweeps," every individual carries the allele as well as identical sequence nearby (red). BOTTOM: a figure from Linnen et al. (2009), demonstrates this pattern in deer mice. Images from Pritchard et al. (2010), fig. 3 and Linnen et al. (2009), fig. 4.The variation introduced by neutral mutations—or, rather its absence—can help identify regions of the genome where selection is active. When a new gene arises by mutation, and it is strongly favored by selection, it can quickly spread through a population. In species that remix their genomes through sexual selection, the region of the genome containing a useful new gene can recombine into many different genetic backgrounds—but the closer a region of DNA is to the favored mutation, the less likely it is to recombine and separate from it. Thus, a region of the genome rather larger than the gene favored by selection is carried along until everyone in the population has the same DNA sequence.

When a new mutation takes over a population in this manner, it's called a selective sweep, and the pattern it produces has been used to identify genes recently favored by selection in many different species, including humans. For instance, Linnen et al. documented reduced genetic variation in the neighborhood of gene variant responsible for light-colored fur in deer mice to demonstrate that it spread rapidly through the population after the mice colonized a region with light-colored sand.

Fuel for the engine of natural selection

Selective sweeps highlight how natural selection acts in opposition to mutation: mutation introduces new variation into populations, and natural selection causes the most fit variants to spread—potentially until the whole population carries the same trait. At the same time, selection requires variation in order to operate. If everyone is identical, then everyone has the same expected number of offspring, and the next generation will look just like the current one.

Because of this, natural selection can only operate as rapidly as mutation can introduce new variation from which to select. We know of specific cases in which selection seems to have "stalled" for lack of heritable variation. For instance, the fly Drosophila birchii lives in rainforest habitats along the northeast coast of Australia. Fly populations from the driest locations in this range have greater tolerance for dry conditions, but they also have virtually no heritable variation for drought tolerance [PDF]—and the authors suggest that this could limit the flies' ability to evolve in response to climate change.

In other cases, though, biologists have found that mutation seems to provide new variation at least as fast as selection can remove it, leading to sustained, long-term evolution of experimental populations [PDF]. One important factor that may determine the outcome of this mutation-selection balancing act is actually the size of the population—more individuals means more opportunities for mutations to occur.

So the rate at which new mutations accumulate in a population depends on many factors, not the least of which are how you choose to measure that rate, and the fitness effects of the counted mutations. (Does a mutation "count" as soon as it occurs in a cell's nucleus, or only when it has passed on to the next generation, or only when it has spread to everyone in a population?) Ultimately, populations evolve through a constant tension between the effects of mutation, natural selection, and the subject of next week's Big Four force: genetic drift.

References

Barton, N., & Keightley, P. (2002). Understanding quantitative genetic variation. Nature Reviews Genetics, 3 (1), 11-21 DOI: 10.1038/nrg700

... Read more »

Barton, N., & Keightley, P. (2002) Understanding quantitative genetic variation. Nature Reviews Genetics, 3(1), 11-21. DOI: 10.1038/nrg700  

Drake J.W., Charlesworth B., Charlesworth D., & Crow J.F. (1998) Rates of spontaneous mutation. Genetics, 148(4), 1667-86. PMID: 9560386  

García-Dorado, A., Ávila, V., Sánchez-Molano, E., Manrique, A., & López-Fanjul, C. (2007) The build up of mutation-selection-drift balance in laboratory Drosophila populations. Evolution, 61(3), 653-65. DOI: 10.1111/j.1558-5646.2007.00052.x  

Hoffmann, A., Hallas R.J., Dean J.A., & Schiffer M. (2003) Low potential for climatic stress adaptation in a rainforest Drosophila species. Science, 301(5629), 100-2. DOI: 10.1126/science.1084296  

Linnen, C., Kingsley, E., Jensen, J., & Hoekstra, H. (2009) On the origin and spread of an adaptive allele in deer mice. Science, 325(5944), 1095-8. DOI: 10.1126/science.1175826  

Pritchard, J., Pickrell, J., & Coop, G. (2010) The genetics of human adaptation: Hard sweeps, soft sweeps, and polygenic adaptation. Current Biology, 20(4). DOI: 10.1016/j.cub.2009.11.055  

Tokuriki, N., & Tawfik, D. (2009) Protein dynamism and evolvability. Science, 324(5924), 203-7. DOI: 10.1126/science.1169375  

  • May 12, 2010
  • 10:05 AM
  • 728 views

The "Big Four," part I: Natural selection

by Jeremy Yoder in Denim and Tweed

This post is the first in a special series about four fundamental forces in evolution: natural selection, mutation, genetic drift, and migration.

Among non-biologists, the best-known of the Big Four forces of evolution is almost certainly natural selection. We've all heard the catchphrase "survival of the fittest," and that's a pretty good, if reductive, summing up of the principle. In more precise terms, here's how natural selection works:Natural populations of living things vary. Deer vary in how fast they can run, plants vary in how much drought they can tolerate, birds vary in their ability to catch prey or collect seeds—no two critters of the same species are exactly alike.
Some of those variable traits determine how many offspring living things have. How well you avoid predators, fight off disease, and collect food all determine how many babies you can make.
Many of those variable traits are heritable, passed on from parents to offspring. Faster deer usually have faster fauns; drought-tolerant plants make drought-tolerant seeds.
With these three conditions in place, natural selection occurs: heritable traits that help make more babies become more common. That is, if you have a trait that lets you support more offspring than your neighbor, you'll have more children than your neighbor, and they'll have more children than your neighbor's children, and so on.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Fitness-versus-phenotype regressions for directional, stabilizing, and disruptive selection. Graphic by jby.Measuring selection

Put this way, natural selection is simply a relationship between fitness, the number of offspring an organism can produce (often reported in comparison to the rest of the local population), and phenotype, the value of one or more traits of that organism (wing length, running speed, number of flowers produced, &c). Biologists can measure selection in natural populations by estimating this relationship between fitness (or a proxy for fitness, like growth rate), and phenotypes. Such an analysis should produce something like the regression graphs to the right, in which the relationship might be directional, with greater- (or smaller-) than-average phenotypes having greater fitness; stabilizing, with the average phenotype value having greater fitness; or disruptive, with extreme phenotype values having greater fitness. The slope of the line, or the shape of the curve, is a measure of the strength of natural selection [PDF] on an organism's phenotype. This approach to measuring selection has been widely applied, and in 2001 a group of biologists led by Joel Kingsolver collected more than 2,500 estimates of the strength of natural selection [PDF].

How strong is selection?

Kingsolver et al. found that selection was usually surprisingly weak. Studies with the largest sample sizes, and the most statistical power to detect selection, mostly found directional selection strength (that is, the slope of the fitness-phenotype regression) less than 0.1, and the strength of stabilizing or disruptive selection was similarly low. Does this mean selection doesn't matter in the short-term evolution of natural populations?

