Adaptive traits, adaptations, and adaptationisms

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One of the main points behind my natural selection lecture is that invoking natural selection as an explanation should only be done when there is evidence that natural selection has acted in a meaningful way. In other words, natural selection is not an automatic null hypothesis, it is an explanatory mechanism for the origin of adaptations only – and it is a grave fallacy to consider all biological traits and attributes as adaptations.

That fallacy is called adaptationism, and I term it a fallacy precisely because it’s extremely easy to commit. As one of my evolution professors back in my undergrad days snarkily remarked, you can explain why stones fall by natural selection: those stones that don’t react to gravity get carried off to space, therefore only those stones that do react to gravity stay on the ground, therefore natural selection. Completely nonsensical, but this is exactly what adaptationist explanations sound like. Natural selection can, theoretically, explain everything. It doesn’t mean those explanations are correct.

In order to clear this issue, an understanding of what adaptations are is necessary. As expected, the issue is rather tangled and complicated, because “adaptation” can be considered both as a process and as a result:

  • The process of adaptation is synonymous with the process of natural selection.
  • The result of natural selection is an adaptation.

The most ideal consideration is to take adaptations as the results of natural selection, as this makes for the least confusion and double-meanings. But in order to avoid the problem of over-reliance on natural selection as an “easy explanation”, we have to use a definition of “adaptation” that’s not circular and tautological. This has been the philosophical conundrum of evolutionary biology for some time, but we have settled on various workable definitions.

  • The functional definition, as explained by Reeve & Sherman (1993), states that “An adaptation is a phenotypic variant that results in the highest fitness among a specified set of variants in a given environment”.
  • There is also the historical definition, which Sober (1984) explains as: “A is an adaptation for task T in population P if and only if A became prevalent in P because there was selection for A, where the selective advantage of A was due to the fact that A helped perform task T.”

At first glance, these two definitions appear very similar, almost identical. In fact, they address two closely-related issues. The functional definition allows us to identify adaptive traits, while the historical definition allows us to identify adaptations. Adaptive traits are those that increase the fitness of an organism [in a given environment]. Adaptations are adaptive traits that got selected for and remained.

To understand the difference: vestigial organs are adaptations, but they are no longer adaptive. In other words, they were adaptive at one point and got selected for, therefore they are adaptations. However, the environment changed and they no longer became adaptive, hence they became reduced and vestigial. But this doesn’t change that they are adaptations – one must never forget the significant historical aspect of evolutionary biology.

This is how we understand adaptations today in evolutionary biology (or, to be fair, how most of us view them). The history of how we came to this point is fairly well-known due to one of the players involved, the inimitable Stephen J. Gould. Gould and his colleague Richard Lewontin published their landmark 1979 Spandrels of San Marco paper.

In architecture, spandrels are triangular areas formed when you have arches or domed structures. In cathedrals and churches, these are used as extra mural painting space, and so the adaptationist thing to say would be that spandrels were developed to maximise how many paintings one could do.

What Gould and Lewontin did in that paper was flip that on its head. Spandrels weren’t developed for the purpose of extra painting space. Spandrels came about ineviatbly because of agriculture, and the artists merely co-opted them. This architectural example is exactly what we observed in evolutionary biology, as they pointed out:

We wish to question a deeply ingrained habit of thinking among students of evolution. We call it the adaptationist programme, or Panglossian paradigm. It is roots in a notion popularised […] towards the end of the nineteenth century: the near omnipotence of natural selection in forging organic design and fasioning the best among possible worlds. This programme regards natural selection as so powerful and the constraints upon it so few that direct production of adaptation through its operation becomes the primary cause of nearly all organic form, function, and behaviour.

The paper pointed out the three major problems of adaptationism: the difference between adaptive traits and adaptations wasn’t recognised; non-adaptive explanations weren’t considered; and adaptive explanations were merely “just-so stories” with no standards for backing evidence.

They had examples for each of the fallacies. For the first, they considered the small forearms of tyrannosaurs. The adaptationist hypothesis of the time was that male tyrannosaurs had such small forearms in order to stimulate the females sexually. Plausible, but it doesn’t explain why they became so small. In other words, there is no distinction made between the adaptive function, and whether there was an adaptation.

For the second, they invoked human evolution, specifically the evolution of the chin. Is there an adaptive explanation for it? Maybe. But it’s also undeniable that the chin evolved as a result of increasing neoteny – it was mere biomechanics, not necessary to invoke natural selection.

Just-so stories are extremely widespread – just look at evolutionary psychology, where you will find adaptationist hypotheses banded out the explain every aspect of human behaviour, without ever considering that the number of unreasonable assumptions (mostly about how ancient humans lived), bad research methods (very biased samples and low sample sizes!), and lack of evidence for natural selection acting all mean that their hypotheses are unscientific.

I already explained thoroughly in the natural selection lecture how we should avoid such mistakes. We need to always consider every possibility for the evolution of an adaptive trait. For example, it may come about due to selection on another trait – remember that biology is all about interconnected modules. It may come about due to physics or biomechanics, much like spandrels. Many traits are subject to constraints. These may be genetic (lack of mutational variability necessary for selection to act), developmental (as with male nipples), functional (as with hydrodynamic body shapes – a bus-like swimming organism won’t evolve), or historical (traits that evolved earlier make it harder to break out of a mold).

I may be a bit unfair to adaptationism. Old-skool adaptationism is all but dead except in a couple of subdisciplines and among overexcited first-year students. Adaptationism nowadays is split along three more or less independent research programs, as Godfrey-Smith (2001) explains.

  • Empirical adaptationism seeks to show that most biological traits are adaptations. Empirical adaptationists take adaptation and natural selection as the working hypothesis for all traits, however they do not claim that natural selection is some sort of all-powerful process as their intellectual forebears did. This may or may not prove to be a good research program: the data will show.
  • Explanatory adaptationism is rather different. It tries to convince us that adaptations are the really interesting things in evolutionary biology. This, in my opinion, is a silly position to take – even if empirical adaptationism turns out to be right, interest is a highly-subjective judgement.
  • Methodological adaptationism states that heuristically, adaptations are more likely to be important, therefore it’s more valuable to concentrate our studies on them. Again, even if they are numerous and valuable, what makes them so important to study that we have to place a focus on them? The use of evolutionary biology for medicine involves no talk of adaptations. The use of evolutionary biology in ecology rarely needs to consider adaptations as a factor. Ditto for palaeontology.

In other words, such blanket statements have no place in a science as diverse and widespread as evolutionary biology. Empirical adaptationism makes sense, as it at least proposes a solid research plan, even if one may find it misguided. But the other two are just projections.

References:

Godfrey-Smith P. 2001. Three kinds of adaptationism. In: Orzack SH & Sober E (eds.). Adaptationism and Optimality.

Gould SJ & Lewontin RC. 1979. The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme. Proc. R. Soc. B 205, 581-598.

Reeve HK & Sherman PW. 1993. Adaptation and the Goals of Evolutionary Research. The Quarterly Review of Biology 68, 1-32.

Sober E. 1984. The Nature of Selection: Evolutionary Theory in Philosophical Focus.

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