In which several evolutionary psychologists still don’t understand evolution

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ResearchBlogging.orgJesse Bering has responded to criticism—by me, Jon Wilkins, and P.Z. Myers, among others—of his post about Gordon Gallup’s hypothesis that fear of homosexuals is favored by natural selection, in the form of an interview with Gallup. The result is informative, but probably not in the way intended.

To recap: Gallup proposed that homophobia could be adaptive if it prevented gay and lesbian adults from contacting a homophobic parent’s children and—either through actual sexual abuse or some nebulous “influence,” making those children homosexual. In support of this, he published some survey results [$a] showing that straight people were uncomfortable with adult homosexuals having contact with children.

I pointed out that all Gallup did was document the existence of a common stereotype about homosexuals—he presents no evidence that believing this stereotype can actually increase fitness via the mechanism he proposes, or that it is heritable.

Homophobia. And, um, everyone-else-phobia, too. Photo by yksin.

So now Gallup and Bering have responded, although they have not, I think, improved their case. There’s a lot for me to address here, so I’ll try to break it up into sections, and follow the order of the interview.

In which Gordon Gallup is not a homophobe

In the response post, Gallup (and Bering, who contributes quite a lot to the argument in his role as interviewer) takes issue with the collective objections of working biologists, but manages not to actually address those objections. Bering starts the conversation on the moral high ground:

BERING: Let’s address the elephant in the room. It’s embarrassing for me to even ask this of you, since the answer is so obviously “no” to me. Is your theory a justification of your own homophobia?

GALLUP: A lot of people think that if a person has a theory it’s a window unto their soul. I have lots of theories. (See CV (pdf).) I have a theory of homophobia, I have a theory of homosexuality, and I have a theory of permanent breast enlargement in women, just to mention a few. So that would make me a homophobic, homosexual who is preoccupied with women’s breasts.

Neither I, nor any of the other critics I’ve seen have called Gallup a homophobe. He may be uniquely bad at understanding how societal homophobia nullifies his interpretation of his survey results, but that doesn’t make him a homophobe. Thanks for clearing that up, though, guys.

Gallup then demonstrates that he either hasn’t actually read any of the latest criticism, or has missed the point entirely:

… It is interesting how my critics tip-toe around the fact that my approach is based on a testable hypothesis, and how they go out of their way to side-step the fact that the data we’ve collected are consistent with the predictions. Whether it is politically incorrect or contrary to prevailing social dogma, is irrelevant. In science, knowing is preferable to not knowing. Minds are like parachutes, they only function when they’re open. If I were a homosexual, I’d want to know about these data.

I certainly didn’t tiptoe around the testability of Gallup’s hypothesis—I wrote that (1) the data he presented do not test his hypothesis, and (2) the data we do have regarding the probable fitness benefits of homophobia and its heritability contradict his hypothesis. I’m entirely prepared to revise my conclusions given new data, but Gallup doesn’t have any.

In which at least one of us doesn’t understand heritability

In his next question to Gallup, Bering accuses me of “bungling” the definition of heritability, linking to evolutionary psychologist Rob Kurzban, who says that my brief definition of heritable as “passed down from parent to child more-or-less intact” is wrong because heritability is actually “the extent to which differences among individuals are due to differences in genes.”

Wow, dude. You are aware that what you just said means exactly the same thing as what I originally said, right?

Let’s go to the textbooks that Kurzban says I’m contradicting. Here’s the passage on heritability from Douglas Futuyma’s gold-standard undergraduate textbook Evolution (page 209):

One way of detecting a genetic component of variation, and of estimating VG [trait variation attributable to genetic differences] and h2 [the proportion of total trait variation explained by genetic variation], is to measure correlations* between parents and offspring, or between other relatives. For example, suppose that in a population, the mean value of a character in the members of each brood of offspring was exactly equal to the value of that character averaged between their two parents (the MIDPARENT MEAN) (Figure 9.20A). So perfect a correlation clearly would imply a strong genetic basis for the trait. [Bold text and bracketed notes mine; otherwise sic.]

The asterisk in that quote leads to a footnote pointing out that regression, rather than correlation, is more typically used. This is the definition of heritability that I learned in my undergraduate and graduate courses. It’s also the definition I’ve just helped teach to a class of third- and fourth-year undergraduate biology students in my capacity as a teaching assistant on a course in population biology.

In non-statistical terms (the kind I try to use on this blog), a positive regression between a parent’s traits and those of their offspring means, in fact, that the parent’s traits are passed on to their offspring, um, more-or-less intact.