Probably not. The average selection strength estimates from the Kingsolver et al. dataset are actually stronger than selection strength assumed in most mathematical models of evolution. Furthermore, the collected estimates of selection had "long tailed" distributions—a small number of studies found quite strong selection, up to ten times as strong as the average. So maybe rare but strong bouts of selection have disproportionate impact over the long term.

.flickr-photo { }.flickr-framewide { float: left; text-align: left; margin-right: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Peter and Rosemary Grant have documented decades of shifting natural selection on Darwin's finches (Geospiza spp.). Photo by Igooch.
Taking the finch by the beak

Part of the problem with assessing selection in nature is that most datasets measure selection over just one or a few years. One exception is the case of Darwin's finches in the Galapagos Islands. The Galapagos offers a wide variety of habitat types, and experiences substantial year-to-year environmental variation—a landscape that should exert all sorts of natural selection on its occupants. Peter and Rosemary Grant have studied Galapagos finches for decades now, and found that selection is continuously at work on these unassuming birds. (The Grants' book How and Why Species Multiply sums up their research program for a lay audience.)

Much of the Grants' work has focused on the finches' beaks, which largely determine what food the birds can eat. The distribution of seed sizes available on different Galapagos islands strongly predicts [PDF] the size of finches' beaks on those islands. In 1989, the Grants published estimates of selection on beak size in the finch species Geospiza conirostris following a drastic wet-to-dry climactic shift that radically changed what foods were available to the finches. They found strong selection [$a], with fitness-phenotype regression slopes as high as 0.37. What's more, the direction of selection changed dramatically from a very wet year to the dry year immediately afterward, as the finches were forced to move from feeding on small seeds and arthropods—which gave the advantage to shorter beaks—to hard-to-crack seeds, which required deep beaks.

The Grants' longer-term study of selection on Galapagos finches confirms this image of selection swinging back and forth unpredictably [PDF]. From 1972 to 2001, they tracked populations of the finch species G. scandens and G. fortis, and saw both more gradual long-term changes in the finches' body size and beak measurements as well as sudden sharp shifts. These changes continually altered the ability of the two species to hybridize, so that some years they were more reproductively isolated than others—and conditions in any one year were poor indicators of what would be going on five, ten, or twenty years later.

So when does selection matter?

The Grants' study makes natural selection look as shifting and impermanent as the wind. How can it shape patterns of evolution over millions of years, then? One possibility is that trends may emerge over longer periods of time, as wobbly selection moves species in new directions in a drunkard's walk, with two steps forward, then one step back, then four steps forward. Another is that lasting trends only occur when speciation intervenes to lock in fleeting changes due to variable natural selection [$a]. Much also depends on how selection interacts with mutation, genetic drift, and migration, as I'll discuss in the rest of this series.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } And here's a shameless plug for a t-shirt. Photo by ... Read more »

Futuyma, D. (1987) On the role of species in anagenesis. The American Naturalist, 130(3), 465-73. DOI: 10.1086/284724  

Grant, B., & Grant, P. (1989) Natural selection in a population of Darwin's finches. The American Naturalist, 133(3), 377-93. DOI: 10.1086/284924  

Grant, P.R., & Grant, B.R. (2002) Unpredictable evolution in a 30-Year study of Darwin's finches. Science, 296(5568), 707-11. DOI: 10.1126/science.1070315  

Kingsolver, J., Hoekstra, H., Hoekstra, J., Berrigan, D., Vignieri, S., Hill, C., Hoang, A., Gibert, P., & Beerli, P. (2001) The Strength of phenotypic selection in natural populations. The American Naturalist, 157(3), 245-261. DOI: 10.1086/319193  

Lande, R. (1976) Natural selection and random genetic drift in phenotypic evolution. Evolution, 30(2), 314-34. DOI: 10.2307/2407703  

Johnson, T., & Barton, N. (2005) Theoretical models of selection and mutation on quantitative traits. Phil. Trans. R. Soc. B, 360(1459), 1411-25. DOI: 10.1098/rstb.2005.1667  

Schluter, D., & Grant, P. (1984) Determinants of morphological patterns in communities of Darwin's finches. The American Naturalist, 123(2), 175-96. DOI: 10.1086/284196  

  • May 5, 2010
  • 10:05 AM
  • 512 views

Back to basics: The "Big Four"

by Jeremy Yoder in Denim and Tweed

The nice thing about a field season away from all regular internet access is that it gives you a real sabbatical of a sort—a chance to reassess plans and set new goals. One of the new goals I set myself this last field season was to introduce a new kind of topic here at Denim and Tweed.

Most of my writing about science at D&T focuses on recently published discoveries in evolution and ecology. It's fun writing, and it coincides neatly with my regular journal reading, and I intend to keep doing it. But I've discovered that when I want to put new work in context, I often need to discuss fundamental concepts of evolutionary biology that aren't necessarily common knowledge, such as genetic drift or sexual selection. However, I rarely have room to explain these concepts in depth within a blog post devoted to something else.

So maybe the solution is to devote some posts to explaining these "basics." I'm going to start with a series of posts on the "Big Four" processes of population genetics. These are the four processes that account, in one way or another, for every change in the frequency of genes within natural populations. In other words, the Big Four account for much of evolution itself. They are:Natural selection, changes in gene frequencies due to fitness advantages, or disadvantages, associated with different genes.
Mutation, the source of new forms of genes;
Genetic drift, or changes in gene frequencies that arise from the way probability works in finite populations; and
Migration, or changes in gene frequencies due to the movement of organisms from site to site.Lay readers may be surprised both by what we know, and what we don't, about how these four processes operate in nature. Natural selection is relatively easy to measure, and apparently ubiquitous [PDF] in natural populations—but we don't know how often the resulting short-term changes impact evolution over millions of years. Mutation, the source of variation on which natural selection acts, seems to vary widely among living things. Genetic drift means that a trait can come to dominate a population even if it has no fitness effect—or sometimes a deleterious one. Finally, migration across variable landscapes can interact with selection, drift, and mutation [$a] to completely alter their effects.

I'll devote one post each to selection, mutation, drift, and migration, discussing classic findings as well as more recent scientific discoveries about the each. They'll arrive as my usual mid-week science posts for the next four weeks, and I'll update this post with links to the others as they go online—so if this looks worth following, you can either bookmark this post, or subscribe to D&T's RSS Feed.

