Category Archives: evolution

How can you tell if a plant is carnivorous? Feed it!

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A Venus flytrap closes on an unfortunate spider. Photo by cheesy42.

ResearchBlogging.orgPlants that eat animals offend our trophic sensibilities. Those of us who can move independently are supposed to eat those of us who can make sugar from sunlight—that’s just the way the food chain works, right?

Well, not really. From a certain perspective, plants prey on animals all the time, using the sneaky strategy of just waiting us out—when we animals stop moving for good, we’re fertilizer. And there are quite a few plants that aren’t so patient. Venus flytraps, sundews, and pitcher plants have been recognized as carnivores since before Charles Darwin devoted a book to their ecology and anatomy. They all have structures—fly-trapping leaves, or sticky hairs, or deep pitfalls full of water—that are uniquely good at catching wayward insects. All of them also grow in particularly nutrient-poor soils, such as bogs, where the nitrogen from trapped insects makes a big difference.

The vast majority of plants lack either adaptations for trapping, or the same kind of need for nitrogen—they either don’t grow where they can’t get the stuff, or they hire symbiotic bacteria to help fix it. Yet there is a third category of plants, which are not exactly carnivorous, but which might just “eat” the occasional stray fly anyway. Many plants have hairy surfaces that can catch insects, or leaf structures that trap water and create pitfalls—and some of these plants can take advantage of the critters caught in these proto-traps.

Sticky purple geranium can trap insects on its sticky leaves, and seems to get some nutrition out of them. Photo by jby.

One such plant is the sticky purple geranium (Geranium viscosissimum), which grows on dry Palouse hillsides around my current hometown of Moscow, Idaho. As its name implies, sticky purple geranium is sticky—its leaves are velvety with tiny glandular hairs, which leave a gummy residue on your hands if you brush against them. These hairs make it difficult for small insect herbivores to get to the leaves—but they also trap some of those insects.

Back in 1999, a biologist in my department at the University of Idaho, George Spomer (who left the department before my arrival), showed that sticky purple geranium leaves would digest a protein film pressed against them, somewhat like the leaves of a sundew. When Spomer placed protein labeled with carbon-14 on geranium leaves, he found elevated levels of carbon-14 elsewhere in the plant, suggesting that geranium leaves could absorb protein as well as digest it [$a].

Spomer demonstrated that the plants he studied could digest and absorb insects caught on their leaves, but his data can’t tell us whether that ability is of any particular use to a geranium growing in a natural population—whether, that is, geraniums actually need the nutrients they might get from trapped insects. A more recent study of another possibly carnivorous plant gets closer to answering that question.

Water collected in the leaves of a teasel plant forms a death trap for insects, and a source of nitrogen for the plant. Photo by HermannFalkner/sokol.

The plant in this second study is fuller’s teasel, Dipsacus fullonum, a widespread European wildflower that has been introduced into North America. The leaves of many teasel plants form catchments (pictured above) that can collect water and form a makeshift pitfall, which catches and drowns small insects. It has been speculated that, like sticky purple geranium, fuller’s teasel can absorb nutrients from these catchments full of rotting insect corpses. British biologists Peter Shaw and Kyle Shackleton set out to test this hypothesis not by tracking protein from trapped insects, but by determining whether teasel plants benefit from the trapping.

To do this, Shaw and Shackleton experimentally manipulated the number of insects trapped in the catchments formed by teasel plants’ leaves. In one treatment, they watched experimental plants and removed insects as soon as they were trapped; in the other, they “fed” the experimental plants an extra bluebottle maggot at set intervals. They compared both treatments to a group of plants that were left un-manipulated as a control. The “fed” plants didn’t necessarily grow bigger or produce more seeds, but they did produce more seeds as a proportion of their total biomass. That is, fuller’s teasel plants that trap more insects can devote more of their resources to making seeds.

Does this make fuller’s teasel carnivorous? Maybe, but probably not in the same sense that a Venus flytrap is. Teasels tend to grow in better soil than carnivorous plants do in general—they like open fields and stream banks, in my experience. Furthermore, we don’t have any evidence that teasels actively attract insects, as most carnivorous plants do. On balance, it seems far more likely that what Shaw and Shackleton found is not carnivory as we usually know it, but plants making sure that a handy source of nitrogen doesn’t go to waste.