Parent-offspring regression is widely used to estimate heritability [PDF], but you can also do similar analyses using trait measurements for siblings, or multiple generations on a pedigree. In all of these cases, known parental or sibling or familial relationships are proxies for genetic similarity—you can estimate heritability without knowing anything about specific genes. (In fact, sometimes biologists use genetic data to reconstruct pedigree relationships, then estimate heritability from the pedigree.) As implied in the quote from Futuyma’s textbook, this approach is statistically equivalent to showing that there is a significant portion of trait (phenotype) variation explained by genetic variation—which is where Kurzban seems to have become confused.

Wild parsnip, mostly here to break up the wall of text. Photo by Bas Kers.

Here’s a specific example near and dear to my field of study, species interactions: To determine whether parsnip webworms could be under natural selection to resist nasty chemicals produced by their food plant, the wild parsnip, May Berenbaum and Arthur Zangerl estimated the genetic component of variation [$a] in the worms’ capacity to choose food with lower levels of the toxins, and to tolerate the toxins they did eat. To do this, they raised webworm larvae of known parentage in the lab, and tested them on controlled diets. Their actual statistical analysis tested for an effect of the worms’ sibling relationships (parentage) on their ability to avoid toxins and survive them.

In all of Gallup’s lengthy response to Bering’s question about heritability, he doesn’t say a word about this kind of data with regard to homophobia. That’s because it doesn’t exist, and, as far as I can tell from the interview, he has no intention to try and collect it. To be completely fair, it’s harder to collect heritability data on humans than on webworms—but it’s hardly impossible. As Gallup notes, there are studies documenting heritability for, of all things, human grip strength [PDF].

Kurzban’s critique is correct in one very specific regard, which Bering doesn’t touch on. It is relatively difficult, both for logistic and resource-related reasons, to estimate a trait’s heritability and determine whether natural selection is acting on it within the same study. (Although there are plenty of exceptions—here’s one example [$a] pulled at random from my reference library.) That’s why I said, in my original post, that biologists expect evidence for heritability or fitness benefits in support of an initial claim that a trait or behavior is adaptive. The study I cited as an example of support for adaptation—which shows that horned lizards’ horns prevent predator attacks [PDF]—demonstrates fitness benefits, but not heritability. This point should be familiar to anyone who regularly reads the evolutionary biology literature.

Grip strength: known to be heritable. Homophobia: not so much. Photo by West Point Public Affairs.

So, again, Gallup has no data on the heritability of homophobia. The rest of his interview shows that he still doesn’t have any data to demonstrate fitness benefits for it, either.

In which evidence of fitness benefits also remains absent

Gallup then comes to the question of whether a child who would otherwise be straight can be “converted” to homosexuality by early same-sex sexual contact.

As detailed in my 1996 reply to Archer, we’ve collected data from male homosexuals that show that most gay males don’t report getting a clear sense of their homosexual orientation until they have their first same-sex postpubertal sexual experience.

Most gay men don’t know for sure that they’re gay until they’ve actually, you know, tried gay sex? Quelle surprise. This is absolutely classic mistaking of correlation for causation, and it suggests that Gallup doesn’t know much about the actual experience of sexual minorities. When you grow up surrounded by straight people, it often takes very direct evidence to convince you that you’re attracted to people of the same sex. If same-sex activity shortly after puberty can cause homosexuality, wouldn’t parents be most concerned about homosexuals having contact with teenagers? At the risk of sounding like a broken record, this is yet another thing we can’t tell from Gallup’s survey data—he asked about pre-pubescent children, and in one case 21-year-old children, but not children who have just passed puberty.

Finally, Gallup deals with the relative risk that homosexuals will molest children. He does this by moving the goalposts for pedophilia:

There is also evidence that shows that the propensity to have sex with minors is positively correlated with promiscuity among homosexual males. Unlike heterosexual pedophiles, homosexuals who have sex with minors target young postpubertal victims.

That’s not pedophilia Gallup is talking about—that’s violation of age-of-consent laws. The comparison between heterosexual-identified pedophiles, who target children, and homosexuals who have sex with post-pubertal teens under the age of consent is, frankly, intellectually dishonest. By definition those are two different groups. The comparison to make is that between all homosexuals who have had sex with minors and all heterosexuals who have had sex with minors. I would imagine that, as Gallup basically admits in his next sentence, those two groups look much more similar.

So that’s where we stand: still no evidence that homophobia is heritable, and still no evidence that it provides a fitness benefit by preventing the homophobe’s children from becoming homosexuals. Gallup’s only data are still, over fifteen years after his initial publication, a set of survey responses that are consistent with any number of hypotheses for the origins of homophobia. Claiming that those data demonstrate an adaptive function for hatred of homosexuals doesn’t just fail the standards of evidence for evolutionary biology, it’s bad scientific reasoning.