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } Natural selection, mutation, genetic drift, and migration act together to shape the evolution of natural populations. Photo by jby.
References

Drake JW, Charlesworth B, Charlesworth D, & Crow JF (1998). Rates of spontaneous mutation. Genetics, 148 (4), 1667-86 PMID: 9560386

Kingsolver, J., Hoekstra, H., Hoekstra, J., Berrigan, D., Vignieri, S., Hill, C., Hoang, A., Gibert, P., & Beerli, P. (2001). The strength of phenotypic selection in natural populations. The American Naturalist, 157 (3), 245-61 DOI: 10.1086/319193

Slatkin, M. (1987). Gene flow and the geographic structure of natural populations. Science, 236 (4803), 787-92 DOI: 10.1126/science.3576198

Wright S (1931). Evolution in Mendelian populations. Genetics, 16 (2), 97-159 PMID: 17246615

... Read more »

Drake JW, Charlesworth B, Charlesworth D, & Crow JF. (1998) Rates of spontaneous mutation. Genetics, 148(4), 1667-86. PMID: 9560386  

Kingsolver, J., Hoekstra, H., Hoekstra, J., Berrigan, D., Vignieri, S., Hill, C., Hoang, A., Gibert, P., & Beerli, P. (2001) The strength of phenotypic selection in natural populations. The American Naturalist, 157(3), 245-61. DOI: 10.1086/319193  

Slatkin, M. (1987) Gene flow and the geographic structure of natural populations. Science, 236(4803), 787-92. DOI: 10.1126/science.3576198  

Wright S. (1931) Evolution in Mendelian populations. Genetics, 16(2), 97-159. PMID: 17246615  

  • February 25, 2010
  • 10:05 AM
  • 502 views

Invasive species runs out of evolutionary "steam" as it invades

by Jeremy Yoder in Denim and Tweed

For invasive plants, flowering time is a trait that may often be under selection during colonization—when a plant flowers determines its climatic tolerances, its vulnerability to herbivores, and its compatibility with the local pollinator community. In a study just released online at Proceedings of the Royal Society B, Colautti and coauthors examined the evolution of this trait in a plant that has swept across eastern North America since its introduction from Europe: purple loosestrife, and found that it may be reaching the evolutionary limits of its invasive-ness.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Purple loosestrife, Lythrum salicaria, may be running out of evolutionary steam as it invades more northerly climes. Photo by Steve_C.Loosestrife, like many organisms, faces a trade-off in establishing a time to reproduce, between early flowering and accumulating resources for seed production. Early flowering means producing fewer seeds, or provisioning them less thoroughly—but as loosestrife colonizes more and more northerly climes, it will be under selection to flower earlier in compensation for shorter and shorter growing seasons.

But natural selection can only do so much. In order for natural selection to operate on a population, individuals in the population must vary in some trait that affects how many offspring they produce—if all individuals have the same trait value, or the same number of offspring, they'll all have equal chances to contribute to the next generation, which will probably look pretty much like the parental generation. Furthermore, species colonizing new territory may actually lose variation, either because new popualtions may be founded by just a few individuals, or because of the action of natural selection itself. Finally, there may be a point at which plants simply cannot flower any earlier, because they must reach a certain developmental point before reproducing.

Add these up for an invasive species moving north, and you might expect that the most recently-arrived (and most northerly) populations would flower earlier, and have less variation in flowering time, than more southerly populations. Using theoretical and experimental approaches, Colautti et al. show that exactly this process is occurring in purple loosestrife. They first built a mathematical model of natural selection acting on flowering time, which behaved as I've outlined above. They followed this by raising loosestrife seeds from northern and southern populations together in experimental sites located at different latitudes, and found that, even raised in southern conditions, seeds from northern sites grew into smaller, less productive plants. Raised in greenhouse conditions, seeds from southern populations produced plants with a much wider range of flowering times than seeds from northern populations. Together, these suggest that loosestrife has evolved earlier flowering times at northern sites—and may be running out of variation, the raw material for natural selection, as it moves north.

Invasive species often evolve in response to their new habitats, and force native species to evolve in response to their arrival. As they colonize Australia, for instance, cane toads (soon to be a major motion picture) have evolved longer legs [PDF] so as to win the race for unoccupied breeding ponds; and exerted selection on native black snakes to tolerate the toads' defensive toxins and to attack the toads less frequently. Better models of how newly introduced species respond to and exert natural selection may help conservation biologists anticipate the results of biological invasions.

References

Colautti, R., Eckert, C., & Barrett, S. (2010). Evolutionary constraints on adaptive evolution during range expansion in an invasive plant. Proc. R. Soc. B DOI: 10.1098/rspb.2009.2231

Phillips, B., Brown, G., Webb, J., & Shine, R. (2006). Invasion and the evolution of speed in toads. Nature, 439 (7078) DOI: 10.1038/439803a

Phillips, B., & Shine, R. (2006). An invasive species induces rapid adaptive change in a native predator: cane toads and black snakes in Australia Proc. R. Soc. B, 273 (1593), 1545-50 DOI: 10.1098/rspb.2006.3479

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification Trends in Ecology & Evolution, 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017

... Read more »

Colautti, R., Eckert, C., & Barrett, S. (2010) Evolutionary constraints on adaptive evolution during range expansion in an invasive plant. Proc. R. Soc. B. DOI: 10.1098/rspb.2009.2231  

Phillips, B., Brown, G., Webb, J., & Shine, R. (2006) Invasion and the evolution of speed in toads. Nature, 439(7078), 803. DOI: 10.1038/439803a  

Phillips, B., & Shine, R. (2006) An invasive species induces rapid adaptive change in a native predator: cane toads and black snakes in Australia. Proc. R. Soc. B, 273(1593), 1545-50. DOI: 10.1098/rspb.2006.3479  

Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007) Effects of exotic species on evolutionary diversification. Trends in Ecology , 22(9), 481-8. DOI: 10.1016/j.tree.2007.02.017  

  • February 17, 2010
  • 10:05 AM
  • 501 views

Caught between birds and squirrels, limber pines go both ways

by Jeremy Yoder in Denim and Tweed

Responding to natural selection often means compromising between different selective forces. A brief paper published online early at Evolution documents one such case – limber pine trees' compromise between protecting their seeds from squirrels, and making them accessible to the birds that disperse them. Pulled between these conflicting selective sources, some limber pine populations grow cones in a wider variety of shapes [$a].

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }

Cones of the limber pine must balance protection against squirrels with accessibility for seed-dispersing Clark's nutcrackers. Photos by Fool-On-The-Hill and almiyi.Seeds of the limber pine are cache-dispersed by Clark's nutcrackers. That is, the birds collect pine seeds to cache as a winter food source, but they collect many more than they need, and forget lots of them, and forgotten seeds are often able to sprout. However, squirrels also like pine seeds, and they harvest them before the birds start caching seeds. These two seed-harvesters generate conflicting selection on limber pine cones [$a]. Nutcrackers go for cones with lots of seeds protected by thinner scales; but so do squirrels.

However, pine-nut-eating squirrels are not present everywhere limber pines grow. The new study's authors, Siepielski and Benkman, take advantage of this quirk of distributions to perform a natural experiment, comparing pines that only need to satisfy their seed dispersers with pines that also need to defend against seed predators. Surveying cone shapes in populations of each class, they found that limber pine populations facing conflicting selection were bimodal, with trees mainly growing either squirrel-defended short, thick-scaled cones, or nutcracker-friendly longer, thin-scaled cones. Populations growing in regions without squirrels produced only nutcracker-friendly cones.