Fuller’s teasel relies on insects for pollination—but does it also rely on them for nutrition? Photo by gynti_46.

References

Darwin, C. (1875.) Insectivorous plants. Google Books link.

Shaw, P., & Shackleton, K. (2011). Carnivory in the teasel Dipsacus fullonum — The effect of experimental feeding on growth and seed set. PLoS ONE, 6 (3) DOI: 10.1371/journal.pone.0017935

Spomer, G. (1999). Evidence of protocarnivorous capabilities in Geranium viscosissimum and Potentilla arguta and other sticky plants. Int. J. Plant Sci., 160 (1), 98-101 DOI: 10.1086/314109

Moths that pass in the night: Reproductive isolation due to pickiness, or just bad timing?

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ResearchBlogging.orgOn a summer night in a Florida corn field, a female armyworm moth emerges from her underground cocoon and spreads her wings to dry in the humid air. Over the next few weeks, she will fly miles away in search of a mate, and a likely-looking patch of host plants on which to lay her eggs.

Her brief adult life will be shaped in many ways by the life she led as a larva, feeding on domestic corn. She could easily find other grasses to feed her offspring, but she’ll probably seek out another cornfield. She may encounter armyworm males who were raised on many other grasses, but the odds are that the males she accepts as mates will also have grown up eating corn. This is so likely to be the case that it has left a mark on the genetics of her species [PDF].

At night in a cornfield, moths mate nonrandomly. Photo by K e v i n.

Yet it isn’t clear how much of this isolation between armyworms from corn (the “corn strain”) and armyworms from other grasses (called the “rice strain”) arises because moths from the different host plants actively prefer mates from their own larval food plant, or because they just don’t encounter moths from the other food plants as frequently. Like many moths, armyworms of both sexes deploy pheromones to attract and woo mates—so maybe armyworms from the same food plant smell better to each other. On the other hand, corn-strain armyworms do more of their mate searching early in the evening (although they’ll keep hunting all night), while rice-strain armyworms wait to search till the last few hours of nighttime.

Disentangling which of these two sources of isolation—preference versus timing—maintains the genetic differences between host plant strains of the armyworm takes some careful experimental work. As in many biological questions, the answer might well be not one or the other, but a little of both [$a].

In a study published in the latest issue of The American Naturalist, a team of entomologists at the Max Planck Institute for Chemical Ecology took on the question of what keeps the armyworm host strains separated. They performed two mating experiments with laboratory-reared moths of both sexes from both strains.

Fall armyworm adult. Photo via Wikimedia Commons.

First was a “no-choice” experiment in which moths were kept in a chamber with a single member of the opposite sex from their own strain, or the other strain. The test was repeated over three nights in a row. On the first night, females from the corn strain were less likely to mate with males of the rice strain than males of their own, and when they did accept rice-strain males, it wasn’t till later in the night. The second and third nights, though, corn-strain females mated about equally with males of both strains. Rice-strain females mated with males of both strains at about equal frequency all three nights, although they did so late in the night.

In the second round of experiments, moths were introduced into flight cages with one member of the opposite sex from each of the two host strains, so they could choose between them. To control for the differences in timing of mate searching between the two strains, the team repeated the experiment twice—in one version, the choosing moth had the entire length of the night to pick a mate, and in the other, the moths were only put into the same cage for the last four hours of the night, when the grass strain prefers to mate.

Fall armyworm larva. Photo by agrilifetoday.

In the all-night experiment, corn-strain males and females were both more likely to choose a mate from their own strain than the other. Rice-strain moths of both sexes mated with moths of both strains about equally—but rice-strain females were less likely to choose any mate at all. On the other hand, when the research team waited till the end of the night to introduce the test moths to their possible mates, rice-strain moths of both sexes mated much more frequently overall, and rice-strain females strongly preferred rice-strain males. Corn-strain males were basically indiscriminate in the late-night experiment, and corn-strain females were also less choosy.