In which we come to a conclusion of sorts

In a coda to the interview, Bering accuses me and his other critics of failing to engage with Gallup’s results. I think my previous discussion, and Bering’s response to it, speak for themselves. Bering has demonstrated to me that he doesn’t understand undergraduate-level biology, and that, as Will Wilkinson suggested, he’s more interested in ginning up controversy than scientific rigor. (On which point he wins, I suppose. D&T’s visit count went through the roof when P.Z. Myers linked here.)

Bering also makes some conspicuously uninformed speculations about my own experience and motivations. I won’t dignify that with a response except to say yes, Jesse, I’m gay, and you don’t know the first thing about what I have or haven’t encountered in the way of “palpable disapproval.” First and foremost, though, I’m a scientist. Contrary to what you seem to think, I love a good counterintuitive, paradigm-shifting hypothesis, but I also expect it to be supported with data.

Bering, however, is convinced that he’s established himself as a hard-nosed scientific iconoclast in opposition to all us stodgy, dogmatic, evidence-demanding biologists. He concludes,

So, I’ll continue to dredge up any old theory, no matter how meager the supporting data …

Clearly, Jesse, I can expect nothing more of you.

References

Arden, N., & Spector, T. (1997). Genetic influences on muscle strength, lean body mass, and bone mineral density: A twin study. Journal of Bone and Mineral Research, 12 (12), 2076-2081 DOI: 10.1359/jbmr.1997.12.12.2076

Berenbaum, M., & Zangerl, A. (1992). Genetics of physiological and behavioral resistance to host furanocoumarins in the parsnip webworm. Evolution, 46 (5), 1373-84 DOI: 10.2307/2409943

Young, K. (2004). How the horned lizard got its horns. Science, 304 (5667) DOI: 10.1126/science.1094790

Campbell, D. (1996). Evolution of floral traits in a hermaphroditic plant: Field measurements of heritabilities and genetic correlations. Evolution, 50 (4), 1442-53 DOI: 10.2307/2410882

Futuyma, DJ. (2005). Evolution. First ed. Sunderland, MA: Sinauer Associates. Google Books.

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

Mousseau, T., & Roff, D. (1987). Natural selection and the heritability of fitness components. Heredity, 59 (2), 181-97 DOI: 10.1038/hdy.1987.113

Parasitism of a different color

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ResearchBlogging.orgThe common cuckoo is such a lazy parent that brood parasitism—laying its eggs in the nests of other birds—is built into its biology.

No bird will willingly adopt cuckoo chicks, which usually out-compete, and sometimes kill, their adoptive siblings. Given any hint that one of the eggs in her nest isn’t hers, a bird will eject the intruder. So cuckoos have evolved eggs that mimic the coloring of their hosts’ eggs—dividing the species into “host races” that specialize on a single host species, and lay eggs that mimic that host’s.

Cuckoo eggs (indicated by arrows) in the nests of three different host species. Illustration via The Knowledge Project.

As you can see from this illustration, the match is often extremely good—the cuckoo egg is really only obvious when the hosts’ eggs are visibly smaller. In fact, a new study by Mary Caswell Stoddard and Martin Stevens shows that this matching is often even better than it looks to the human eye [$a].

Birds see the world differently than humans—where we have three kinds of color-sensitive cells in our eyes, they have four. This allows them to see colors in the ultraviolet range, which is invisible to us. Birds’ eyes also have an additional class of sensory cell that may help them perceive and discriminate among textures. So to study the match between cuckoo and host eggs, Stoddard and Stevens first had to figure out what each egg looked like to a bird.

A reed warbler feeds the cuckoo chick that has taken over its nest. Photo via Wikimedia Commons.

To do this, they developed a mathematical model of each host species’ vision. The model estimated how similar two eggs should look to a bird, given raw data about what colors of light the eggs reflect and the specific colors the bird can detect. Using the model, Stoddard and Stevens could then calculate the “overlap” between the colors and patterning of a host egg and the egg of a cuckoo specializing on that host species.

Stoddard and Stevens then applied the vision model’s measure of similarity to museum specimens of eggs from the cuckoo-parasitized nests of eleven European bird species. They found that cuckoo eggs matched their hosts’ quite well overall, but the match was best for cuckoos specialized on especially vigilant hosts. For each host, the authors looked up studies of egg rejection behaviors to calculate the probability that each species would eject eggs that didn’t look like their own. Species with higher ejection probabilities were parasitized by cuckoo host races whose eggs were better mimics.

That suggests host rejection behavior exerts strong natural selection on cuckoos, which makes sense given that successfully fooling a host is essential to cuckoo reproduction. In light of evidence that cuckoos can also exert selection on their hosts, it looks as though brood parasitism is a truly coevolutionary interaction between cuckoos and their hosts—one that can cause both to evolve greater diversity.

