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Robert Kurzban

The Evolutionary Psychology Blog

By Robert Kurzban

Robert Kurzban is an Associate Professor at the University of Pennsylvania and author of Why Everyone (Else) Is A Hypocrite. Follow him on Twitter: @rkurzban

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.

  • Jesse Marczyk

    It seems to be the classic confusion of terms lots of people run into, where one form of the word refers to a trait being inherited (like number of eyes) while the other refers to a trait being heritable (the extent to which difference in a trait can be explained by differences in genes). While similar and often used as substitutes, the two are not interchangeable. A trait can be inherited without being heritable.

    I take it that Yoder would say that this counts as preliminary evidence in favor of the view that homophobia is adaptive after all.

    Of course you know this, but it’s probably worth mentioning explicitly: While inheritance of a trait is a base requirement for establishing possible adaptive value, heritability itself isn’t the greatest way of determining adaptive value – since byproducts are inherited in the same fashion as adaptations are – nor required in some cases, as the number of eyes example illustrates.

    • Robert Kurzban

      Right, and just to clarify: I put it that way – why he would say – to indicate what his view entails, rather than what I myself would infer from the data.

      • Jesse Marczyk

        Of course. I was agreeing with the sentiment that heritability estimates, while nice, aren’t the end-all-be-all

  • Marco DG

    There’s another reason why “heritable” and “passed down from parent to child more-or-less intact” are defintely not the same, and it has to do with the distinction between narrow-sense vs. broad-sense heritability. Narrow heritability (h^2), the one discussed by Yoder in his post, is the proportion of additive genetic variance, the one that makes parents and children resemble each other. However, broad heritability also includes non-additive genetic variance, which (in sexual species) does not contribute much to making children resemble their parents.

    Note that broad heritability is a better measure of the overall proportion of variation of a trait accounted for by genetic differences. Biologists are often interested in narrow heritability beacuse it is the component that responds to directional selection in the short-term. Thus, h^2 is useful because it can be used to predict the response of a trait to selection, but it only represents part of the genetic variance of a trait. For example, most personality traits in humans show substantial non-additive genetic variance, and only looking at h^2 undersetimates the genetic contribution to differences in personality.

    Another quick note on Yoder’s post. He emphasized the following sentence by Futuyma: “so perfect a correlation clearly would imply a strong genetic basis for the trait” in support of his definition of heritability. But there’s a logical fallacy here: a strong parent-offspring correlation does imply high heritability, but the converse is not true; high broad-sense heritability needs not imply a stronbg parent-offspring correlation, and lack of strong parent-offspring correlation does not negate heritability of a trait.

    • Jesse Marczyk

      Here’s another little contradiction from Yoder’s article:
      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.

      So on the one hand, Yoder seems to imply that evidence of heritability doesn’t exist, though it would seem Rob did dig up some evidence of it, meaning Yoder probably didn’t do his homework. That alone kind of makes his criticism fall a bit flat.

      But there’s also this:
      (2) the data we do have regarding the probable fitness benefits of homophobia and its heritability contradict his hypothesis

      If there are no studies of heritability, that doesn’t contradict Gallup’s hypothesis; the lack of research on the subject would just not saying anything about his hypothesis one way or the other. In this case, absence of evidence is not evidence of absence, as Yoder seems to imply it is.

      Not that I’m saying Gallup’s hypothesis is or isn’t correct, just that Yoder seems to be a touch too eager to criticize it (and evolutionary psychology) to the point where he extends beyond his ability to do so.

      • Robert Kurzban

        Again, fair enough, but I can’t take credit for “digging,” really, which sounds like I had to make some sort of effort. I typed “homophobia heritability” into Google Scholar, and it’s the first one…

        • Jesse Marczyk

          As evolutionary psychologists, we might not understand evolution, but at least we get Google.

  • Marco DG

    BTW: interesting how people like Yoder are anxious to show the world that “evolutionary psychologists don’t understand evolution.” Even better, evolutionary psychologists don’t even understand basic, textbook evolution. Wouldn’t it be great if it were true? That would make it legitimate to dismiss the whole field without even bothering with reading the papers. I’ve seen variations on this claim repeated so often that I’m almost bored.

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