In short, when mating during their usual activity periods, females of both strains were choosy about their mates; but when offered mates at the wrong time, they didn’t discriminate as much. The authors suggest that these mistimed matings were less discriminating because they were more likely to be initiated by the males, who showed relatively weak preferences even during their own usual mating times.

So the genetic differentiation between armyworm host strains is probably due to both timing and mate choice, and the two isolating factors affect males and females differently. Females, particularly rice-strain females, are quite picky about mating with a male of their own strain. Males, on the other hand, seem mainly to be prevented from pursuing females of the other strain by the fact that their respective schedules don’t line up. As the study’s authors conclude, all these individual rejections and missed connections, added up across entire armyworm populations, bring these moths a little bit closer to speciation.

References

Prowell, D., McMichael, M., & Silvain, J. (2004). Multilocus genetic analysis of host use, introgression, and speciation in host strains of fall armyworm (Lepidoptera: Noctuidae). Annals Entomol. Soc. America, 97 (5), 1034-44 DOI: 10.1603/0013-8746(2004)097[1034:MGAOHU]2.0.CO;2

Schöfl, G., Dill, A., Heckel, D., & Groot, A. (2011). Allochronic separation versus mate choice: Nonrandom patterns of mating between fall armyworm host strains. The American Naturalist, 177 (4), 470-85 DOI: 10.1086/658904

In which I try to explain why “heritability” is not quite the same thing as “heritable”

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ResearchBlogging.orgRobert Kurzban responds in the ongoing “adaptive” homophobia kerfuffle (henceforth, the O.A.H.K.) with continued confusion about how biologists identify possible adaptations and test to see whether natural selection is responsible for them. He notes that one effect of natural selection is to remove heritable variation in traits under selection, so that many traits which are probably adaptations—arguably, sometimes the best-adapted traits—actually have zero heritability.

This is true. But it’s important to note that a trait having zero heritability, or no genetic variation, is not the same thing as that trait not being heritable, or having no genetic basis. If the trait has zero heritability, the observed variation in the trait may not be heritable, but the trait still may be. Kurzban’s confusion over this distinction may be a fault of the terminology, as was pointed out to me in a couple independent conversations following the last round of the O.A.H.K.

That aside, reduced heritable variation in a trait—relative to appropriate standards for comparison, like other traits in the same species or the same trait in closely related species—is sometimes used to infer that selection has acted on that trait in the past. This is what my lab has done in the case of Joshua tree and its pollinators, which Kurzban cites. This sort of approach provides only indirect evidence of natural selection’s activity—but it’s often the best you can do when your focal species isn’t amenable to growing in a lab or greenhouse within the span of a single grant cycle.

The two varieties of Joshua tree, because apparently these are part of the discussion now. Photo by jby.

The comparison to other traits or to other species is the critical point here. Without it, you can’t determine whether a lack of genetic variation is due to strong selection, or due to the fact that there is no genetic basis for the trait. In isolation, the observation that there is no heritable variation for a single trait or behavior in a single species doesn’t tell you much except that natural selection cannot currently be acting on the observed range of variation in that trait. If there’s no genetic basis for the trait at all, then it cannot have been under selection in the past, either.

Forming hypotheses versus testing them

Regarding Kurzban’s broader point about how biologists identify adaptations:

Futuyama’s textbook, which Yoder cites for the discussion of heritability, indicates the following: “Several methods are used to infer that a feature is an adaption for some particular function” (p. 261), and lists the criteria that evolutionary psychologists rely on, including complexity, evidence of design, experiments, and so on. From the material I quoted in my prior post, it seems to me that by indicating the two kinds of evidence that are necessary for inferring a feature is an adaptation, Yoder is rejecting Futuyama’s claim that one can infer adaptation from its form, complexity, and so on.

Here Kurzban is confusing how we initially infer that a trait or behavior might be an adaptation with how we actually demonstrate that a trait or behavior is an adaptation. Forming a hypothesis is not the same thing as testing it, as Jon Wilkins explained so well. If Kurzban is accurately representing evolutionary psychology’s standards of evidence, then he’s confirmed Wilkins’s accusation that evo psych usually doesn’t go beyond the step of forming a plausible hypothesis to collecting the data that can test it.

