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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
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September 1, 2010
09:05 AM
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New cooperation theory has major Mommy issues

by Jeremy Yoder in Denim and Tweed

The cover article for last week's issue of Nature promised to be the last word in a long-running scientific argument over the evolution of cooperation—but it really just rejiggers the terms of the debate. Instead of solving the problem of how cooperative behavior can evolve, the new paper presents a model of maternal enslavement [$a]. These are not, it turns out, quite the same thing.

Group selection versus kin selection

Let's start with some background. Unselfish, cooperative behavior has long been a puzzle in evolutionary biology, because natural selection should never favor individuals who make significant sacrifices for the benefit of others. Sure, an unselfish individual might expect those she helps to reciprocate later; but a population of the unselfish would be easily overrun by those who don't reciprocate.

There have historically been two answers to the problem of the selfish out-competing the unselfish. The first case is basically an extension of logic we all learned in kindergarten: cooperative groups can do things that uncooperative groups can't. Like, for instance, start a neighborhood garden.

Under this model, neighborhoods of cooperative, garden-making people are nicer places to live, and their inhabitants can collectively out-compete other neighborhoods that can't get it together to start a community garden. In evolutionary terms, this is group selection—even if individuals sacrifice to build the garden, the group as a whole benefits. Unfortunately, this breaks down if the new garden attracts selfish people to move to the neighborhood, buy up all the cheap real estate, and open Urban Outfitters franchises.

There's another possibility, though. What if unselfish behavior isn't always truly unselfish? For instance, if you help your relatives, you're actually helping some of your own genes. You share half your genes with your siblings, a quarter of your genes with half-siblings, an eighth of your genes with first cousins, and so on. This means that Michael Bluth might be on to something.

Evolutionarily speaking, it doesn't matter if Michael spends all his time helping his feckless family, as long those efforts help someone in the family (G.O.B., most likely) reproduce and perpetuate some of the genes that Michael shares with him or her. This idea was advanced by W.D. Hamilton in two 1964 papers, one mathematical [PDF], and one more focused on real-world examples [PDF]; we now know it as kin selection. It doesn't hold up so well for maintaining the kind of complex society humans have today, where we interact with lots of completely unrelated people—but it might have got the ball rolling toward the wheel, war, New York and so forth by selecting for cooperative behaviors within small tribes back at the dawn of history.

The group selection versus kin selection debate has gone back and forth for decades, and the new paper is a shot across the bow of kin selection. The authors, Martin Nowak, Corina Tarnita, and E.O. Wilson, aim to do two things: first, prove that kin selection is wrong; and second, describe an alternative explanation. For the first, they argue that kin selection only applies in narrow circumstances, that those circumstances never show up in nature, and that empirical studies just don't support the model. Johnny Humphreys makes some reasonable objections to these arguments, and so do several folks interviewed by Carl Zimmer, and I'll refer you there rather than try to improve on them.* I'm more interested in the second part: the alternative explanation.

Enslaved by Mom
.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; } No individual fitness for you—you're cogs in the Superorganism. Photo by jby.Nowak et al. propose to explain the evolution of unselfishness as it applies to eusociality—organisms like ants or bees or naked mole rats, in which colonies of (closely related) individuals defer most or all of their opportunities to reproduce, in order to support one or a few individuals that reproduce a lot. As Johnny points out in his critique, it's not clear that eusociality is the same thing as unselfishness at all, even though it's historically cited as an example of unselfishness [$a]. The new model that Nowak et al. develop actually makes the difference between eusociality and unselfishness even clearer. Under their model, it's not that worker ants give up reproductive opportunities to help their mother, the Queen, reproduce—it's that the Queen takes away their reproductive opportunities.

The key insight of the new model is that, in evolving from a non-social insect to a eusocial one, the natural selection that matters affects not the individuals evolving into workers, but the individual who would be Queen. Consider an insect similar to the probable ancestor of ants: females build nests, provision them with food, and lay eggs inside. Nowak et al. propose that a female who evolved the ability to lay "worker" eggs—females that grow up not to found their own nest, but to help in their mother's—would have greater fitness than females without such helpful offspring.

Aside from the probability of evolving "worker" eggs (which is not a small issue, I think), this shift in perspective from the fitness of the worker to the fitness of the Queen makes all sorts of sense to me. I've often wondered why myrmecologists don't treat ant colonies as single organisms, rather than collections of cooperating individuals.

But this approach also seems to sidestep the key question biologists hope to answer with kin selection and group selection models—these models aim to explain how individuals can come together to cooperate, but Nowak et al. have built a model that looks more like enslavement. I can't learn anything about how unselfish behavior can spontaneously evolve in a population by looking at a population that has had unselfishness imposed upon it. To indulge in one last especially geeky pop culture reference, it'd be like trying to learn about market economics by studying The Borg.

Nowak, Tarnita, and Wilson might have come up with a very good model for the evolution of eusociality; but if so, it means that eusociality is a bad model for the evolution of cooperation as we usually conceive it.

------------
* I will, however, note that Nowak et al. do something I've never seen in a scholarly paper before—in dismissing empirical studies of kin selection, they defer substantive discussion to the Supplementary Information. There are, in fact, 43 pages of SI for this 6-page paper, including two major mathematical models and the discussion of empirical kin selection studies. This is a problem, but one that is beyond the scope of this already-long post.

References

... Read more »

Axelrod, R., & Hamilton, W. (1981) The evolution of cooperation. Science, 211(4489), 1390-1396. DOI: 10.1126/science.7466396

Hamilton, W.D. (1964) The genetical evolution of social behaviour. I. Journal of Theoretical Biology, 7(1), 1-16. DOI: 10.1016/0022-5193(64)90038-4

Hamilton, W.D. (1964) The genetical evolution of social behaviour. II. Journal of Theoretical Biology, 7(1), 17-52. DOI: 10.1016/0022-5193(64)90039-6

Nowak, M., Tarnita, C., & Wilson, E. (2010) The evolution of eusociality. Nature, 466(7310), 1057-62. DOI: 10.1038/nature09205

August 30, 2010
09:05 AM
34 views

On competition, ecological opportunity, and Sahney et al.

by Jeremy Yoder in Denim and Tweed

There's already been a lot of blogospheric discussion of the BBC's recent declaration that "Darwin may have been wrong" based on a recently-published paleontology paper. I hadn't paid it much attention, because while sloppy science journalism irritates me, it's not quite in my wheelhouse, expertise-wise. Then I actually got around to reading the paper, and it turns out that it's directly related to some of my own work—and the conclusion that led to the sensationalistic sub-headline doesn't make any sense.

Coauthors Sahney, Benton, and Ferry analyze the fossil record of four-limbed vertebrates—tetrapods—to show that in general, as more species evolve, they also evolve to fill a wider variety of ecological roles [$a]. Ecological roles are here defined by combinations of body size, diet, and habitat. (Sahney et al estimate there are 207 such combinations possible, though only 75 are "occupied.") That's a straightforward and mostly unsurprising result—the number of tetrapod species increases as tetrapods evolve new ways to make a living. But then we get to the conclusions of the paper, and things get weird.

Sahney et al. conclude that because diversification is associated with finding unoccupied ecological roles, competition is mostly unimportant in the diversification of tetrapods: "Given the unrestricted access tetrapods have to ecospace, perhaps there is little need for competitive interactions to shape diversification." In other words, if diversification happens by finding ways to make a living that aren't already occupied, competition isn't important.

Except that the very reason species diversify following an ecological opportunity like the development of a new ecological role is the lack of competition the new role provides. As my coauthors and I documented in a recently published literature review, competition shapes the kind of diversification documented by Sahney et al. in two ways: first, by its absence following the evolution of a new lifestyle; then in spurring an adaptive radiation as new species evolve to partition up the newly-available "ecospace."

