This visit is significant since the Spandrels of San Marco are famous in evolutionary biology. They are part of the attack on adaptationism launched in 1979 by Gould and Lewontin. This is a paper that every student of evolution should read. Here's an online version: The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.
John has been struggling with adaptationism for almost fifteen years. When he first began studying evolutionary biology he, like many others, was unaware of the importance of random genetic drift and other anti-adaptationist perspectives. He certainly didn't know that random genetic drift is by far the dominant mechanism of evolution in terms of frequency of allele fixation. Over time John has developed an unusual perspective on adaptionism—one that I don't really understand.
Here's how he explains it in his latest posting ...
This is interesting, I think, in the context of Gould’s and Lewontin’s paper. It shows that claims of things being adaptive or not depend crucially on what one counts as the “task” of a structure. Since I think that everything is subjected to selection pressure at all times (sometimes not enough to overcome the noise of statistical properties), counting what is, and what isn’t, adaptive is a bit of a personal call, in the absence of access to the historical processes of particular traits. I am becoming more of an adaptationist these days.The idea that many alleles might be slightly beneficial or slightly detrimental isn't very controversial. But that's not what John is saying. As I understand him, he's saying there can be no such thing as a truly neutral allele. He seems to be saying that anyone who believes otherwise is making a "personal call." A personal call that he believes is wrong since he thinks (i.e. his personal call) that everything is subject to natural selection.
He's also making a somewhat trivial point that doesn't contribute to the debate, as far as I'm concerned. Many alleles that are slightly beneficial are lost due to random genetic drift and many alleles that are slightly deleterious are fixed by random genetic drift. To me, that says that adaptationism can't explain all of evolutionary biology. To call yourself an adaptationist while knowing that slightly deleterious alleles can be fixed by random genetic drift seems somewhat unsatisfying.
John has a paper in the latest issue of Biology and Philosophy, an issue devoted to Adaptationism. It's not a very enlightening issue, from my perspective. The main problem with adaptationism isn't that it can't explain adaptation and it isn't that some just-so stories are wrong. The main problem is that adaptationists don't even consider any other alternatives to fixation by natural selection. Everything, especially everything with a visible phenotype, is automatically assumed to be adaptive and the arguments proceed from there.
One of the papers I liked was Seven Types of Adaptationism by Tim Lewens (Lewens, 2009). The seven types are:
A Empirical adaptationismsThere are problems with all seven forms of adaptationism but the nice thing about Lewens' paper is that he effectively refutes #4, #5, and #7. In the case of methodological adaptationism it's just not true that the default assumption has to be adaptation. Evolutionary biology will be just as productive in the long run if drift is the default assumption and adaptation has to be proven.
1. Pan-selectionism–natural selection is the most significant of the evolutionary forces that act on populations.
2. Good-designism–evolutionary processes tend to result in organisms with suites of well-designed traits. Most lineages are highly evolvable.
3. Gradualism–adaptation is always the result of selection acting on gradual
B Methodological Adaptationisms
4. Weak heuristic adaptationism–those traits that are adaptations are likely to be correctly recognised as such only if we begin by assuming that all traits are adaptations.
5. Strong heuristic adaptationism–only by beginning to think of traits as adaptations can we uncover their true status, whether they are adaptations or not.
C Disciplinary Adaptationism
6. Explanatory adaptationism–an evolutionary biologist’s proper business is the study of adaptations.
D Epistemological Adaptationism
7. Epistemological optimism–investigators have access to the data that reliably discriminate between conflicting evolutionary hypotheses.
In explanatory adaptationism, the assumption is that all of the interesting parts of evolution are adaptations and fixation of alleles by random genetic drift is so boring that it might as well not even be evolution. This is the stance taken by many adaptationists, like Richard Dawkins. As you might imagine, it doesn't take much effort to refute that kind of argument. One's personal opinion about what's interesting and what's not interesting should not play a role in determining how everyone else should go about studying evolution.
Gould, S.J. and Lewontin, R.C. (1979) The Spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proc. R. Soc. Lond. B 205:581-598.
Lewens, T. (2009) Seven types of adaptionism. Biol. Philos. 24:161–182. [doi: 10.1007/s10539-008-9145-7]