Demonstrating that an adaptive hypothesis is well supported by data is, as I’ve previously said, a lot of work—usually enough for more than one scientific article. Depending on what is easiest to do, building the case that a trait is an adaptation might start with a paper that merely demonstrates a trait’s function—but that trait has not been conclusively shown to be an adaptation until we know that its demonstrated function is selectively important, and that the trait itself has a genetic basis.

While familiar to anyone who reads the evolutionary biology literature, this maybe isn’t so obvious to non-biologists. This may be because popular science accounts don’t always differentiate between hypotheses with good scientific support and those with none. Walk through a zoo or a natural history museum, and you’ll read nothing but adaptive hypotheses all day—but you’ll rarely see good, deep discussion of how well they’re supported.

This is why, since I started graduate school, I’ve became rather tiresome company on trips to museums and zoos. But one of the great things about popular writing by working scientists (from my perspective as a scientist) is that it lets specialists explain exactly such finicky details of our fields directly to the public. Doing so clearly and accessibly is challenging, to be sure, but naïve, uncritical endorsements of unsupported hypotheses—about the adaptive values of human behavior, or anything else—are available in just about every major media outlet. If scientists don’t do better than that in our own science communication, what value do we have to add to the discussion?

And now something new: relevant data

Your reaction to this image might be in your genes, but the evidence is that it can change, too. Photo source unknown, presumed public domain.

Which brings us back to evaluating Gordon Gallup’s “adaptive” homophobia hypothesis. Kurzban also points to evidence (ye gads! data!) that natural selection actually could have something to work with in the case of attitudes towards homosexuals. A 2008 Australian twin study, which finds a genetic component of variation in responses to a questionnaire about attitudes towards homosexuality.

This is indeed, as Kurzban suggests, preliminary data in support of the idea that natural selection could operate on homophobia. As Neuroskeptic pointed out in the comments on my last O.A.H.K. post, it also means that natural selection could be operating on tolerance of homosexuals. It’s an interesting and important question, actually, why the authors of that study chose to frame their results as showing the heritability of intolerance, rather than the heritability of tolerance.

However, as I noted all the way back at the beginning of the O.A.H.K., we also know that homophobic attitudes can change considerably over the course of an individual’s lifetime. It’s hard to say how survey responses taken at a single point in time relate to what natural selection would actually have to work with, if homophobic attitudes or lack thereof somehow shape an individual human being’s expected reproductive fitness. Even if there is some solid genetic basis to homophobia, we still don’t have data that can rigorously determine whether or how natural selection might act on that variation.

References

Godsoe, W., Yoder, J.B., Smith, C.I., Drummond, C., & Pellmyr, O. (2010). Absence of population-level phenotype matching in an obligate pollination mutualism Journal of Evolutionary Biology, 23 (12), 2739-2746 DOI: 10.1111/j.1420-9101.2010.02120.x

Verweij, K., Shekar, S., Zietsch, B., Eaves, L., Bailey, J., Boomsma, D., & Martin, N. (2008). Genetic and environmental influences on individual differences in attitudes toward homosexuality: An Australian twin study. Behavior Genetics, 38 (3), 257-265 DOI: 10.1007/s10519-008-9200-9

The evolution of homophobia, continued

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On Twitter, hectocotyli just pointed me to another discussion of the problems with Gordon Gallup’s case for an adaptive function to homophobia (and linked to my take in connection, for which, thanks). Jon Wilkins goes into more detail on the general problem that evolutionary psychology too often accepts plausibility as the standard of proof for adaptive hypotheses.

In fact, it is trivially easy to come up with a plausible-sounding evolutionary argument to describe the origin of almost any trait. More importantly, it is often just as easy to come up with an equally plausible-sounding argument to describe the origin of a hypothetical scenario involving the exact opposite trait.

I think Wilkins is a little too polite in some regards; Gallup’s hypothesis doesn’t even qualify as “plausible” in the context of what we know today about its ugly component assumptions. (And what, by the way, Jesse Bering should have known before dredging up Gallup’s articles from well-deserved obscurity.) Nevertheless, Wilkins broadens the discussion to address scientific reasoning more generally, and the post is worth reading in its entirety.