What makes this doubly odd is that Sahney et al. refer to another kind of ecological opportunity, the extinction of competitors, as a good example of competition-driven diversification. But a central insight of the literature on ecological opportunity is that diversifying because a whole bunch of ecological roles have just opened up is not fundamentally different from diversifying after a new mutation makes a never-before-seen ecological role possible. Think of it like starting a new business: to avoid competition, you could either sell an existing product in a place where no one else sells that product, or you can invent a product no one else offers. Both approaches give you a market all to yourself, and both are defined by competition.

It's hard for me to understand why Sahney et al. don't make this conceptual connection—which, for what it's worth, has its roots in The Origin of Species.

References

Sahney, S., Benton, M., & Ferry, P. (2010). Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters, 6 (4), 544-7 DOI: 10.1098/rsbl.2009.1024

Yoder, J.B., Des Roches, S., Eastman, J.M., Gentry, L., Godsoe, W.K.W., Hagey, T., Jochimsen, D., Oswald, B.P., Robertson, J., Sarver, B.A.J., Schenk, J.J., Spear, S.F., & Harmon, L.J. (2010). Ecological opportunity and the origin of adaptive radiations Journal of Evolutionary Biology, 23 (8), 1581-96 DOI: 10.1111/j.1420-9101.2010.02029.x

... Read more »

Sahney, S., Benton, M., & Ferry, P. (2010) Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters, 6(4), 544-7. DOI: 10.1098/rsbl.2009.1024

Yoder, J.B., Des Roches, S., Eastman, J.M., Gentry, L., Godsoe, W.K.W., Hagey, T., Jochimsen, D., Oswald, B.P., Robertson, J., Sarver, B.A.J.... (2010) Ecological opportunity and the origin of adaptive radiations. Journal of Evolutionary Biology, 23(8), 1581-96. DOI: 10.1111/j.1420-9101.2010.02029.x

August 24, 2010
09:05 AM
42 views

Are mutualists monogamists, while antagonists play the field?

by Jeremy Yoder in Denim and Tweed

Two of the most diverse groups of living things on Earth are flowering plants and the insects that make their living from flowering plants. Biologists have long thought that the almost incessant, intimate interactions between plants and plant-eating insects might be the evolutionary cause of each group's spectacular diversity. On a smaller scale, this means that we're interested in the reasons that specific insects and plants interact in the first place—what evolutionary trails leads one insect species to specialize on a single host while others eat pretty much any plant they land on.

A new study of one group of plant-eating insects suggests that the kind of interaction between insects and their host plants also determines how specific those interactions are. Examining a group of moths that, like the yucca moths I study, pollinate their host plant and then eat some of its seeds, the authors of the new study find that related, non-pollinating moths use more host plant species than the pollinators [$a]. I think it makes a particularly nice companion piece to my post about the evolutionary origins of yucca moths, because it provides an example of one or two other things biologists can deduce from phylogenies—and, as we'll see, some things they can't.

Epicephala: like a yucca moth without the snappy name

The moths in question are in the genus Epicephala, and they have an obligate pollination relationship with trees in the genus Glochidion, a diverse group of plant species found in southern Asia. That is, female moths carry pollen between Glochidion flowers in special mouthparts, deliberately apply pollen to the flower, and then lay eggs in the flower so that, when it develops into a fruit, her larvae can eat some of the seeds inside. Epicephala species are highly specialized, with most species only using one species of Glochidion [$a]. That's a higher degree of specialization than what's seen in yucca moths, in fact.
.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; } Pollinating moths (genus Ephicephala, left) use fewer host plant species than related non-pollinating moths (genus Caloptilia, right). Photos by CharlesLam and Bettaman.The family of which Epicephala is a member happens to include other moths that interact with Glochidion, but only as herbivores: species in the genera Caloptilia and Diphtheroptila, whose larvae all eat Glochidion leaves. Do these antagonistic moths use more, or fewer, species of the host plant than the mutualistic Epicephala? Kawakita and his coauthors set out to answer that question by reconstructing the phylogenies of Caloptilia and Diphtheroptila.

Finding species in evolutionary trees

Most biologists agree that two groups of organisms are separate species if there is no gene flow between them. A consequence of genetic isolation between species is that, if they're isolated long enough, they become monophyletic within phylogenies. That is, all the individuals within each species share a common ancestor that is not shared with any other species. You can see this by contrasting two monophyletic species (on the left in the figure below) with two groups that turn out to be paraphyletic—some individuals of the red species are more closely related to individuals of the blue species than to other individuals of their own species.

.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; } Monophyletic and paraphyletic groupings. Image by jby.
The reasoning behind this is a bit subtle. Paraphyletic groups might still be separate species—they just haven't been isolated long enough to become monophyletic. As a good example, I'm a coauthor on a recent study that did this kind of analysis on non-pollinating "bogus" yucca moths that use three different yucca species. In that case, the moths were paraphyletic with respect to which yucca species they used, but more analysis showed that there is currently very little gene flow between moths using different hosts [PDF].

In the case of the Glochidion-using Caloptilia and Diphtheroptila, Kawakita et al. found something more complicated. Each genus broke up into several monophyletic groupings, or clades of genetically similar individuals—but in most cases each clade included moths collected from at least two different Glochidion species. Kawakita et al. note that the clades also correspond to differences in the moths' wing coloration, larval feeding behavior, and genitalia, and conclude that each clade is a different species. That would mean that the two antagonist genera tend to use multiple host plants.

Interesting question, but is this the way to answer it?

Except I'm not sure I buy this usage of phylogenies to define species. Kawakita et al. have shown that within the clades they call species, the individuals all have very similar genetics, but only for the two commonly-used genetic markers from which the phylogenies are reconstructed. It's not impossible that within each clade the moths might be adapted to individual host plant species, and reproductively isolated by that adaptation—and this could have happened recently enough that not many genetic differences would have built up in the two markers.

To really answer the question Kawakita et al. have posed would require a study of each clade in the two antagonist genera at a much finer scale. The question of how specialized Caloptilia and Diphtheroptila are hinges on how many species are in each genus, and that's better addressed by examining population genetics, not ancient relationships among these genera.

Reference

Drummond, C., Xue, H., Yoder, J., & Pellmyr, O. (2009). Host-associated divergence and incipient speciation in the yucca moth Prodoxus coloradensis (Lepidoptera: Prodoxidae) on three species of host plants. Heredity, 105 (2), 183-96 DOI: 10.1038/hdy.2009.154

Kawakita, A., & Kato, M. (2006). Assessment of the diversity and species specificity of the mutualistic association between Ep... Read more »

Drummond, C., Xue, H., Yoder, J., & Pellmyr, O. (2009) Host-associated divergence and incipient speciation in the yucca moth Prodoxus coloradensis (Lepidoptera: Prodoxidae) on three species of host plants. Heredity, 105(2), 183-96. DOI: 10.1038/hdy.2009.154

Kawakita, A., & Kato, M. (2006) Assessment of the diversity and species specificity of the mutualistic association between Epicephala moths and Glochidion trees. Molecular Ecology, 15(12), 3567-81. DOI: 10.1111/j.1365-294X.2006.03037.x

Kawakita, A., Okamoto, T., Goto, R., & Kato, M. (2010) Mutualism favours higher host specificity than does antagonism in plant-herbivore interaction. Proc. Royal Soc. B, 277(1695), 2765-74. DOI: 10.1098/rspb.2010.0355

August 17, 2010
09:05 AM
38 views

Turning up the alarms makes aphids careless

by Jeremy Yoder in Denim and Tweed

The oven in my apartment needs a serious deep-cleaning. A really serious deep-cleaning. To the point that, when I want to do some baking, smoke is more or less inevitable. As a result, I've developed the habit of responding to the apartment's smoke alarm by reaching up and un-mounting it from the ceiling, which completely disables it. If a fire were to start somewhere else in the apartment while I'm baking, I'd probably be in trouble.