This apparently simple pattern conceals more complicated dynamics – in fact, as the authors disclose in the Discussion section, many other limber pine populations are solely composed of trees producing squirrel-defended cones. This is because, when pines establish in areas with large squirrel populations, nutcrackers may never colonize the area, or may visit less frequently and disperse fewer seeds. Without nutcracker dispersal, seeds are mainly dispersed after the cones fall, by rodent species that (unlike squirrels) forage on the ground. This makes squirrel defense the only selective priority. Populations displaying both cone types probably only arise in unique conditions, the authors say, where squirrels are present but not at high density.

References

Siepielski, A., & Benkman, C. (2007). Convergent patterns in the selection mosaic for two North American bird-dispersed pines. Ecological Monographs, 77 (2), 203-20 DOI: 10.1890/06-0929

Siepielski, A., & Benkman, C. (2009). Conflicting selection from an antagonist and a mutualist enhances phenotypic variation in a plant. Evolution DOI: 10.1111/j.1558-5646.2009.00867.x

... Read more »

Siepielski, A., & Benkman, C. (2007) Convergent patterns in the selection mosaic for two North American bird-dispersed pines. Ecological Monographs, 77(2), 203-20. DOI: 10.1890/06-0929  

Siepielski, A., & Benkman, C. (2009) Conflicting selection from an antagonist and a mutualist enhances phenotypic variation in a plant. Evolution. DOI: 10.1111/j.1558-5646.2009.00867.x  

  • February 11, 2010
  • 10:05 AM
  • 781 views

Bats can hold their liquor

by Jeremy Yoder in Denim and Tweed

A new paper in PLoS ONE tests the alcohol tolerances of nectar-eating bats. Believe it or not, there is a scientific purpose.

Alcohol isn't a vice exclusive to humans. Animals that eat fruit or nectar may accidentally imbibe if they eat past-ripe fruit or nectar that has had time to ferment. Some species, like the pentail treeshrew, have evolved tolerances that surpass our own capacities – and some, like cedar waxwings, get distinctly tipsy after a few bad berries. Alcohol tolerance effectively expands the potential food supply, by allowing consumption of fruit or nectar that's a little past the metaphorical sell-by date.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Artibeus lituratus, one of the bat species tested for alcohol tolerance by Orbach et al. Photo by Setsuo Tahara.In the new study, Dara Orbach and colleagues captured several individuals each from a variety of nectar-feeding bat species found near their field station in Belize. They fed the bats a sugar-water solution approximating nectar, spiking a randomly-chosen subset with ethanol. The bats were then released to fly through an obstacle course, where microphones were placed to record their echolocation calls. The authors figured that intoxication would manifest in difficulty navigating the obstacles, possibly in connection with distortions of echolocation.

However, bats fed alcohol performed as well as bats fed plain sugar water. Different species varied in how they handled the alcohol, though. The authors dosed the bats with alcohol in proportion to their body weight, but saliva tests just before the experiment revealed quite a bit of variation in actual blood alcohol concentration for individuals from different species. Evidently some species metabolized the alcohol, and others were able to fly straight even with a BAC that would put a human in jail for DUI.

Reference

Orbach, D., Veselka, N., Dzal, Y., Lazure, L., & Fenton, M. (2010). Drinking and flying: Does alcohol consumption affect the flight and echolocation performance of Phyllostomid bats? PLoS ONE, 5 (2) DOI: 10.1371/journal.pone.0008993

... Read more »

Orbach, D., Veselka, N., Dzal, Y., Lazure, L., & Fenton, M. (2010) Drinking and flying: Does alcohol consumption affect the flight and echolocation performance of Phyllostomid bats?. PLoS ONE, 5(2). DOI: 10.1371/journal.pone.0008993  

  • February 3, 2010
  • 10:05 AM
  • 706 views

Dethroning the Red Queen?

by Jeremy Yoder in Denim and Tweed

Regular readers of Denim and Tweed know that I'm fascinated by the evolution of species interactions: interactions between plants and nitrogen-fixing bacteria, Joshua trees and yucca moths, parasitoid wasps and butterflies, and between ants and the trees they guard. I tend to think that coevolutionary interactions not only determine the health of natural populations, but shape their evolutionary history. But would I feel that way if I were a paleontologist?

Running just to stay in place

The idea that interactions between species matter goes all the way back to the origins of evolutionary biology in the writing of Charles Darwin:What a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand; what war between insect and insect – between insects, snails, and other animals with birds and beasts of prey – all striving to increase, and all feeding on each other or on the trees or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees! (On the Origin of Species, 1859: 74-5)This image of constant struggle among living things was more formally encapsulated in a 1973 paper by Leigh Van Valen (which paper is not, alas, available online), who proposed that constant coevolution with other species should mean that natural populations of living things are constantly adapting – in response to competitors, mutualists, predators, parasites – without gaining ground in the struggle, because the other species are also adapting. Van Valen lifted an image from Lewis Carroll's Through the Looking-Glass, in which the Red Queen tells Alice that, in the strange world of Looking-Glass Land, "... it takes all the running you can do, to keep in the same place."They were running hand in hand, and the Queen went so fast that it was all she could do to keep up with her ... The most curious part of the thing was, that the trees and the other things around them never seemed to changed their places at all..flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
"... it takes all the running you can do, to keep in the same place." Image from Through the Looking-Glass, via VictorianWeb.Thus, this idea that fuels much of my research, and a great deal of scientific study over the last three decades, is often identified with the Red Queen. What is interesting about this result is that Van Valen wasn't interested in species interactions as such; he was trying to explain a pattern in the fossil record – that, for a wide variety of living things, the probability that a species would go extinct was independent of its age. That is, species that have been around for ten million years are no better adapted to their environments than species that have just formed; the probability of extinction is constant.

Van Valen's explanation for this result was that something must constantly act to prevent living things from becoming better adapted, and better able to resist extinction, over time – specifically, the Red Queen's race against other living things. Whenever a species "loses" the race, it goes extinct, regardless of how long the race has been up to that point. A similar pattern applies to the creation of new species – if coevolutionary interactions often help create reproductive isolation, then new species should also form at a roughly constant rate [$a]. Since this is what we observe, many biologists conclude that coevolution is responsible for the diversity of life on Earth.

What if the race doesn't matter?

Fortunately for the advance of knowledge, however, not all evolutionary biologists have the same perspective. Paleontologists, for instance, tend to think that the year-to-year dynamics of the Red Queen race don't make much difference in the longer run, over millions of years. They'd argue that most of the evolutionary change induced by coevolution between species is too variable and fleeting to have much effect on the rates at which species are formed and go extinct. Under this view, random geological events – continents splitting, mountain ranges rising, volcanoes erupting – are more likely to create new species and force them to extinction.

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
What matters more in the history of life, the biological environment, or the physical environment? Photos by Martin Heigan and Cedric Favero.This competing model should also lead to a roughly constant rate of species formation and extinction, but it predicts a different pattern of variation around that constant rate than the coevolutionary Red Queen does. If most speciation and extinction events are caused by coevolution, then the time periods between speciation events should follow a normal distribution – forming a "bell curve" with most periods close to the average length, and symmetrical tails of longer and shorter periods of time. On the other hand, if many different, individually rare geological events are the most common cause of speciation and extinction, the periods between speciation events should follow an exponential distribution, with most periods being shorter than the average, but a long tail of longer periods as well.