An adaptive fairytale with no happy ending

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ResearchBlogging.orgThe evolution of human traits and behaviors is, as I’ve noted before, a contentious and personal subject. This is enough of a problem when there’s some data to inform the contentiousness. In the absence of meaningful data, it’s downright dangerous.

Take, for instance, Jesse Bering’s recent post about the evolution of homophobia, which Steve Silberman just pointed out to me.

A grim fairy tale indeed. Photo by K Wudrich.

When evolutionary biologists say a trait or behavior is “adaptive,” we mean that the trait or behavior is the way we see it now because natural selection has made it that way. That is, the trait or behavior is heritable, or passed down from parent to child more-or-less intact; and having it confers fitness benefits, or some probability of producing more offspring than folks who lack the trait. Lots of people, including some evolutionary biologists, speculate about the adaptive value of all sorts of traits—but in the absence of solid evidence for heritability or fitness benefits, such speculation tends to get derided as “adaptive storytelling.”

Evolutionary biology wasn’t always so rigorous, once upon a time. Then Stephen Jay Gould and Richard Lewontin buried adaptive storytelling under an avalanche of purple prose in their landmark 1979 essay “The Spandrels of San Marco” [PDF]. Norman Ellstrand made a similar point with better humor in a satirical 1983 article for the journal Evolution proposing adaptive explanations for why children always start life smaller than their parents [PDF]. Nowadays, when evolutionary biologists want to, say, argue that horned lizards’ horns are an adaptation for defense against predators, they have to demonstrate the claimed fitness benefit [PDF].

Evolutionary psychologists, however, seem not to have gotten the memo.

Bering’s post focuses on a series of studies by the evolutionary psychologist Gordon Gallup. Gallup was interested in the question of whether there might be an adaptive explanation for homophobia—which, given the fact that many (although far from all) human cultures treat homosexuality as a taboo—is a fair question for research. He hypothesized that treating homosexuality as taboo helped to prevent homosexual adults from contacting a homophobic parent’s children, which would reduce, however slightly, the prospects of those children growing up to be homosexual, and ensure more grandchildren for the homophobe.

Gallup supported this adaptive hypothesis with … evidence that straight people were uncomfortable about homosexuals coming into contact with children [$a]. Here’s the opening sentence of that paper’s abstract:

In a series of four surveys administered either to college students or adults, reactions toward homosexuals were found to vary as a function of (1) the homosexual’s likelihood of having contact with children and (2) the reproductive status (either real or imagined) of the respondent.

If you’ve noticed that this doesn’t mention evidence of heritability or a fitness benefit to homophobia, that’s not because I left it out—that’s because Gallup’s work contains no data to support either.

What this amounts to is arguing that homophobia is an adaptation favored by natural selection because homophobia is a thing that exists.

Could a complex behavior like homophobia have a genetic basis? Sure. Homosexuality itself is a complex behavior, and it certainly does have some genetic basis. However, the fact that attitudes toward homosexuality have shifted as far and as fast as they have in the last few decades suggests that any genetic effects underlying homophobia are pretty easy to overcome. Behaviors can be inherited culturally, too, since human children learn from their parents. But—note, again, lots of change in the last thirty years or so—cultural inheritance is more fleeting and malleable than biological inheritance.

Careful, Red Riding Hood—that wolf might be gay. Photo by crackdog.

What about Gallup’s proposed fitness benefit for homophobes? Well, that would require homophobia to, you know, actually prevent homosexuality. Gallup’s argument there hangs on two distasteful assertions. First, that gay men are more likely to be pedophiles, and second, that boys sexually abused by gay men are themselves more likely to grow up gay. In spite of Gallup’s assertions otherwise [$a], we have strong evidence from multiple studies that gay men are no more likely to be sexually attracted to children than straight men.

And there is, to my knowledge, no evidence to suggest that abuse can cause homosexuality. Bering cites a recent study that does document an association between childhood abuse and later homosexuality in men. However, the study’s authors point out that, “The reason for the connection between childhood sexual abuse and same-sex partnerships among men is not clear from our findings.”