That's more or less the idea behind an approach to agricultural pest control proposed in a paper just released online at PNAS: if you saturate insect pests with a predator warning signal, they become used to the signal, and more vulnerable to predators [$a]. Aphids are the target pest—they form huge, clonal swarms to literally suck the life out of plants, as described very nicely in this BBC Nature video.

As the video notes, those clonal swarms are vulnerable to all sorts of predators, most famously ladybird beetles. So when attacked, the aphids emit an alarm pheromone to warn the rest of the clone. But it's possible to habituate aphids to the alarm pheromone—if they're surrounded by it long enough, they won't respond to it by running away. The new study's authors proposed genetically engineering crop plants to produce the alarm pheromone, to automatically produce that habituation.

To see if this would work, they raised aphids on a line of Arabidopsis thaliana (the white lab mouse of the plant world) that had been engineered to produce the alarm pheromone. And, indeed, habituated aphids were much less likely to be repelled by the alarm pheromone—and were even in some cases attracted to it. Perhaps the most telling test involved leaving habituated and non-habituated aphids on an experimental plant with ladybird beetles introduced—habituated aphids were less likely to survive 24 hours with the beetles.

This is a pretty clever approach to pest control, but there's an obvious caveat. I don't see any reason why aphids couldn't evolve a way around this attempt to swamp out their own alarm signals—the paper notes that different aphid species have different responses to the particular alarm pheromone tested, so engineering one pheromone into crop plants doesn't leave the aphids without evolutionary options. Unless it's very cleverly designed, any pest-control strategy creates strong natural selection—and the better the strategy is, the stronger the selection is—to evolve resistance. Alarm-pheromone-producing crops might be another tool for pest control, but they won't be the last one we need.
.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; }
A ladybird beetle makes short work of some aphids. Photo by kenjonbro.
See also this press release from Cornell University, which discusses the paper's results.

Reference

de Vos, M., Cheng, W., Summers, H., Raguso, R., & Jander, G. (2010). Alarm pheromone habituation in Myzus persicae has fitness consequences and causes extensive gene expression changes. Proc. Nat. Acad. Sci. USA DOI: 10.1073/pnas.1001539107

... Read more »

de Vos, M., Cheng, W., Summers, H., Raguso, R., & Jander, G. (2010) Alarm pheromone habituation in Myzus persicae has fitness consequences and causes extensive gene expression changes. Proc. Nat. Acad. Sci. USA. DOI: 10.1073/pnas.1001539107

August 14, 2010
06:59 PM
142 views

Cite more papers, get more citations?

by Jeremy Yoder in Denim and Tweed

Nature News is reporting some interesting results presented as a paper at a meeting of the International Society for the Psychology of Science & Technology last week: articles published in the journal Science with longer "Works Cited" sections are themselves more frequently cited [$$]. A plot of the number of references listed in each article against the number of citations it eventually received reveal that almost half of the variation in citation rates among the Science papers can be attributed to the number of references that they include. And — contrary to what people might predict — the relationship is not driven by review articles, which could be expected, on average, to be heavier on references and to garner more citations than standard papers.The same authors did a similar analysis of papers published in the journal Evolution and Human Behavior over 30 years, and found similar results [PDF]. Here's the relevant figure from that paper:

.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; } Cite more, be cited more. Figure 2 from Webster et al. (2009) [PDF].
The lack of a "review effect" is surprising, but I don't think this overall result is. Academia, as much as we might describe it as cutthroat, also runs on reciprocal altruism. Authors notice when their papers are cited, and are more likely to cite papers that build on or relate to their own work. I'd be interested to see the network of citation underlying the pattern Webster et al. have found—I suspect that there's a lot of clustering around disciplines and sub-disciplines and sub-sub-sub-disciplines that contributes to all this mutual back-scratching citing.

Reference

Webster, G.D., Jonason, P.K., & Schember, T.O. (2009). Hot topics and popular papers in evolutionary psychology: Analyses of title words and citation counts in Evolution and Human Behavior, 1979-2008. Evolutionary Psychology, 7 (3), 348-348 Other: http://www.epjournal.net/filestore/ep07348362.pdf

... Read more »

Webster, G.D., Jonason, P.K., & Schember, T.O. (2009) Hot topics and popular papers in evolutionary psychology: Analyses of title words and citation counts in Evolution and Human Behavior, 1979-2008. Evolutionary Psychology, 7(3), 348-348. info:other/http://www.epjournal.net/filestore/ep07348362.pdf

August 13, 2010
10:05 AM
79 views

How Wright was wrong: When is it genetic drift?

by Jeremy Yoder in Denim and Tweed

Science is often said to work in three easy steps: (1) observe something interesting, (2) formulate a hypothesis for why that something is interesting in the way it is, and (3) collect more observations to see if they also support that hypothesis. Wash, rinse, repeat, and you eventually get from Newton to Einstein, from Aristotle to Darwin.

.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; } Sewall Wright, pioneer of population genetics. Photo from Wikimedia Commons.Except, of course, it's never that straightforward. Sometimes scientists come up with a hypothesis without a clear-cut example to support it, and then go looking that example. Sometimes observations that support a hypothesis turn out not to, if you look closer. And—here's the funny thing—this can even happen with hypotheses that are, in the end, pretty much correct.

In the spirit of this month's Giants Shoulders blog carnival, which focuses on "fools, failures, and frauds" in the history of science, I'm going to recount a case in which one of the greatest biologists of the Twentieth Century was fooled by a small desert flower. Sewall Wright was no fool or failure, and he certainly didn't commit fraud, but he does seem to have been totally wrong about his favorite example of a particular population genetic process, one he discovered. That process, isolation by distance, is widely documented in natural populations today—but it also doesn't seem to have worked the way Wright thought it did for Linanthus parryae.

Isolation by distance and Linanthus parryae

I've discussed isolation by distance before—it's a population genetic phenomenon related to genetic drift, the random change in the frequencies of genes in populations when natural selection is weak or not acting altogether. When you play out the random evolution of genetic drift across space as well as time, you get isolation by distance—populations that are far enough apart evolve different gene frequencies just because it takes so long for migrants to move between them. IBD, as I'll call it from here on out, is important to population geneticists because it gives us a "null" expectation, or baseline for comparison, to take to natural populations—if we see more differentiation in the traits or gene frequencies of two populations than we would expect under IBD, then we have reason to think natural selection could be creating that differentiation.

.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; }
Are these little flowers a textbook case of genetic drift, or natural selection? Photos by naomi_bot.Sewall Wright conceived of IBD in the midst of the Modern Synthesis, the period in the mid-Twentieth Century when biologists were putting together Darwin's thinking about natural selection with Mendel's discoveries in genetics, mixing them both with probability theory, and coming up with mathematical models of evolution that we still use today. Wright presented the math underlying IBD in a 1943 paper in the journal Genetics [PDF], as an extension of his earlier work on the effects of inbreeding, migration, and genetic drift.

But showing that something makes mathematical sense isn't the same as showing that it happens in the natural world. Wright needed a concrete example of IBD in action, and he found it—he thought—in Linanthus parryae, a small flowering plant found in the Mojave Desert. Two other biologists, Carl Epling and Theodosius Dobzhansky (yep, that Theodosius Dobzhansky), had reported the previous year that there was something funny going on with the color of L. parryae's flowers [PDF]. The little plants, which flower briefly in the desert springtime, have either blue or white flowers. But meticulous surveys across 30-some miles of desert around Victorville, California, found no pattern as to which color of L. parryae grew where. Patches of the flowers were often all blue or all white, but lots of patches were mixed. Plants with each flower color grew in the same kinds of locations, as Epling and Dobzhansky noted:No relation between the composition of populations of Linanthus Parryae and the environment in which these populations live is detectable. ... the region studied is homogeneous both with respect to physical factors and with respect to plant associations. The boundaries between the variable and the predominantly white areas do not coincide with any perceptible migration barriers.The only pattern Epling and Dobzhansky found was that patches of L. parryae were very likely to have frequencies of the two flower colors similar to other, nearby patches—and the correlation between color frequencies weakened as the distance between patches increased.