Reference

Stoddard, M., & Stevens, M. (2011). Avian vision and the evolution of egg color mimicry in the common cuckoo. Evolution DOI: 10.1111/j.1558-5646.2011.01262.x

Open Lab 2010 available for purchase!

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The Open Lab 2010 is here!.

Bora Zivkovic has just announced that the Open Lab 2010, the latest edition of the annual collection of online science writing, is now available for print on demand. Congrats to the hard-working team who put it all together: Bora, Jason Goldman, Andrea Kuszewski, and Blake Stacey.

OL2010 features my first-ever contribution to the collection, the tale of J.B.S. Haldane’s role in Soviet scientific propaganda, as well as top-notch work by Eric Michael Johnson, Carl Zimmer, Deborah Blum, Steve Silberman, Kate Clancey, and many others. So what are you waiting for? Go buy a copy or three.

Science online, spring break edition

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The weather was lousy, but the coffee was excellent. Photo by andrewyang.

I spent most of my final spring break as a graduate student in Portland, Oregon, where I am not sure I saw direct sunlight even once. Who wants to get a tan over spring break, anyway? Regular posting resumes when I’m back in Moscow next week. If you “like” D&T on Facebook, you’ll get an alert about that right in your News Feed (TM). That’s a good thing, right?

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.

Science online, mnemonic rats edition

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Bat in flight. Photo by tarotastic.

Blah blah blah, Facebook, blah blah blah.

  • When life gives you parasites … Ancient ammonoids—forerunners of modern squid and nautiluses, dealt with parasites by encasing them in pearl.
  • Evolutionary baby pictures. Bats’ evolution from flightless ancestors, illustrated.
  • Note to my students: no human testing planned yet. Enhancing the levels of a particular enzyme in rats’ brains helps them retain memories.
  • Discredited more than a century ago, but you get to use a “cephalometer of Anthelme.” Want to make a living reading people’s personalities by the bumps on their head? Maybe phrenology is for you.
  • Using foremost legs as antennae, even. Spider mimics ant surprisingly well.
  • Sensory metaphor hijinks. People are more likely to identify an ambiguously-gendered face as female when touching something soft.
  • Glass ceilings are durable. Active discrimination may not be preventing women from advancing in the sciences, but institutional biases sure are.
  • Don’t panic. Panic Virus author Seth Mnookin understands the parental worries underlying vaccine denialism, but he still thinks it’s a problem.
  • In case you missed it. Jesse Bering thinks homophobia might be adaptive. He’s wrong.

And now, after a string of weeks without video in my Friday roundup, I give you a slow loris with a tiny umbrella.

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

Science online, falling coconuts edition

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Waiting for the next one to drop? Photo by KhayaL.

What? You still haven’t told Facebook you like Denim and Tweed? But then how else will it know to send you ads for, um, obligate pollination mutualism?

  • Gotta start somewhere. The simplest possible biological eye—and the starting point for the evolution of more complex models—may have been found in brachiopod larvae.
  • Look out below! In Palmyra Atoll National Wildlife Refuge, introduced coconut palms are literally bombing the natives into extinction.
  • In case you missed it. The Carnival of Evolution is out at Genome Engineering.
  • Herpetologist porn. Anolis osa has just been differentiated from Anolis polylepis based on what a leading Anole scholar calls their “man parts.”
  • Big cats, but no longer the top dogs. With population declines of more than 90 percent since 1960, lions are in danger of extinction in the wild.
  • But wait, there’s more. It’s not looking so great for every other known species, either.

Testimony from the front lines, Exhibit B.

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Via the Hairpin’s sister site The Awl this time: Queer students at the very Christian Harding University have published a ‘zine trying to explain themselves to the rest of the student body. It’s pretty damn’ hard to read, although maybe just because it sounds pretty damn’ familiar to me:

Our voices are muted, our stories go unheard, and we are forced into hiding. We are threatened with re-orientation therapy, social isolation, and expulsion. We are told stories and lies that we are disgusting sinnners who are dammed [sic] to hell, that we are broken individuals and child abusers. We are told we will live miserable lives and are responsible for the collapse of civilization. …. We are good people who are finished being treated as second-class citizens at Harding. We have done nothing wrong and we did not choose this suppression.

From the vantage point of someone for whom it got better, it’s hard not to see a certain amount of cognitive dissonance underlying the attempts to engage the intended audience with Biblical exegesis. But you know what, Harding University queers? Whether or not God hears your “cries for liberation from harsh oppression,” the rest of us do.

Naturally, Harding University has blocked access to the ‘zine website on its campus.