The Adaptation Program

Ready for some evolutionary theory?

Yesterday, I read a famous scientific article on adaptation by evolutionary biologists Stephen J. Gould and Richard Lewontin, titled “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme” (Gould & Lewontin, 1979).  Gould & Lewontin criticize what they call the “adaptationist programme” school of thought within evolutionary biology.  The adaptationist program is:

“The notion of near omnipotence of natural selection in forging organic design and fashioning 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.”

— Stephen J. Gould & Richard Lewontin

As a result, Gould & Lewontin claim that proponents of the adaptationist program are unwilling to consider alternatives to adaptation as an explanation for an organism’s morphology or behaviour.  Furthermore, they accuse evolutionary biologists of creating adaptive stories that cannot be validated by empirical testing.  Within this school of thought, adaptive argument after adaptive argument are employed, without consideration for any non-adaptive argument.  And in the absence of any adaptive explanation, Gould & Lewontin claim that researchers would rather attribute this failure to an “imperfect understanding of where an organism lives and what it does” as opposed to considering a non-adaptive explanation (e.g., genetic drift).  Consider the following example for the type of thinking that Gould and Lewontin are critiquing:

“How Tyrannosaurus used its tiny front legs is a scientific puzzle; they were too short even to reach the mouth. They may have been used to help the animal rise from a lying position.”

— (Gould & Lewontin, 1987)

a.aaa-Arms-being-to-Short

 

Gould and Lewontin point out that just because the Tyrannosaurus front legs cannot reach the mouth, does not mean you can just create an untestable adaptive story about what other potential ways they may have used them.  The Tyrannosaurus front legs may not have been adaptive at all.  They may have simply been allometrically scaled down homologues in an allosaur ancestor.

Even though this paper was published in 1979, it still resonates with evolutionary theorists today.  TheWeb of Knowledge website indicates that it has been cited over 2,000 times, and is generally considered a “citation classic” throughout academia.  I can definitely understand why: while I was reading it, I started to get nervous.  I consider myself a human evolutionary theorist, and I frequently attempt to employ adaptive theory that will explain contemporary morphology and behaviour.  I started to wonder: “Am I a part of the adaptationist program that Gould and Lewontin were denouncing over forty years ago?”

In order to find out I had to go back and look at some of my old papers, including a recently accepted publication for Folia Primatologica and my nearly completed Master of Science thesis.  If I had been a part of the adaptationist program, I had become so unknowingly, and I would have to question my entire evolutionary theory education.

My initial fear quickly turned to relief.  After analyzing both papers, I realized that I had unknowingly benefitted from reading (and implementing) decades of evolutionary research that had built on the suggestions of Gould and Lewontin.  Instead of falling victim to the adaptationist program, I had incorporated a pluralistic approach to theory that considered non-adaptive explanations.  In my Folia Primatologica paper I collected data on chimpanzee night nesting patterns between two different forest blocks in Cameroon.  What I discovered was that there were differences in patterns of night nesting that were dependent on the level of human presence.  I concluded that in areas where humans were present, chimpanzees felt less safe, and consequently were less likely to nest terrestrially during the night.  In terms of theory, this means that terrestrial night nesting is likely an adaptation to an environment with low predation levels.  I did not just create an adaptation story; I had empirical evidence to support my claim for an evolutionary adaptation.

Likewise, I did not assume adaptation in my Master of Science thesis while investigating the potential for ring-tailed lemur cathemeral behaviour.  An animal can be considered cathemeral if “the activity of an organism […] is distributed approximately evenly throughout the 24 hours of the daily cycle, or when significant amounts of activity, particularly feeding and/or traveling, occur within both the light and dark portions of that cycle” (Tattersall, 1987: 201).  In my analysis I consider both an adaptive (stable evolutionary strategy) and non-adaptive (evolutionary disequilibrium) hypothesis for cathemerality.  The adaptive hypothesis proposes that cathemerality is a stable and deep-rooted activity pattern among ring-tailed lemurs that is dependent on environmental and ecological variables.  The non-adaptive hypothesis proposes that cathemerality is a transitional state between nocturnality and diurnality made possible by the rapid extinction of subfossil lemurs and raptors approximately 2,000 years ago (Van Schaik & Kappeler, 1996).  Therefore, we should expect there to be a mismatch between cathemeral lemurs activity pattern and biological adaptation.

The fact that I had been unaware of the adaptationist program critiqued by Gould & Lewontin in 1987, and yet still avoided the pitfalls of logic that accompanied it, reveal how powerful and influential their paper has become.  I strongly recommend reading it.  In today’s field of evolutionary biology, theorists have realized that not all organisms are perfectly adapted to their environments, and that you cannot simply create an adaptive story without sufficient evidence to indicate probability.  However, even though I personally had not become part of an adaptationist program, it is still important that I read this paper.

I have started to think about adaptation, and adaptive evolutionary theory in a different way.  It is now clear to me that adaptation is a concept that can only completely solve evolutionary puzzles for extant organisms.  As soon as we apply evolutionary theory to extinct organisms, we can only solve evolutionary puzzles on a gradation of probability.  It is true that we can collect empirical evidence supporting adaptive theory, but that can only reveal that a certain adaptation is the likely cause of a trait.  For example, evolutionary theorists believe that humans became bipedal as an adaptive response to an increasingly terrestrial existence in a woodland-mosaic environment.  This can be tested via evidence from fossilized remains, paleoclimatic data, and modeling the behaviour of our closest relatives (e.g., chimpanzees and bonobos).  These data all strongly suggest that there were real environmental pressures for a transition to bipedality after the split from our common ancestor with chimpanzees and bonobos.  These pressures likely remained strong until the emergence of our contemporary genus (e.g, Homo).  However, these data can only allow us to conclude with a high probability of certainty.  They cannot be used to allow us to conclude with 100% certainty.  Therefore, if a certain trait like bipedalism appears to be adaptive, we will always have competing hypotheses.  A complete theory of bipedality will always be near-completion, but never definitive.  The more data we collect, the stronger our current hypothesis may become.  However, that may be the best we can do since our early-bipedal ancestors are now extinct.

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We may never have 100% certainty of the adaptive selection pressure for bipedality

In the future, I will likely be a better researcher for being cognizant of Gould & Lewontin’s landmark paper “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.”  I will still attempt to use adaptive theory, but I will be aware that adaptation cannot explain all morphology and behaviour.  I should probably use this experience as evidence that I need to read more Stephen J. Gould.

References:

Gould, S.J. & Lewontin, R.C.  1979.  The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.  Proc. R. Soc. Lond. B.  205, doi: 10. 1098/rspb.1979.0086

Tattersall, I.  1987.  Cathemeral activity in primates: A definition.  Folia Primatologica, 49: 200-202.

Van Schaik, C.P., & Kappeler, P.M.  1996.  The social systems of gregarious lemurs: lack of convergence with anthropoids due to evolutionary disequilibrium?  Ethology, 102: 915-941.

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About Cadell Last
Hello. I'm probably drinking coffee and reading.

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