… gay men tend, on average, to be more gender non-conforming as boys (Bailey & Zucker, 1995). This tendency could increase their appeal or conspicuousness to sexual predators, which might make them more likely to be victims of abuse (B. Mustanski, personal communication, February 11, 2008). Similarly, it is possible that boys who are developing and exploring a same-sex sexual orientation are more likely to enter situations where they are at risk for being sexually abused (Holmes & Slap, 1998). [In-text citations sic]

Why on Earth would Bering dredge up Gallup’s adaptive fairytale a decade and a half after it was published, if it was baseless to begin with and no new evidence supports it? Well, according to Bering, because he’s a hard-nosed scientist who isn’t afraid to consider uncomfortable possibilities.

Sometimes, science can be exceedingly rude—unpalatable, even. The rare batch of data, especially from the psychological sciences, can abruptly expose a society’s hypocrisies and capital delusions, all the ugly little seams in a culturally valued fable. I have always had a special affection for those scientists like Gallup who, in investigating highly charged subject matter, operate without curtseying to the court of public opinion.

Of course, says Bering, Gallup’s work isn’t conclusive, but it sure would be interesting if someone tested it.

Except, when Gallup was forming his hypotheses about the evolutionary benefits of gay-hating—he first proposed the idea in a 1983 article—he was hardly thumbing his nose at public opinion. He was, in fact, giving natural selection’s approval to the prevailing ugly stereotypes about gay men. And, as any competent evolutionary biologist would recognize, he did it without a shred of relevant evidence.

References

Ellstrand, N. (1983). Why are juveniles smaller than their parents? Evolution, 37 (5), 1091-4 DOI: 10.2307/2408423

Gallup GG Jr, & Suarez SD (1983). Homosexuality as a by-product of selection for optimal heterosexual strategies. Perspectives in Biology and Medicine, 26 (2), 315-22 PMID: 6844119

Gallup, G. (1995). Have attitudes toward homosexuals been shaped by natural selection? Ethology and Sociobiology, 16 (1), 53-70 DOI: 10.1016/0162-3095(94)00028-6

Gallup, G. (1996). Attitudes toward homosexuals and evolutionary theory: The role of evidence. Ethology and Sociobiology, 17 (4), 281-284 DOI: 10.1016/0162-3095(96)00042-8

Gould, S., & Lewontin, R. (1979). The Spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proc. Royal Soc. B, 205 (1161), 581-98 DOI: 10.1098/rspb.1979.0086

Wilson, H., & Widom, C. (2010). Does physical abuse, sexual abuse, or neglect in childhood increase the likelihood of same-sex sexual relationships and cohabitation? A prospective 30-year follow-up. Archives of Sexual Behavior, 39 (1), 63-74 DOI: 10.1007/s10508-008-9449-3

Young, K., Brodie, E.D., Jr., & Brodie, E.D., III (2004). How the horned lizard got its horns. Science, 304 (5667) DOI: 10.1126/science.1094790

One snout to rule them all: Does migrating help weevils win the arms race of coevolution?

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ResearchBlogging.orgNatural selection and gene flow have a sort of love-hate relationship. Natural selection acts, on average, to make a population better fit to its environment. Gene flow, the movement of individuals and their genes, can counter the optimizing effect of selection if it introduces less-fit individuals from somewhere a different environment. On the other hand, not all new immigrants are necessarily less fit—sometimes they’re better suited to their new environment than the locals.

This gets more complicated, and more interesting, when the environment in question is another living species. Then, the question is not just how movement of one species changes its response to natural selection, but how movement of the other species changes the nature of that natural selection. That’s the focus of the latest study of a Japanese weevil species and its favorite food plant. The two species are locked in a coevolutionary arms race—but who wins the arms race in any given location depends on the gene flow each species is receiving from elsewhere [$a].

Male and female camelia weevils, caught at an indelicate moment. Evidently he doesn’t find her much longer rostrum intimidating. Photo from Toju et al. (2011), figure 1.

