To Which Organisms, If Any, Does The Logic Of Adaptationism Apply? (Part II)Published 27 March, 2011
Jeremy Yoder has replied to my prior remarks about heritability. In his post, he defends his view that saying a trait is “heritable” means the same as “passed down from parent to child more-or-less intact.”
I have no issue with his account of heritability in this latter post. However, I wasn’t convinced by his defense of his gloss of the term. He writes, reiterating his argument, that “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.”
To see why I don’t think that high heritability means more or less intact, consider, to take a simple example, that if you were to measure the heritability of the trait “eye number,” it will be zero. Variation in eye number is due to environmental factors, like accidents.
(This is largely an aside. My argument here turns only on the claim that some adaptations have zero heritability, not that they all do. Since Fisher, the relationship between heritability and adaptations has been much discussed; Yoder illustrates the idea: “Natural selection against less-well-matched moths and trees should eliminate heritable variation in moth ovipositor length and Joshua tree flower shape from natural populations. This would leave only non-heritable variation due to causes like developmental errors and environmental effects, which are random with respect to the local plant or pollinator population.” It seems from this that the ovipositor length is an adaptation that shows low or perhaps even zero heritability. Mark Ridley, in his textbook, writes that traits such as “number of legs per individual” is (trivially) zero (p. 236), and, more generally: “Directional selection unambiguously should continue to alter a character until its heritability is zero” p. 250.)
So, it seems to me – and maybe I’m wrong about this – that these traits – having two eyes, a particular ovipositor length, etc. – 1) are adaptations, 2) have zero heritability, 3) are passed down more or less intact. So it seems to me that if you think that these three are all the case, then traits that have a heritability of zero are passed down more or less intact; similarly traits that are not heritable (heritability = 0) are adaptive. I think that these combinations contradict Yoder’s views, but perhaps I have misunderstood his position (or something else). (Again, I’m not saying either that all adaptations will show very low heritability or that all traits with zero heritability are adaptations (e.g., the color of bones or other byproducts.))
The link to the larger issue here is the role that computing heritability plays in establishing adaptation. Yoder’s claim seems to be that it is essential. My claim is that it is not, as illustrated by the work on seahorses. There, a claim regarding a trait being adaptive was made without either of the two criteria – computing heritability, showing fitness effects of the trait in question. Instead, the authors of this piece make a claim about adaptation by making an inference from form to function on the basis of their model.
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.
That is, by the way, fine. My point is (and was) that if one rejects the logic that Futuyama describes in his textbook, then one ought to reject the logic uniformly across species.
Postscript: Yes, I know that I said I didn’t really care about the homophobia issue. And I still don’t. I just thought that I would mention a finding in a paper published in Behavior Genetics that reports that “variation in homophobia scores could be explained by additive genetic (36%), shared environmental (18%) and unique environmental factors (46%)” and concludes that “variation in attitudes toward homosexuality is substantially inherited, and that social environmental inﬂuences are relatively minor” (Verweij et al., 2008). I take it that Yoder would say that this counts as preliminary evidence in favor of the view that homophobia is adaptive after all.
Verweij, K. J. H., Shekar, S. N., Zietsch, B. P., Eaves, L. J., Bailey, J. M., Boomsma, D. I., & Martin, N. G. (2008). Genetic and Environmental Influences on Individual Differences in Attitudes Toward Homosexuality: An Australian Twin Study. Behavior Genetics, 38(3), 257-265.