That pattern of decreasing similarity with distance is the definition of IBD. And so, directly after his paper introducing IBD, in the same issue of Genetics, Wright published his analysis of the strange case of L. parryae [PDF]. "It seemed of interest," Wright wrote, "to make an analysis of [Epling and Dobzhansky's] data from the standpoint of the theory of isolation by distance discussed in the preceding paper." He fed Epling and Dobzhanky's data into his newly-developed model of IBD, and concluded, with some caveats, that the observed variation in flower color frequencies "could be a by product [sic] of the development of different genetic systems by the process of random differentiation."

Digging up complications

So Linanthus parryae looked like the textbook case of isolation by distance, selected by the guy who thought up IBD in the first place. Except that, as Epling continued studying the little plant, he found some complicating factors. Specifically, that the frequencies of blue and white L. parryae flowers are predictable across time even as they were unpredictable across space [$a]. Over 17 years, Epling and his coauthors observed that the total number of flowering L. parryae shifted wildly from year to year, as they reported in 1960:In 1941, Linanthus formed an almost continuous carpet over hundreds of square miles, resembling a light fall of snow amongst the Larrea bushes. In 1950, 1951, and 1959 it was completely absent from the region ... Its dispersion, even in good years, is highly variable even over distances of a few feet.Wild population size fluctuations like that should exacerbate the effects of genetic drift, because when the population size drops sharply—biologists refer to this as going through a "bottleneck"—the small sample of plants that survive is more likely to be unrepresentative of the pre-bottleneck population. But Epling and his coauthors found that the frequency of blue flowers was much more stable than the total number of L. parryae flowering from year to year.

In the soil of their study sites, they discovered thousands of L. parryae seeds waiting for the right conditions to germinate—a pretty common strategy for desert plants. In any given year, only a small fraction of this "seed bank" sprouted, so the effective population of L. parryae... Read more »

Epling, C., & Dobzhansky, T. (1942) Genetics of natural populations. VI. Micro-geographic races in Linanthus parryae. Genetics, 27(3), 318-32. info:other/

Epling, C., Lewis, H., & Ball, F.M. (1960) The breeding group and seed storage: A study in population dynamics. Evolution, 14(4), 238-55. info:/

Schemske, D., & Bierzychudek, P. (2001) Evolution of flower color in the desert annual Linanthus parryae: Wright revisited. Evolution, 55(7), 1269-82. DOI: 10.1111/j.0014-3820.2001.tb00650.x

Schemske, D., & Bierzychudek, P. (2007) Spatial differentiation for flower color in the desert annual Linanthus parryae: Was Wright right?. Evolution, 61(11), 2528-43. DOI: 10.1111/j.1558-5646.2007.00219.x

August 3, 2010
09:05 AM
45 views

Double the mutualists, double the fun?

by Jeremy Yoder in Denim and Tweed

For all living things, information is critical to survival. Where's the best food source? Is there a predator nearby? Will this be a good place to build a nest? It probably shouldn't be surprising, then, that lots of animals do what humans do when faced with a host of hard-to-answer questions—they take their cues from their neighbors.

Red-backed shrikes place their nesting sites near where other shrike species have set up territories. Many bird species recognize each other's predator alarm calls, and respond appropriately. And a new natural history discovery published in the latest issue of The American Naturalist shows that treehoppers let one species of butterfly know where to find ants that will tend its larvae [$a].

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The ant-tended butterfly (Parrhasius polibetes, above) looks for ant-tended treehoppers (Guayaquila xiphias, below) to know where to lay her eggs. Photos from Kaminski et al. (2010), figure 2.The treehoppers help out the butterfly inadvertently, because both of them are dependent on a common resource: ants. Like many true bugs, treehoppers make their living sucking the sap of a host plant. This gives them a surplus of simple sugars and water, which they excrete as "honeydew" to attract ants for protection. As it happens, the larvae of the butterfly Parrhasius polibetes do the same thing—so the new study's authors hypothesized that P. polibetes females might prefer to lay their eggs on plants where treehoppers were already present, since those would likely already have ants ready to protect butterfly larvae.

To test this, the authors set up experimental pairs of host-plant branches, one occupied by ant-tended treehoppers, and one not. They excluded ants from accessing the unoccupied branch with Tanglefoot, a water-resistant glue used in insect traps. After 48 hours, they checked the experimental plants for newly-laid butterfly eggs, and found that P. polibetes was both more likely to lay eggs, and laid more eggs at a time, on branches occupied by treehoppers.

To assess the fitness benefit of laying eggs on treehopper-occupied plants, the authors compared the survival of newly hatched P. polibetes larvae artificially introduced onto branches occupied by treehoppers to the survival of larvae introduced to branches unoccupied by treehoppers (and with ants excluded, again, using Tanglefoot). The larvae placed with treehoppers had substantially better odds of survival—about six times better.

These two experiments confound the effect of treehoppers with the effect of ants, however—so the authors performed one additional experiment. In this one, they set up paired branches with and without treehoppers, but allowed ants to reach both the occupied and unoccupied branches—and the general result from the earlier experiment held. Larvae placed near treehoppers were three times more likely to survive for the duration of the experiment even when larvae placed on a branch without treehoppers were able to attract ants on their own.

So it looks like P. polibetes is able to freeload on the treehoppers' ant-attracting efforts, and benefits from that freeloading. What effect does that freeloading have on the treehoppers, or the ants, or the host plant? It's hard to say based on the data presented in the current paper, but I'd guess that the treehoppers don't lose much—in fact, they might gain from having another ant-attracting insect nearby, just as the butterfly larvae do. Similarly, it's probably helpful for the ants to have more honeydew-producing species in the same location. It's almost like that commercial for ... what was the product?

(I'll leave it to you, dear reader, to decide which insects correspond to which gendered pair in that video.)

I'd think, though, that this pile-on isn't so good for the host plant, if plants already hosting treehoppers are more likely to have to deal with butterfly larvae, too. Untangling all the different ways these four species—ants, treehoppers, butterflies, host plants—exert direct and indirect natural selection on each other should keep the authors busy for a long time to come.

References

Hromada, M., Antczak, M., Valone, T., & Tryjanowski, P. (2008). Settling decisions and heterospecific social information use in shrikes. PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003930

Kaminski, L., Freitas, A., & Oliveira, P. (2010). Interaction between mutualisms: Ant‐tended butterflies exploit enemy‐free space provided by ant‐treehopper associations. The American Naturalist DOI: 10.1086/655427

Magrath, R., Pitcher, B., & Gardner, J. (2007). A mutual understanding? Interspecific responses by birds to each other's aerial alarm calls. Behavioral Ecology, 18 (5), 944-51 DOI: 10.1093/beheco/arm063
... Read more »

Hromada, M., Antczak, M., Valone, T., & Tryjanowski, P. (2008) Settling decisions and heterospecific social information use in shrikes. PLoS ONE, 3(12). DOI: 10.1371/journal.pone.0003930

Kaminski, L., Freitas, A., & Oliveira, P. (2010) Interaction between mutualisms: Ant‐tended butterflies exploit enemy‐free space provided by ant‐treehopper associations. The American Naturalist, 2147483647. DOI: 10.1086/655427

Magrath, R., Pitcher, B., & Gardner, J. (2007) A mutual understanding? Interspecific responses by birds to each other's aerial alarm calls. Behavioral Ecology, 18(5), 944-51. DOI: 10.1093/beheco/arm063

July 29, 2010
09:05 AM
94 views

Global warming roundup: There's bad news, and weird news, but no really good news

by Jeremy Yoder in Denim and Tweed

Regardless of what James Inhofe thinks, global climate change is going to dramatically reshape the natural systems our civilization depends upon. Unfortunately, even as we embark on the radical experiment of turning our planet's temperature up to 11, we're just figuring out what results to expect. A whole series of papers released in the last week exemplify this point, showing that living communities' response to the changing planet may often be counter-intuitive.