These are camelia weevils, Curculio camelliae. As their name suggests, they like to eat camelias, at least when they’re young. Specifically, weevil larvae eat camelia seeds, which are protected by a thick layer called a pericarp. To deal with camelia pericarps, the weevils have evolved prodigious proboscises, or rostrums, which female weevils use to drill through the pericarp so they can lay their eggs inside. Note that the female in the picture above is the one with the rostrum longer than the rest of her body.

Camelias can reduce their risk of losing seeds to weevil larvae by evolving thicker pericarps; weevils can make sure they’re able to feed their young by evolving longer rostrums. Both species are constrained by costs, though—the cost of producing more pericarp tissue, or carrying around a Pinocchio-grade snout. These costs vary somewhat with climate—camelias grow thinner pericarps in cooler conditions [$a]. This means the arms race won’t proceed equally far in all camelia populations, and introduces the possibility that the way in which weevils and camelias (well, camelia seeds and pollen) move across the landscape may very well determine which species has the upper hand.

A female weevil drills into a camelia fruit. Photo from Toju et al. (2011), figure 1.

The new study sets out to see whether gene flow among populations of the two species determines how far the arms race proceeds in each population. Rather than directly track weevils and camelia seeds, the authors use genetic markers for each species—the more migrants move between two weevil (or camelia) populations, the more similar those two populations’ genetics will be. The populations in question were seven sites on a small island at the south end of the Japanese archipelago, and presumably relatively free from the influence of immigration from the larger islands.

It looks like the movement of weevils, but not camelias, affects how the arms race proceeds. As the genetic difference between weevils at two different sites increased, the difference in how far the arms race had proceeded—that is, how long the local rostrums were, and how thick the local pericarps—increased too. That suggests weevils may be prevented from evolving rostrums of the optimum length for their local camelias by the arrival of less-than-optimal migrants. On the other hand, there was no statistically significant relationship between the genetic similarity of camelia populations and their place in the arms race.

This is where the relationship between selection and gene flow gets complicated, though. Even given the relationship between weevil gene flow and how far the arms race seems to have proceeded, the genetic differences between weevil populations were consistent with very low actual rates of migration. A female weevil arriving in a population of camelias with pericarps too long for her rostrum isn’t going to contribute many offspring to the next generation of weevils at that site. So it’s not impossible that what we’re seeing is selection constricting gene flow rather than gene flow slowing down selection.

Alternatively, weevils from a population with super-long rostrums should be able to lay eggs in any population of camelias they meet. In fact, an analysis that uses the genetic data to estimate rates of immigration and emigration suggests that one of the weevil populations with the longest snouts contributes more migrants to the other sites than it receives from each of them. In arms-race coevolution, size is all that matters—and so the weevils with the longest snouts may be winners no matter where they go.

Reference

Toju, H. (2008). Fine-scale local adaptation of weevil mouthpart length and camellia pericarp thickness: Altitudinal gradient of a putative arms race. Evolution, 62 (5), 1086-102 DOI: 10.1111/j.1558-5646.2008.00341.x

Toju, H., Ueno, S., Taniguchi, F., & Sota, T. (2011). Metapopulation structure of a seed-predator weevil and its host plant in arms race coevolution. Evolution DOI: 10.1111/j.1558-5646.2011.01243.x

Carnival of Evolution No. 32

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Barnacles, one of Darwin’s first study organisms. Photo by Minette Layne.

The 32nd Carnival of Evolution, collecting online writing about exactly what it says on the tin, went live yesterday at Genome Engineering, with contributions from yours truly, Zen Faulkes, Bite Sized Biology, Dr. Bik, and Kevin Zelnio. Go have a look!

One of these mutualists is not like the other

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ResearchBlogging.orgOver the last few months I’ve been writing a lot about how different species interactions have different evolutionary effects. The studies I’ve looked at so far focus on effects over just a few generations—barely time to take notice, in evolutionary time. The February issue of The American Naturalist remedies this short-term perspective with a paper showing that over millions of years, two different kinds of mutualists had very different effects on the history of one group of orchids [$a].