This contrast is the crux of a study recently published in Nature. The paper's authors, Venditti et al., examined 101 evolutionary trees estimated from genetic data, including groups like the dog family, roses, and bees. For each group's evolutionary tree, they determined the distribution of the lengths of time periods between speciation events. A majority of the trees – 78% – supported the exponential model. That is, 78% of the groups of organisms examined had evolved and diversified in a fashion best explained by geology, not coevolution. None of the groups fit the normal distribution, and only 8% fit the related lognormal distribution.

The Red Queen is dead, long live the Red Queen!

This result suggests that within many groups of organisms, the physical environment is a more common cause of reproductive isolation or extinction than the biological environment. However, this isn't to say that species interactions don't matter. As Van Valen originally noted, extinction rates may be roughly constant within large groups of organisms, like those examined by Venditti et al., but those constant rates vary from group to group. These differences in rate may still depend on species interactions, because species interactions can shape how prone a population is to reproductive isolation.

For instance, a group of plants that has lousy seed dispersers may form new species in response to much smaller, and more common, geological barriers than a group of plants whose seeds can travel for hundreds of miles. Additionally, species interactions that promote diversity within the interacting species may mean that when geology creates ... Read more »

Benton, M. (2010) Evolutionary biology: New take on the Red Queen. Nature, 463(7279), 306-7. DOI: 10.1038/463306a  

Futuyma, D. (1987) On the role of species in anagenesis. The American Naturalist, 130(3), 465-73. DOI: 10.1086/284724  

Stenseth, N., & Maynard Smith, J. (1984) Coevolution in ecosystems: Red Queen evolution or stasis?. Evolution, 38(4), 870-80. DOI: 10.2307/2408397  

Van Valen, L. (1973) A new evolutionary law. Evolutionary Theory, 1(1), 1-30. info:/

Venditti, C., Meade, A., & Pagel, M. (2009) Phylogenies reveal new interpretation of speciation and the Red Queen. Nature, 463(7279), 349-52. DOI: 10.1038/nature08630  

  • January 27, 2010
  • 10:05 AM
  • 926 views

"Chemical camouflage" lets leafhoppers hide from their own bodyguards

by Jeremy Yoder in Denim and Tweed

Many insects in the order Hemiptera – the "true" bugs – have evolved a way to hire their own protection by excreting sugary "honeydew." Honeydew attracts ants, who tend honeydew-producing bugs like livestock, protecting them from predators and even disease. Honeydew is cheap to make because honeydew producers typically make a living sucking the sap of their host plants; they're trading sugar and water, which they have in abundance, for safety.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Camponotus crassus ants protect Guayaquila xiphias leafhoppers, apparently mistaking them for part of their host plant. Detail of Silveira et al., figure 1.But there's a catch. Ants make good bodyguards because they are carnivorous – and they're perfectly willing to start eating their flock. A natural history note in the latest issue of The American Naturalist suggests that one group of ant-protected bugs deals with this problem by cloaking themselves in chemicals that make the ants think they're part of the host plant [$a].

The study's authors determined that organic compounds on the cuticle of the honeydew-producing leafhopper Guayaquila xiphias, which is often tended by the ant Camponotus crassus, were similar to compounds on the surface of the leafhoppers' preferred host plant. They presented ants with freeze-dried leafhoppers whose cuticles were washed clean with solvent, and found that the ants were much more likely to attack washed leafhoppers than unwashed ones; the ants were also more likely to attack leafhoppers they found on plants other than the preferred host. Finally, the authors replicated the earlier experiments using moth larvae coated with leafhopper cuticle compounds, and found that the "chemical camouflage" conferred the same protection on a different insect species.

This neat result shows how hazardous honeydew-producers' relationship with their ant bodyguards can be – they have to hide from the ants even as they offer them an inducement to stick around!

Reference

Silveira, H., Oliveira, P., & Trigo, J. (2010). Attracting predators without falling prey: Chemical camouflage protects honeydew‐producing treehoppers from ant predation The American Naturalist, 175 (2), 261-8 DOI: 10.1086/649580

... Read more »

Silveira, H., Oliveira, P., & Trigo, J. (2010) Attracting predators without falling prey: Chemical camouflage protects honeydew‐producing treehoppers from ant predation. The American Naturalist, 175(2), 261-8. DOI: 10.1086/649580  

  • January 19, 2010
  • 10:05 AM
  • 855 views

Evolving from pathogen to symbiont

by Jeremy Yoder in Denim and Tweed

Recently the open-access PLoS Biology published a really cool study in experimental evolution, in which a disease-causing bacterium was converted to something very like an important plant symbiont. The details of the process are particularly interesting, because the authors actually used natural selection to identify the evolutionary change that makes a pathogen into a mutualist.

Life as we know it needs nitrogen – it's a key element in amino acids, which mean proteins, which mean structural and metabolic molecules in every living cell. Conveniently for life as we know it, Earth's atmosphere is 78% nitrogen by weight. Inconveniently, that nitrogen is mostly in a biologically inactive form. Converting that inactive form to biologically useful ammonia is therefore extremely important. This process is nitrogen fixation, and it is best known as the reason for one of the most widespread mutualistic interactions, between bacteria capable of fixing nitrogen and select plant species that can host them.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Clover roots, with nodules visible (click through to the original for a nice, close view. Photo by oceandesetoile.In this interaction, nitrogen-fixing bacteria infect the roots of a host plant. In response to the infection, the host roots form specialized structures called nodules, which provide the bacteria with sugars produced by the plant. The bacteria produce excess ammonia, which the plant takes up and puts to its own uses. The biggest group of host plants are probably the legumes, which include the clover pictured to the right, as well as beans – this nitrogen fixation relationship is the reason that beans are the best source of vegetarian protein, and why crop rotation schemes include beans or alfalfa to replenish nitrogen in the soil.

For the nitrogen-fixation mutualism to work, free-living bacteria must successfully infect newly forming roots in a host plant, and then induce them to form nodules. The chemical interactions between bacteria and host plant necessary for establishing the mutualism are pretty well understood, and in fact genes for many of the bacterial traits, including nitrogen-fixation and nodule-formation proteins thought to be necessary to make it work are conveniently packaged on a plasmid, a self-contained ring of DNA separate from the rest of the bacterial genome, which is easily transferred to other bacteria.

This is exactly what the new study's authors did. They transplanted the symbiosis plasmid from the nitrogen-fixing bacteria Cupriavidus taiwanensis into Ralstonia solanacearum, a similar, but disease-causing, bacterium. With the plasmid, Ralstonia fixed nitrogen and produced the protein necessary to induce nodule formation – but host plant roots infected with the engineered Ralstonia didn't form nodules. Clearly there was more to setting up the mutualism than the genes encoded on the plasmid.