.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; } Temperature stress may offset trees' ability to soak up carbon dioxide. Photo by Wade Franklin.Let's start with the bad news:

A study out in last week's PLoS ONE suggests that, rather than growing more rapidly and absorbing more carbon dioxide as the planet warms, forest trees may actually grow more slowly. More carbon dioxide in the atmosphere should generally increase plants' growth rates, since carbon dioxide is the raw material for photosynthesis. On the other hand, rising temperatures may put plants under so much stress that it offsets the benefits of more carbon dioxide.

Silva et al. examined core samples from four tree species—black spruce, red pine, red oak, and red maple—growing in Ontario forests, and found that the trees' growth rings were narrower in more recent years, as atmospheric carbon dioxide increased. Comparison of the growth rings to carbon isotope ratios (which capture a tree's response to temperature stress) suggested that the growth declines were due to less hospitable temperatures.

A large-scale historical study just out in Nature shows similar results for phytoplankton, microscopic photosynthetic organisms that form the base of ocean food chains. Working from historical records of ocean water transparency—phytoplankton makes water cloudy—going back to 1899, Boyce et al. found widespread declines in phytoplankton density [$a]. That's bad news on multiple levels, implying that phytoplankton growth isn't helping to absorb carbon dioxide, and that the oceans' productivity is declining with its foundational food sources, not just from overfishing. (See also coverage of this result by the BBC and NPR.)

.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; } Earlier springs mean bigger marmots. Photo by Blake Matheson.Now, the weird news:

The rule of thumb for plants' response to climate change has been that they'll respond to warmer temperatures by starting the growing season earlier. But a new survey of plant populations in Florida finds that as global warming progressed, most species flowered later. The authors suggest that this is because many Florida plant communities that are already adapted to warm conditions, and because climate change across much of Florida has meant not just warmer temperatures overall, but also greater seasonal variation in temperatures—areas where summer temperatures increased also tended to have decreasing winter temperatures. Faced with the possibility of more late frosts, Floridian plants are waiting till later in the spring to start flowering.

Another weird result of climate change received lots of press last week: a thirty-year study of yellow-bellied marmots in Colorado found that, as their alpine habitats grew warmer, the marmots grew bigger and more numerous [$a]. Warmer overall temperatures mean earlier spring thaws, so the marmots are emerging from hibernation earlier, have more time to grow and pack on fat reserves before hibernation in the fall, and can make more babies the next spring. Is this good or bad? Co-author Dan Blumstein's answer to that question in an interview with NPR is worth quoting:I don't know if I'm worried as much as I'm intrigued by it and I want to continue following the story. ... it's only through these long-term studies that we can gain important insights into what's happening, what's happened and ultimately identify mechanisms through which we may be able to predict what might happen in the future.Climate change is essentially a global gamble, with the function of ecological communities everywhere as the stakes. Even as we humans are unable to muster the will to stop it, we're finding out daily how many changes are on the way as the planet warms.

References

Boyce, D., Lewis, M., & Worm, B. (2010). Global phytoplankton decline over the past century. Nature, 466 (7306), 591-6 DOI: 10.1038/nature09268

Ozgul, A., Childs, D., Oli, M., Armitage, K., Blumstein, D., Olson, L., Tuljapurkar, S., & Coulson, T. (2010). Coupled dynamics of body mass and population growth in response to environmental change. Nature, 466 (7305), 482-5 DOI: 10.1038/nature09210

Silva, L., Anand, M., & Leithead, M. (2010). Recent widespread tree growth decline despite increasing atmospheric CO2. PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011543

Von Holle, B., Wei, Y., & Nickerson, D. (2010). Climatic variab... Read more »

Boyce, D., Lewis, M., & Worm, B. (2010) Global phytoplankton decline over the past century. Nature, 466(7306), 591-6. DOI: 10.1038/nature09268

Ozgul, A., Childs, D., Oli, M., Armitage, K., Blumstein, D., Olson, L., Tuljapurkar, S., & Coulson, T. (2010) Coupled dynamics of body mass and population growth in response to environmental change. Nature, 466(7305), 482-5. DOI: 10.1038/nature09210

Silva, L., Anand, M., & Leithead, M. (2010) Recent widespread tree growth decline despite increasing atmospheric CO2. PLoS ONE, 5(7). DOI: 10.1371/journal.pone.0011543

Von Holle, B., Wei, Y., & Nickerson, D. (2010) Climatic variability leads to later seasonal flowering of Floridian plants. PLoS ONE, 5(7). DOI: 10.1371/journal.pone.0011500

July 27, 2010
09:05 AM
262 views

Before they were yucca moths

by Jeremy Yoder in Denim and Tweed

Yuccas and yucca moths have one of the most peculiar pollination relationships known to science. The moths are the only pollinators of yuccas, carrying pollen from flower to flower in specialized mouthparts and actively tamping it into the tip of the pistil. Before she pollinates, though, each moth lays eggs in the flower—the developing yucca seeds will be the only thing her offspring eat. How does such a specialized, co-adapted interaction evolve in the first place? My coauthors and I attempted to answer this question in a paper just published in the Biological Journal of the Linnean Society, by reconstructing the ecology of yucca moths before they were yucca moths [PDF].

Using the present to reconstruct the past

Before I describe our study's results, let me explain a little about how biologists can reconstruct the characteristics of extinct species using what we know about living ones. First, we use DNA data to reconstruct evolutionary relationships between our favorite living species—this gives us an evolutionary tree, or phylogeny, like the ones in the illustration below. A phylogeny diagrams the branching evolutionary history that led to the living species at the tips of the tree. If we map the different states of some character that all those species have—say, the color of their feathers, onto the tips, we can infer what the ancestors at each of the inner branch points might have been like.

For instance, consider the possible scenarios for species A, B, C, and D in the illustration below. In the first case, if A and B are both red, then their common ancestor was probably red, too. However, C is blue—what does that mean for the common ancestor of C, A, and B? Because D is blue, we infer that the common ancestor of C, A, and B was also blue, as was the common ancestor of all four species. This is the most parsimonious reconstruction—it minimizes the number of times that color changes in the evolutionary history of the four species.
.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; } Knowing evolutionary relationships between living species helps us estimate the characteristics of their ancestors. Image by jby.In the second scenario, species D is red, so the same logic infers that the common ancestor of A, B, and C was red. In the third scenario, adding another red species (E) to the tree might also alter the most likely character states for the ancestral species on the tree—but this depends on where the DNA suggests that the new species fits on the tree. Most modern reconstructions of ancestral character states are more statistically complex than what I've just described, but the underlying logic is the same.

What did the ancestors of yucca moths do for a living?

.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 small sample of Prodoxid diversity: Greya politella (above) and Tegeticula synthetica (below). Photos by jby.So in order to reconstruct what yucca moths were like before they became yucca moths, we need to know the evolutionary relationships between yucca moths and their close relatives, the other members of the moth family Prodoxidae. This is a diverse group, including The yucca-pollinating genera Tegeticula and Parategeticula;
The genus Prodoxus, moths that lay eggs on various parts of yuccas (and other related plants) without pollinating them;
The genus Mesepiola, which lay eggs on the flowers of plants similar to yuccas—woody desert monocots;
The genus Greya, which lay eggs in the flowers of a number of different plants, and pollinates some in the process [PDF]; and
The genus Lampronia, which lay eggs in another wide assortment of plants.
This diversity offers some intriguing possibilities—depending on how these genera are related to each other, the moths that would colonize yuccas and evolve obligate pollination mutualism might have lived on anything from roses to saxifrages, and their larvae might have eaten leaf tissue, woody twigs, fruit, or flowers. However, the last study to reconstruct the evolutionary relationships among these groups included only one species of Lampronia, leaving a number of current host plant associations and larval feeding habits unrepresented.

