28 March 2011

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

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 na├»ve, 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

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