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Wild-type colonies of Ralstonia (tagged with fluorescent green) are unable to enter root hairs (A), but colonies with inactivated hrcV genes are able to enter and form "infection threads," like symbiotic bacteria (B). Detail of Marchetti et al. (2010), figure 2.
This is where the authors turned to natural selection to do the work for them. They generated a genetically variable line of plasmid-carrying Ralstonia, and used this population to infect host plant roots. If any of the bacteria in the variable population bore a mutation (or mutations) necessary for establishing mutualism, they would be able to form nodules in the host roots where others couldn't. And that is what happened: three strains out of the variable population successfully formed nodules. The authors then sequenced the entire genomes of these strains to find regions of DNA that differed from the ancestral, non-nodule-forming strain.

This procedure identified one particular region of the genome associated with virulence – the disease-causing ability to infect and damage a host – that was inactivated in the nodule-forming mutant strains. As seen in the figure I've excerpted above, plasmid-bearing Ralstonia with this mutation were able to form infection threads, an intermediate step to nodule-formation, where plasmid-bearing Ralstonia without the mutation could not. Clever use of experimental evolution helped to identify a critical step in the evolution from pathogenic bacterium to nitrogen-fixing mutualist.

References

Amadou, C., Pascal, G., Mangenot, S., Glew, M., Bontemps, C., Capela, D., Carrere, S., Cruveiller, S., Dossat, C., Lajus, A., Marchetti, M., Poinsot, V., Rouy, Z., Servin, B., Saad, M., Schenowitz, C., Barbe, V., Batut, J., Medigue, C., & Masson-Boivin, C. (2008). Genome sequence of the  beta-rhizobium Cupriavidus taiwanensis and comparative genomics of rhizobia. Genome Research, 18 (9), 1472-83 DOI: 10.1101/gr.076448.108

Gitig, D. (2010). Evolving towards mutualism. PLoS Biology, 8 (1) DOI: 10.1371/journal.pbio.1000279

Marchetti, M., Capela, D., Glew, M., Cruveiller, S., Chane-Woon-Ming, B., Gris, C., Timmers, T., Poinsot, V., Gilbert, L., Heeb, P., Médigue, C., Batut, J., & Masson-Boivin, C. (2010). Experimental evolution of a plant pathogen into a legume symbiont. PLoS Biology, 8 (1) DOI: ... Read more »

Amadou, C., Pascal, G., Mangenot, S., Glew, M., Bontemps, C., Capela, D., Carrere, S., Cruveiller, S., Dossat, C., Lajus, A.... (2008) Genome sequence of the  beta-rhizobium Cupriavidus taiwanensis and comparative genomics of rhizobia. Genome Research, 18(9), 1472-1483. DOI: 10.1101/gr.076448.108  

Gitig, D. (2010) Evolving towards mutualism. PLoS Biology, 8(1). DOI: 10.1371/journal.pbio.1000279  

Marchetti, M., Capela, D., Glew, M., Cruveiller, S., Chane-Woon-Ming, B., Gris, C., Timmers, T., Poinsot, V., Gilbert, L., Heeb, P.... (2010) Experimental evolution of a plant pathogen into a legume symbiont. PLoS Biology, 8(1). DOI: 10.1371/journal.pbio.1000280  

  • January 13, 2010
  • 01:05 PM
  • 462 views

Why make your own food when it doesn't pay?

by Jeremy Yoder in Denim and Tweed

We humans like to think we're pretty complex – what with having invented the wheel, wars, New York, and so on – so we tend to forget that evolution doesn't care about complexity. All that matters to natural selection is who makes the most babies, and sometimes complex adaptations can get in the way of that criterion. A study recently published on the always open-access PLoS ONE provides a good example of this principle in action – given the right selective pressures, photosynthetic organisms will give up on the whole photosynthesis thing.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }

Tiny Indianpipe (Monotropa) and giant Rafflesia, two plants that gave up photosynthesis. Photos by Bemep and Tamara van Molken.Photosynthesis is clearly a complex adaptation, requiring specialized cellular structures and biochemical processes that can use light to power the synthesis of sugars. Complex enough for a whole additional organism, in fact, since the chloroplast, the cellular structure in which most eukaryotes conduct photosynthesis, probably originated as a symbiont that never left its host cell [$a]. (In some organisms, this process of becoming photosynthetic is still underway.) There are clear advantages to the ability to make your own food conferred by photosynthesis. Yet there are numerous examples of non-photosynthetic organisms with photosynthetic ancestors. For instance, plants as varied as the big, exotic Rafflesia or Monotropa, whose small white flowers are easy to spot in North American woods, have inactive chloroplasts and parasitize other plants. These cases are good reason to think that there may be selective conditions in which the cost of maintaining the mechanisms of photosynthesis outweighs the benefit of independent food production.

The new paper describes just such a set of selective conditions. The authors build a mathematical model of competition between microorganisms, such as flagellates, that can either be mixotrophs, able to conduct photosynthesis or capture prey to feed themselves, or heterotrophs, only able to sustain themselves by eating other critters. The model's result hinges on two key facts of life for single-celled predators: (1) it turns out that the size of a flagellate cell determines what size of prey it is best able to capture [$a]; and (2) chloroplasts take up space in a cell, limiting the evolution of cell size.

The relative advantage of retaining photosynthesis, then, is directly related to the size range of available prey. Mixotrophs, whose cells are big enough to accommodate chloroplasts, are most efficient predators of larger prey; with no chloroplasts, heterotrophs can be small enough to take advantage of smaller prey. The question of which form wins out, then, relies on the distribution of available prey sizes and the light environment. If there's lots of light for photosynthesis, mixotrophs can out-compete heterotrophs even if they don't hunt very efficiently; but if there's not much light and mostly small prey, the more efficient heterotrophs win.

The fact is, it's rare for any given adaptation to be useful under all possible conditions. Biological structures or metabolic processes that become disused are no longer under selection for efficient performance of their original function – they are free to accumulate mutations that may make them degenerate into uselessness, or to be co-opted for entirely new functions. But if an adaptation is actually costly to maintain, then natural selection may eradicate it altogether.

References

de Castro, F., Gaedke, U., & Boenigk, J. (2009). Reverse evolution: Driving forces behind the loss of acquired photosynthetic traits. PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008465

Hansen, B., P. K. Bjornsen, & P. J. Hansen (1994). The size ratio between planktonic predators and their prey.
Limnology and Oceanography, 39, 395-403

McFadden, G. (2001). Chloroplast origin and integration Plant Physiology, 125 (1), 50-3 DOI: 10.1104/pp.125.1.50

... Read more »

de Castro, F., Gaedke, U., & Boenigk, J. (2009) Reverse evolution: Driving forces behind the loss of acquired photosynthetic traits. PLoS ONE_id, 4(12). http://dx.plos.org/10.1371/journal.pone.0008465

Hansen, B., P. K. Bjornsen, & P. J. Hansen. (1994) The size ratio between planktonic predators and their prey. . Limnology and Oceanography, 395-403. info:/

McFadden, G. (2001) Chloroplast Origin and Integration. Plant Physiology, 125(1), 50-3. DOI: 10.1104/pp.125.1.50  

  • January 7, 2010
  • 10:05 AM
  • 988 views

Masquerading caterpillars hide in plain sight

by Jeremy Yoder in Denim and Tweed

Insects that have evolved elaborate mimicry of inanimate objects – leaves, twigs, even bird droppings – to hide from predators are a staple of nature documentaries. But do these masquerades work because they help insects blend into the background, or because predators actually see the insects and then dismiss them as inedible leaves, twigs, or bird droppings? It's a tricky question to answer, but a brief paper in this week's Science presents an experiment that tries to do just that [$a].