So we collected new DNA sequences from another dozen species in the genus Lampronia, reconstructed their relationships to the rest of the Prodoxidae, and used the resulting phylogeny to estimate the host plant association and larval feeding habit of the ancestral species that gave rise to the yucca moths. The results are presented in the large, color-coded figure below. Interpretation of this figure is similar to the example I gave above, except that the reconstruction method we used allows us to estimate the relative probability of each character state at the ancestral nodes, which we present in color-coded pie charts.

This gives us a better picture of the evolutionary changes in the lineage that would become yucca moths. The ancestral moths probably fed inside floral ovaries all the way back to the origin of the Prodoxidae. Before colonizing woody monocots (the Agavaceae, the family including yuccas, and possibly the Ruscaceae, the family fed on by Mesepiola), they most likely fed on plants in the rose family.
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A simplified phylogeny of the Prodoxidae, with reconstructions of ancestral host plant associations (by family) and larval feeding habits. Image from Yoder et al., Figure 2.This reconstruction gives us the best picture we've had to date of the conditions under which yucca moths evolved obligate mutualism—before they were active pollinators, the moths were already feeding inside developing flowers. This suggests that active pollination evolved to help ensure a larval food supply. We might imagine, then, that plants used by these pollinating seed parasites would evolve greater dependence on their highly efficient pollen delivery, moving toward the yucca-yucca moth mutualism we see today.

References

Brown, J., Leebens-Mack, J., Thompson, J., Pellmyr, O., & Harrison, R. (1997). Phylogeography and host association in a poll... Read more »

Brown, J., Leebens-Mack, J., Thompson, J., Pellmyr, O., & Harrison, R. (1997) Phylogeography and host association in a pollinating seed parasite Greya politella (Lepidoptera: Prodoxidae). Molecular Ecology, 6(3), 215-24. DOI: 10.1046/j.1365-294X.1997.t01-1-00171.x

Pellmyr, O. (1999) Forty million years of mutualism: Evidence for Eocene origin of the yucca-yucca moth association. Proc. Nat. Acad. Sci. USA, 96(16), 9178-83. DOI: 10.1073/pnas.96.16.9178

Yoder, J.B., Smith, C.I., & Pellmyr, O. (2010) How to become a yucca moth: minimal trait evolution needed to establish the obligate pollination mutualism. Biol. J. Linnean Soc., 100(4), 847-55. DOI: 10.1111/j.1095-8312.2010.01478.x

July 19, 2010
07:05 AM
106 views

Sex after dawn: Marriage and natural selection

by Jeremy Yoder in Denim and Tweed

The book Sex at Dawn, by Christopher Ryan and Cacilda Jethá, has had a lot of press in the last month—it first popped up on my radar with Eric Michael Johnson's review for SEED, and then it became unavoidable (for me, anyway) when Dan Savage devoted a whole column and podcast to it. The thesis of Sex at Dawn is that early humans were highly promiscuous, and that modern expectations of monogamy are probably not consistent with our biology. I haven't read the book yet, but the discussion surrounding it has largely missed an important detail—human evolution didn't stop when we invented agriculture.

In fact, we've evolved in response to agriculture. My capacity to digest milk proteins at age 28—most other mammals lose this ability as soon as they're old enough for solid food—is the result of natural selection acting on my northern European ancestors. Sex at Dawn coauthor Christopher Ryan acknowledges exactly this, citing the same example, in a recent response to a question on his blog. I'm not aware of a study that documents human evolution in response to marriage customs, but conveniently enough, an article in the current issue of The American Naturalist does show that a population's marital customs can shape its response to natural selection [$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; } Vintage postcard via chicks57.The intensity and nature of natural selection often varies with age—it's strongest on traits expressed prior to and during the period of life when most reproduction happens, and weaker on traits having to do with life after reproduction is mostly complete.

The new paper examines the effect of marriage on this relationship between age, reproductive activity, and the strength of natural selection. The authors are able to do this thanks to church records of births (via christenings), marriages, and deaths from four Finnish towns during the nineteenth century—a deep multigenerational dataset. The society described is probably as far away from the Sex at Dawn world of communal relationships within hunter-gatherer tribes as Western society has ever gotten—the births recorded are all in the context of monogamous marriages. How monogamous these marriages actually were is debatable; this was also a world before paternity testing. But the study follows women, who would probably have had less opportunity, and certainly had less social leeway, for affairs outside marriage.

Within that society, women's reproductive success depended strongly on their husbands' economic status, as approximated by whether or not they owned land. Women who married landowners married almost three years earlier, on average, than those who married non-landowners (between 24 and 25 years old, compared to 27). Women who married at an earlier age generally had more children survive to age 15, the paper's benchmark for lifetime fitness—and this effect was stronger for women who married landowners.

.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; } Vintage wedding portrait via freeparking.This meant that the intensity of potential natural selection acting on women in the study group peaked around their 30th birthday, declined slowly for around a decade, and then dropped off sharply. By comparison, an estimate of selection intensity based only the women's probability of survival to a given age (i.e., without accounting for the need to marry before having children) just shows a steady decline with age. So marriage customs probably shaped the way natural selection could act on this population. (The comparison made, though, is a pretty facile one. I'd love to know what the intensity of selection looks like under other post-agricultural mating systems like, say, polygyny.)

Does that mean that these nineteenth-century Finns were evolving in response to the strictures of monogamy? Not necessarily. This study only estimates how strong selection would be on a trait relative to the age at which it's expressed. That is, traits that reduced (or improved) a woman's ability to bear children would be more strongly selected against (or favored), if they were expressed while she was between the ages of 30 and 40.

So the fact that marriage customs shape natural selection doesn't mean that we've evolved to be better adapted to current marriage customs than we are to those of pre-agricultural hunter-gatherers. Marriage customs vary considerably among cultures, and over time—I don't know of any culture that has maintained strict monogamy since the origin of agriculture. Even if a single culture did prefer monogamy that long, natural selection to adapt to that mating system would be working from a pool of genetic variation evolved from hundreds of generations of our earlier polyamorous lifestyle. It doesn't matter how strongly natural selection would favor a perfectly monogamous person, if such a person doesn't exist.

In other words, the key insight of Sex at Dawn—which is also a key insight of evolutionary biology in general—is right: What we can become is shaped by what we used to be. That's certainly an important thing to keep in mind when considering a commitment that lasts till death do you part.

(For example, you might want to makes sure your significant other is of at least the same genus as you. I mean, talk about biological impediments.)

References

Gillespie, D., Lahdenperä, M., Russell, A., & Lummaa, V. (2010). Pair-bonding modifies the age-specific intensities of natural selection on human female fecundity. The American Naturalist, 176 (2), 159-69 DOI: 10.1086/653668

... Read more »

Gillespie, D., Lahdenperä, M., Russell, A., & Lummaa, V. (2010) Pair-bonding modifies the age-specific intensities of natural selection on human female fecundity. The American Naturalist, 176(2), 159-69. DOI: 10.1086/653668

July 14, 2010
10:05 AM
112 views

When ecological opportunity knocks, does adaptive radiation answer?

by Jeremy Yoder in Denim and Tweed

One of the most basic questions in evolutionary ecology is, "why are there more kinds of this kind of critter than that kind of critter?" As in, why are there more than twenty thousand species of orchids, but only one species of ginkgo? Why are there hundreds of thousands of species of beetles, but only four species of horseshoe crab? In a literature review just released online—and my first publication as lead author!—my coauthors and I assess the support for one hypothesis: that species multiply because of ecological opportunity.