The new study examines the evolutionary history of coryciinae orchids, a group of South African orchids that rely on two major groups of mutualists. The first, and perhaps most obvious, are pollinating bees, which coryciinae orchids attract not with nectar but with oils. Like most other orichids, this group of flowers interacts with its pollinators in very specific ways, to the point that different coryciinae species can share a single pollinator by placing pollen on different parts of the pollinator’s body, as seen in the image below.

Double duty: This bee is carrying pollen from one orchid species on its forelegs, and pollen from another orchid species on its abdomen. Photo from Waterman et al (2011), figure 1.

The second important group of mutualists are mycorrhizae, a class of fungus found in soil, which colonize plants’ roots. Mycorrhizae aid their hosts in taking up minerals, particularly phosphorus, in exchange for sugar supplied by the host. In certain kinds of soil, having the right mycorrhizae is the difference between life and death for a plant.

Although both pollinators and mycorrhizae are vital to an orchid’s success, they should contribute to forming new orchid species in very different ways. Evolving a new pollinator relationship can directly create reproductive isolation for a flowering plant, independent of other ecological considerations. On the other hand, mycorrhizae are closely linked to basic ecology, because the mycorrhizae in a plant’s roots determine what kinds of soils it can use—wet or dry, acidic or alkaline. If new orchid species usually form by adapting to new habitats, they probably acquire new mycorrhizae while doing so.

If changing a trait—in this case, a mutualistic relationship—is related to forming new species, then closely related orchid species will be more likely to differ in that trait. This turns out to be the case for pollinators—the more closely related two orchid species are, the more likely they are to use different pollinators, or different parts of the same pollinator. However, the reverse is true for mycorrhizae. The more closely related two orchids are to each other, the more likely they are to have the same mutualistic fungi in their roots. This finding that pollination matters most to species formation is right in keeping with Verne Grant’s classic study noting that animal pollinated plants tend to differ more in their floral structures—the parts that interact with pollinators—than in other traits.

The authors followed up on these results with field experiments on a few selected species, and found that co-occurring orchids could often successfully pollinate each other, if the pollen was deliberately placed. In these cases, differences in specialized pollination interactions are most of what maintains the orchid species as separate genetic entities. On the other hand, closely related orchids that grow in adjacent habitats did just fine when transplanted into each others’ soil—and mycorrhizae.

Biologists studying the effects of pollination on plant species formation have recently become more aware that correlation does not necessarily imply causation. New pollinator interactions certainly might form new species—but it is also possible that new orchid species created by other forces must rapidly evolve new pollinator interactions to compete with existing species.

References

Armbruster, W., & Muchhala, N. (2008). Associations between floral specialization and species diversity: cause, effect, or correlation? Evolutionary Ecology, 23 (1), 159-79 DOI: 10.1007/s10682-008-9259-z

Waterman, R., Bidartondo, M., Stofberg, J., Combs, J., Gebauer, G., Savolainen, V., Barraclough, T., & Pauw, A. (2011). The effects of above- and belowground mutualisms on orchid speciation and coexistence. The American Naturalist, 177 (2) DOI: 10.1086/657955

Scientific American guest blog: Ecological opportunity is all around us

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ResearchBlogging.orgThe latest entry in the wide-ranging Guest Blog at Scientific American is a post by yours truly, about a subject I’ve discussed before:

Since the Origin was first published, biologists have come to use the phrase ecological opportunity to describe the processes that can produce a diverse group of species from a single colonizing ancestor. Islands provide colonizing species with new food resources and an escape from predators and competitors. Under these highly favorable conditions, island species can live at much higher population densities than possible on the mainland—a phenomenon called density compensation. This increase in population size is often accompanied by increased variation among individuals, and greater competition from crowding neighbors creates strong benefits for individuals that try new ways to make a living.

Given enough time, one big, variable population will begin to fracture into smaller populations with different lifestyles. Given even more time, those smaller populations will stop interbreeding, and become different enough to call separate species. If that seems like a stretch of the imagination, consider that the processes of ecological opportunity are occurring all around us—as invasive species spread across the landscape, and viruses multiply in a new victim’s bloodstream.

To learn how ecological opportunity really is all around us, you’ll have to go check out the whole post.

Kudzu, taking advantage of ecological opportunity. Photo by Suzie T.

Reference

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