The paper's authors reasoned that if mimicry-based camouflage works through disguise rather than invisibility, a predator's experience might determine their response to mimic camouflage. They trained three experimental groups of young domestic chicks by introducing them into trial arenas containing either natural hawthorn branches, empty arenas, or hawthorn branches wrapped in purple thread. The wrapped branches were used to test whether the chicks would be more or less likely to attack something twig-like but differently colored (though this is only clear from the supplementary online material).

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Larva of the brimstone moth Opisthograptis luteolata, looking distinctly twig-like. Photo by Michael E. Talbot.
The authors then presented chicks from each "training" group with either one of two species of hawthorn-twig-mimicking moth larvae (the brimstone moth, or the early thorn moth), or a hawthorn twig about the size of a caterpillar. Chicks that had previously encountered natural twigs waited longer to attack the caterpillars than chicks that hadn't previously seen twigs, or that saw the colored hawthorn branches. So, apparently, the chicks were reasoning (inasmuch as chicks reason) that the twig-like object in front of them was the same as the inedible twigs they had tried before.

This is an elegant experimental test of the effect of mimicry as mimicry – what the authors propose to call camouflage by "masquerade." However, it doesn't actually show that what the authors term camouflage by crypsis – blending into the background – isn't also contributing to the benefits that these caterpillars receive from their unique shape and coloration. There's no reason to think that twig-shaped caterpillars can't benefit in both ways, by being less visible in the first place, and then easily mistaken for a twig if they are seen.

In conclusion, here's some video footage of another natural mimic, the leaf insect.



Reference

Skelhorn, J., Rowland, H., Speed, M., & Ruxton, G. (2010). Masquerade: Camouflage without crypsis Science, 327 (5961), 51 DOI: 10.1126/science.1181931

... Read more »

Skelhorn, J., Rowland, H., Speed, M., & Ruxton, G. (2010) Masquerade: Camouflage without crypsis. Science_id, 327(5961), 51-51. http://www.sciencemag.org/cgi/doi/10.1126/science.1181931

  • December 30, 2009
  • 10:05 AM
  • 956 views

Escaping the "poverty trap" of infectious disease

by Jeremy Yoder in Denim and Tweed

Even in the twenty-first century, infectious diseases such as malaria, dengue fever, cholera, and AIDS remain widespread in much of the developing world, at tremendous cost to human life and economic productivity. Poorer nations lack the resources for more effective public health measures; but widespread infectious disease may slow or prevent the economic development that can provide those resources. A new paper in Proceedings of the Royal Society tries to sort out this chicken-and-egg problem, and finds that economic development is the fastest route out of the "poverty trap" [$a].

The paper's authors, Bonds et al. start with a classic model of infectious disease, in which susceptible (healthy) members of a population have a chance of becoming infected whenever they encounter an infected person, and infected people have a chance to recover to susceptible condition if they survive the effects of the disease. The first probability is the rate of transmission from person to person; the second is the rate of recovery from disease caused by the infection.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
A woman receives tetanus vaccine in the Central African Republic. Photo by hdptcar.Bonds et al. insert economics into this basic model by reasoning that the rate of recovery is a function of per-capita income – well-fed people are better able to fight off infection – and that income is a function of the proportion of the population that remains uninfected at any given time. This yields a mathematical version of the poverty trap I outlined above: high-income populations are easily able to fight off infection and remain near 100 percent susceptible, but highly infected societies are unable to increase their per-capita income to reduce their rate of infection. However, there is an internal equilibrium point – a level of income and infection from which a population could easily move in either direction, towards high income and low infection or high infection and low income.

The question then becomes how best to push a developing nation's population toward that threshold condition – or how best to bring the threshold closer. Bonds et al. compare two options: reducing the rate of disease transmission, and boosting individual economic productivity. The former captures the effect of boosting public health – vaccination, better sewage treatment, food aid. The latter captures the effect of improving infrastructure or financial institutions – making the economy more developed. They found that the threshold condition is more sensitive to economic productivity. Even at low transmission rates, a society can be caught in the poverty trap if its productivity is low enough, but at high enough productivity levels, societies can avoid the trap created by even highly transmissible diseases.

This suggests that, although medical aid can help the acute problem of infectious disease, it's investment in economic development that can ultimately solve it.

Reference

Bonds, M., Keenan, D., Rohani, P., & Sachs, J. (2009). Poverty trap formed by the ecology of infectious diseases Proc. R. Soc. B DOI: 10.1098/rspb.2009.1778

... Read more »

Bonds, M., Keenan, D., Rohani, P., & Sachs, J. (2009) Poverty trap formed by the ecology of infectious diseases. Proc. R. Soc. B. DOI: 10.1098/rspb.2009.1778  

  • December 23, 2009
  • 10:05 AM
  • 783 views

Why aren't there more sickle-cell anemics in the Mediterranean?

by Jeremy Yoder in Denim and Tweed

The story of sickle-cell anemia and its malaria-protective effects is a textbook case how environmental context determines the fitness of a given genetic profile. However, the evolution of human blood disorders in response to selection from malaria parasites might be more complicated than that textbook story.

.flickr-photo { }.flickr-frameright { float: right; text-align: left; margin-left: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }

Malaria-causing parasites (dark-stained) among human red blood cells (top), and "sickled" red blood cells (bottom). Photos via WikiMedia Commons.Malaria is caused by mosquito-spread parasites that attack their hosts' oxygen-bearing red blood cells. A particular mutation in the gene that codes for part of the hemoglobin molecule – the molecule that actually stores oxygen inside red blood cells – leads to deformed, sickle-shaped, blood cells. People who carry two copies of the sickle cell gene develop sickle-cell disease, in which the sickle-shaped cells reduce oxygen transport efficiency and interfere with blood circulation. People with only one copy of the sickle-cell gene are healthy, and better able to resist malaria infection than those with no copies. The textbook story is that, in regions where malaria is common, such as sub-Saharan Africa, the advantage of malaria resistance is enough to offset the fitness risk of carrying the sickle-cell gene – that one-fourth of children born to parents who each have one copy of the gene will themselves have two copies and develop sickle-cell disease.

However, there are regions like the Mediterranean where malaria has historically been prevalent, but in which the human population hasn't evolved the higher frequency of sickle-cell genes that you'd expect from the scenario outlined above. A new paper in PNAS demonstrates that this may be because of interactions between the sickle-cell gene and another genetic blood disorder, thalassemia [$a].