Biologists interested in the origins of species diversity frequently focus on the phenomenon of adaptive radiation, the process by which a single species rapidly gives rise to many new species, each with different traits adapted to different lifestyles. Darwin's finches, with their beaks shaped to suit to different foods [$a], are a classic case; the Anolis lizards of the Caribbean, which have repeatedly evolved into a handful of "ecomorphs" with different body sizes and shapes adapted to different perching locations [PDF], are another.
.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; } Why are there so many [insert taxon here]? Photos by Bill & Mark Bell (1 & 2), fturmog (3 & 4).
The two most influential theories of adaptive radiation—by G.G. Simpson and Dolph Schluter—have suggested that it results when a species encounters ecological opportunity. Ecological opportunity might be a newly-evolved trait, or a new habitat, or the extinction of a species' competitors or predators. For instance, a butterfly might evolve a way to overcome the chemical defenses of an abundant plant species, or a plant introduced by humans to a new habitat might find that local pathogens aren't as deadly to it as the ones in its native range. Ecological opportunities have the effect of granting access to new resources. We have pretty good evidence that this can allow individual populations to increase in number, and even evolve greater diversity—but is that enough to spur the rapid speciation that forms adaptive radiation?

Ecological opportunity ? adaptive radiation

.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; } We're pretty sure about steps 1 and 3. We're still trying to figure out step 2.Readers in certain demographic groups may think this sounds like an underpants gnome problem. But it isn't, exactly. The gnomes' business model can't get to from step 1 (collect underpants) to step 3 (profit) because they don't have a step 2. Evolutionary ecologists, on the other hand, already have their step 3 in the phenomenon of adaptive radiation. Ecological opportunity looks like a good prospect for step 1 precisely because it suggests some plausible options for step 2.

When a population encounters ecological opportunity, the new habitat, new trait, or extinction of antagonists provides access to new resources, and relaxes natural selection on the population. This leads to three phenomena usually grouped together under the term ecological releaseThe population experiences density compensation—more individuals can live in a particular area, creating stronger competition within the population.
Because of this stronger competition within the population, or because there isn't much competition from other species, members of the population venture into new habitats, or use new food resources.
The population becomes more diverse, either because of the relaxed selection, or because of competition-driven selection for using new habitat and new resources.
One or more of these three aspects of ecological release turn up whenever populations find new food resources, or escape predators and/or competitors. Density compensation has been widely observed in populations colonizing new habitats, especially islands; and experiments with sticklebacks and fruit flies [$a] suggest that the stronger competition resulting from density compensation can spur the population to become more diverse in its use of resources. Bacterial populations can even evolve different specialized forms—adaptive radiations in microcosm—when introduced to new food resources.

.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; } Anoles show signs of density compensation on Caribbean islands—is that the reason behind their diversification? (Pictured: Anolis oculatus.) Photo via WikiMedia Commons
But where's the speciation?

However, the evolution of bigger, more diverse populations is not the same thing as the evolution of new species—and that's what adaptive radiation is really all about. These changes resulting from ecological opportunity might directly promote speciation if stronger competition leads to disruptive natural selection. Similarly, the competition-driven incentive to colonize new habitats or exploit new food sources could expose some parts of the population to different forms of natural selection, eventually causing them to evolve into specialists on the new resources. Finally, even if speciation only happens when natural barriers cut off migration, maybe larger, more variable populations provide more diversity for vicariance events to divvy up.

This is all pretty speculative, though. We still don't know how often—or how rarely—divergent natural selection contributes to making new species. One way to deal with this is to approach the question from the other direction: look backward at the history of existing species, rather than follo... Read more »

Alfaro, M., Santini, F., Brock, C., Alamillo, H., Dornburg, A., Rabosky, D., Carnevale, G., & Harmon, L. (2009) Nine exceptional radiations plus high turnover explain species diversity in jawed vertebrates. Proc. Nat. Acad. Sci. USA, 106(32), 13410-4. DOI: 10.1073/pnas.0811087106

Bolnick, D. (2001) Intraspecific competition favours niche width expansion in Drosophila melanogaster. Nature, 410(6827), 463-6. DOI: 10.1038/35068555

Blumenthal, D., Mitchell, C., Pysek, P., & Jarosik, V. (2009) Synergy between pathogen release and resource availability in plant invasion. Proc. Nat. Acad. Sci. USA, 106(19), 7899-904. DOI: 10.1073/pnas.0812607106

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

Kassen, R. (2009) Toward a general theory of adaptive radiation: Insights from microbial experimental evolution. Annals New York Acad. Sci., 1168(1), 3-22. DOI: 10.1111/j.1749-6632.2009.04574.x

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

Svanbäck, R., & Bolnick, D. (2007) Intraspecific competition drives increased resource use diversity within a natural population. Proc. Royal Soc. B, 274(1611), 839-44. DOI: 10.1098/rspb.2006.0198

Wheat, C., Vogel, H., Wittstock, U., Braby, M., Underwood, D., & Mitchell-Olds, T. (2007) The genetic basis of a plant insect coevolutionary key innovation. Proc. Nat. Acad. Sci. USA, 104(51), 20427-31. DOI: 10.1073/pnas.0706229104

Yoder, J.B., Des Roches, S., Eastman, J.M., Gentry, L., Godsoe, W.K.W., Hagey, T., Jochimsen, D., Oswald, B.P., Robertson, J., Sarver, B.A.J.... (2010) Ecological opportunity and the origin of adaptive radiations. Journal of Evolutionary Biology. DOI: 10.1111/j.1420-9101.2010.02029.x

July 5, 2010
10:05 AM
102 views

Losing the scientific lede

by Jeremy Yoder in Denim and Tweed

Over at SEED, Dave Munger reflects on how online publishing and dissemination methods can strip the nuance from scientific news:I thought I was being careful to explain the results of several studies, showing that suicide is a difficult problem with many potential contributing factors and confounding variables, including mental illness, depression, and the seemingly contradictory influences of intelligence. Yet on social-networking sites, many readers latched on to one finding: That countries with higher average IQ tend to have higher suicide rates.Munger suggests that this problem can be mitigated by careful consideration of both the nut graf sent out via Twitter and RSS and the audience receiving them, and that's clearly right. But I think it's also worth considering whether some subjects are less appropriate for blogs.

.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; } Consider your medium! Photo by K!T.Blog posts are best when they're less than 700 or 800 words long, and their contents are readily summed up in a headline and only slightly expanded upon by the first paragraph. Think newspaper, not magazine articles. Do people read posts longer than that? Sure they do. But the longer a post is, the more possibility there is that some fraction of the readers will quit reading before the end, and maybe even pass on links or comments based on that incomplete understanding. I realize I'm not in the majority of online science writers in taking this position, but I think this better reflects how the average online reader reads.

Posts about individual, straightforward results work well in that context. For example, my colleague Jeanne Robertson recently discovered that desert lizards under divergent selection for camouflage have also become confused about visual mating signals. It's simple—one lizard population moved to white sand dunes and evolved lighter coloration, so now light males think that dark males from the ancestral population look like females—and it supports a lot of catchy headlines that don't sacrifice accuracy. The title of the talk at Evolution 2010 in which Robertson presented the discovery was "Dude looks like a lady." I'd say the Wired Science article I linked to above captures all the interesting details.

Complexity doesn't work so well. Scientific papers based on broad surveys of the literature, or many interrelated experiments, are inevitably going to lose some potentially important nuance when translated into an RSS-suitable post title, and explaining them accurately may take a lot more than 700 words. I've run into exactly this trying to write about complicated papers—either I go on for longer than I think my readers are likely to follow, or I have to omit detail and rely on readers to follow up with the links to the literature.