Thalassemia is a class of genetic disorders affecting the protein subunits that comprise hemoglobin. Each hemoglobin molecule is formed by binding together two "alpha"-type subunits, and two "beta"-type subunits. If there is a shortage of correctly-formed subunits of either type, then hemoglobin formation is impaired, resulting in anemia or (if the mutation stops subunit production altogether) death. However, like sickle-cell genes, thalassemic mutations can confer resistance to malaria; and if alpha-thalassemia is paired with beta-thalassemia, the reduced production of both subunits can balance out.

As it happens, in combination with alpha-thalassemia, the sickle-cell gene's malaria protection is neutralized. Using population genetic models, the new study's authors show that this effect may have actively prevented the sickle-cell gene from establishing in the Mediterranean, where alpha- and beta-thalassemias are more common than in Africa. In the Mediterranean, the presence of beta-thalassemia genes reduces the fitness cost of (mild) alpha-thalassemia genes; and in the presence of alpha-thalassemia genes, the sickle-cell gene confers no protection to people with one copy but still induces sickle-cell disease in people with two copies.

These interactions between genes are called epistasis, and they can have dramatic impacts on evolution. Although I haven't seen many cases as well-characterized as this one, epistasis is probably widespread in the complex systems of genomes, where thousands of regulatory and protein-coding genes interact to build living things.

References

Penman, B., Pybus, O., Weatherall, D., & Gupta, S. (2009). Epistatic interactions between genetic disorders of hemoglobin can explain why the sickle-cell gene is uncommon in the Mediterranean Proc. Nat. Acad. Sci. USA, 106 (50), 21242-6 DOI: 10.1073/pnas.0910840106

... Read more »

Penman, B., Pybus, O., Weatherall, D., & Gupta, S. (2009) Epistatic interactions between genetic disorders of hemoglobin can explain why the sickle-cell gene is uncommon in the Mediterranean. Proc. Nat. Acad. Sci. USA, 106(50), 21242-6. DOI: 10.1073/pnas.0910840106  

  • December 16, 2009
  • 10:05 AM
  • 757 views

Cuckholding crows don't necessarily have fitter chicks

by Jeremy Yoder in Denim and Tweed

Birds are bad at monogamy. There are a number of good evolutionary reasons to cheat on your mate, and it's not clear which one is the most likely explanation. A new study of American crows, however, suggests that, for females, cheating isn't necessarily the best choice [$-a].

Avian infidelity isn't obvious, because many birds are socially monogamous, forming couples for one or more breeding seasons to raise chicks. However, DNA-based paternity testing has overturned this intuition -- a 2002 review of such studies [PDF] estimated that "cheating" occurs in 90% of bird species, and an average of 11% of chicks are "illegitimate."

The biological term for this non-monogamy is "extrapair copulation," often abbreviated to EPC. Evolutionary reasons for EPC behavior break down by which parent benefits from the cuckoldry: Females benefit if EPC means their chicks will be less inbred, which can make them less prone to disease or recessive genetic disorders. Males benefit if EPC means they will have more chicks than they would otherwise. Perhaps more importantly, EPC might impose real costs on females, if it leads mated males to invest less in caring for the chicks in their nests because they can't be sure the chicks are theirs [PDF].

.flickr-photo { }.flickr-frameleft { float: left; text-align: left; margin-right: 15px; margin-bottom: 15px; width:40%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; }
Crows in flight. Photo by wolfpix.In the new study, Townsend et al. evaluated the costs and benefits of EPC for female American crows, which have a social structure that adds a twist to the cost-benefit analysis. Mated pairs of crows live in larger family groups, which include "auxiliary," unmated males who may help feed and protect chicks -- perhaps especially if those chicks are the result of their own EPC. Females also engaged in EPC with males from outside the family group, who should be less closely related than within-group males, and whose chicks would be more genetically healthy than those sired by any within-group male, mated or not.

Townsend et al. observed several such family groups over four years, using DNA fingerprinting methods to identify the parents of chicks as they were born, and tracking the chicks' health and survival as well as how frequently mated crows and auxiliary males tended them. Contrary to what might have been expected, chicks produced by EPC were more, not less, inbred; they didn't grow faster or have a higher probability of survival than chicks produced by mated parents. On the other hand, cuckholded males tended chicks sired by others as often as they did their own.

The most telling result is that broods containing chicks produced by EPC were more frequently tended by auxiliary males -- but only when the EPC was with a within-group male. This suggests that EPC mainly benefits male crows, not females. From a mated female's perspective, EPC produces chicks that are less genetically fit, and no more or less likely to survive, than chicks sired by her mate. On the other hand, an unmated male can only have offspring through EPC, and if he does, it makes sense for him to give them extra assistance. Males from outside the family group don't stick around to offer that help, but auxiliary males from within the group can, and do.

References

Arnqvist, G., & Kirkpatrick, M. (2005). The evolution of infidelity in socially monogamous passerines: The strength of direct and indirect selection on extrapair copulation behavior in females. The American Naturalist, 165 (s5) DOI: 10.1086/429350

Griffith, S.C., Owens, I.P.F., & Thuman, K.A. (2002). Extrapair paternity in birds: A review of interspecific variation and adaptive function. Molecular Ecology (11), 2195-212 : 10.1046/j.1365-294X.2002.01613.x

Townsend, A., Clark, A., & McGowan, K. (2010). Direct benefits and genetic costs of extrapair paternity for female American Crows (Corvus brachyrhynchos). The American Naturalist, 175 (1) DOI: 10.1086/648553

... Read more »

Arnqvist, G., & Kirkpatrick, M. (2005) The evolution of infidelity in socially monogamous passerines: The strength of direct and indirect selection on extrapair copulation behavior in females. The American Naturalist, 165(s5). DOI: 10.1086/429350  

Griffith, S.C., Owens, I.P.F., & Thuman, K.A. (2002) Extrapair paternity in birds: A review of interspecific variation and adaptive function. Molecular Ecology, 2195-212. info:/10.1046/j.1365-294X.2002.01613.x

Townsend, A., Clark, A., & McGowan, K. (2010) Direct benefits and genetic costs of extrapair paternity for female American Crows (Corvus brachyrhynchos). . The American Naturalist, 175(1). DOI: 10.1086/648553  

join us!

Do you write about peer-reviewed research in your blog? Use ResearchBlogging.org to make it easy for your readers — and others from around the world — to find your serious posts about academic research.

If you don't have a blog, you can still use our site to learn about fascinating developments in cutting-edge research from around the world.

Register Now

News

Editor's Selections: Beautiful eclogite, Poker studies, and Astrobiology

Editor's Selections: Sooty astrophysics, Kids vs. Crows, and Cell Phone Anxiety

Editor's Selections: Dark Galaxies Illuminated, Exploring the Uncanny Valley, and the Hippies of Physics

Research Blogging is powered by SMG Technology.

To learn more, visit seedmediagroup.com.