Mind you, this length-versus-content balance is a universal problem in disseminating scientific results—just look at the short-form journals Science and Nature. Some results are perfectly suited to the three-pages-and-online-supplement format, like an experimental result showing that sexually-reproducing lines of the worm Caenorhabditis elegans maintain more fitness in the face of mutation than asexual lines [PDF]. (I've posted about another result in this experimental system.) It's a simple result easily understood even without getting into the Supplementary Material. Compare that to a recent statistical survey of evolutionary trees that concluded species interactions weren't important in the history of life [$a]. That, too, fits into three pages of Nature, but the result is deceptively simple—even after delving into the Supplementary Material, the statistical reasoning underlying the core result isn't clear, as the comments thread on my post about the piece reveals.

Which isn't to say that online science writers should stick to covering simple experimental results or flashy natural historical notes, any more than scientists should never tackle complicated projects. They do, however, need to consider the limitations of the medium in which they report scientific results. Is a topic too complicated to fit in a single post? Maybe it's suitable for a series of posts. I like how Slate handles this, building collections of interrelated articles that can stand alone, but link into something like a long-form magazine article—see Will Saletan's great series on memory manipulation for a recent example.

And now I've blown through 700 words in the service of an extended, hopefully nuanced, discussion. Take from that what you will.

References

Morran, L., Parmenter, M., & Phillips, P. (2009). Mutation load and rapid adaptation favour outcrossing over self-fertilization. Nature, 462 (7271), 350-2 DOI: 10.1038/nature08496

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

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Morran, L., Parmenter, M., & Phillips, P. (2009) Mutation load and rapid adaptation favour outcrossing over self-fertilization. Nature, 462(7271), 350-2. DOI: 10.1038/nature08496

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

June 29, 2010
08:06 PM
87 views

#evol2010 day 4: In which the race is not always to the swift, and giving up on sex isn't a dead end

by Jeremy Yoder in Denim and Tweed

The final day of Evolution 2010 featured a fantastic series of talks in the ASN Young Investigators Symposium, and marked the premiere of the iEvoBio sister conference, which ran concurrently today. Perhaps not surprisingly, the #ievobio tag quickly outran the #evol2010 tag on Twitter.

I'm ending the conference with a final wrap-up audiocast with the crew from Evolution, Development, and Genomics, and then hopefully a quick run before the closing banquet.

.flickr-photo { }.flickr-framewide { float: right; text-align: left; margin-bottom: 15px; width:100%;}.flickr-caption { font-size: 0.8em; margin-top: 0px; } A western bluebird arrives at its nest box. Photo by kevincole.Joel McGlothlin demonstrated how testosterone's influence on dark-eyed junco males' aggressiveness and parental contributions produces alternate strategies for fitness: either be a good father, or sire more chicks.
Marc Johnson presented evidence that species of evening primrose which evolve asexual reproduction are less able to defend against herbivory, but also more likely to form new species.
Renee Duckworth described how the availability of nesting sites—these days, usually human-made nest boxes—determines whether Western bluebirds are selected to move away from the site of their birth [PDF], and that female bluebirds can manipulate their sons' likelihood of doing so. She also demonstrated that bluebirds are exceptionally photogenic.

Primary literature referenced

Duckworth, R., & Kruuk, L. (2009). Evolution of genetic integration between dispersal and colonization ability in a bird. Evolution, 63 (4), 968-77 DOI: 10.1111/j.1558-5646.2009.00625.x

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Duckworth, R., & Kruuk, L. (2009) Evolution of genetic integration between dispersal and colonization ability in a bird. Evolution, 63(4), 968-77. DOI: 10.1111/j.1558-5646.2009.00625.x

June 29, 2010
12:49 PM
102 views

#evol2010 day 3: In which butterflies self-medicate and Orr conjectures

by Jeremy Yoder in Denim and Tweed

How do you know it's getting to be the end of the Evolution 2010 meetings? Because I didn't get to this until this morning, in the back rows of the SSE symposium on evolutionary prediction. But the third day of the meetings were great, with cool natural history and a great address by SSE president H. Allen Orr.

And don't forget to check out the daily wrap-up audiocast over at Evolution, Development, and Genomics, which was just endorsed by none other than Carl Zimmer.
.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 monarch butterfly. Photo by MrClean1982.Thierry Lefevre presented evidence that female monarch butterflies infected with a microbial parasite lay their eggs on host plants with more toxins that can fight the parasite.
Susan Dudley presented new work on kin recognition in the small annual plant Cakile edentula, in which the plants grow less aggressively if planted next to close relatives.
Ian Pearse presented evidence that introduced oak species were more likely to be attacked by a native herbivore if they were more closely related to native oak species.
Finally, H. Allen Orr capped the day with an SSE presidential address that focused on what we know—and what we don't—about how reproductive isolation evolves and creates new species. Orr concluded with three conjectures:Extrinsic postzygotic isolation is usually due to adaptation to ecological conditions,
Intrinsic postzygotic isolation is usually due to adaptation to the intrinsic environment within the genome, and
Prezygotic isolation is usually due to sexual selection.
The idea, of course, is to collect the data to test these conjectures. But I'd say these make pretty good sense based on what we already know.Primary literature referenced

Dudley, S., & File, A. (2007). Kin recognition in an annual plant. Biology Letters, 3 (4), 435-8 DOI: 10.1098/rsbl.2007.0232

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Dudley, S., & File, A. (2007) Kin recognition in an annual plant. Biology Letters, 3(4), 435-8. DOI: 10.1098/rsbl.2007.0232

June 28, 2010
01:20 AM
85 views

#evol2010 day 2: In which sexes diverge and reptiles are disparate

by Jeremy Yoder in Denim and Tweed

In day two, Evolution 2010 is already feeling a mite overwhelming. I started the morning in the SSE symposium on speciation and the origin of dimorphism, then spent the rest of the day bouncing from talk to talk and preparing for my own presentation, which is tomorrow at 9:30. I'm going to bed early tonight, I think.

There's a new daily wrap-up podcast over at Evolution, Development, and Genomics, and, if you haven't been following the conference on Twitter, check hashtag #evol2010 or this list of twittering attendees I've compiled.

.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; } What's going on with snakes, anyway? Photo by Tambako the Jaguar.In the SSE symposium, Jenny Boughman and Dan Bolnick discussed ways in which the evolution of sexual dimorphism was like the evolution of different ecological specialists [PDF]—and how males and females evolving different ecological roles can counteract divergent selection acting on the population as a whole.
In the same symposium, Marguerite Butler showed that correlated neutral evolution of male and female traits may have shaped the diversification of Anolis lizards, using OUCH, a statistical package for R [PDF] that Marguerite demonstrated at the seminar I just attended.
Luke Harmon presented an impressive but not very conclusive exploration of morphological evolution in squamate reptiles. One insight: once a reptile loses its limbs, its head is free to evolve in new ways, but not always the same ways. Why? Good question.

Primary literature referenced

Bolnick, D. I. & Doebeli, M. (2003). Sexual dimorphism and adaptive speciation: Two sides of the same ecological coin. Evolution 57(11):2433-49 DOI: 10.1111/j.0014-3820.2003.tb01489.x.

Butler, M., & King, A. (2004). Phylogenetic comparative analysis: a modeling approach for adaptive evolution. The American Naturalist, 164 (6), 683-95 DOI: 10.1086/426002

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Butler, M., & King, A. (2004) Phylogenetic comparative analysis: a modeling approach for adaptive evolution. The American Naturalist, 164(6), 683-95. DOI: 10.1086/426002

June 26, 2010
08:03 PM
98 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

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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
121 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

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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
91 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

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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
96 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

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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

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