Recently a few blogs I follow have been having a back and forth "debate" which seem to recapitulate in the most general sense the "selectionist vs. neutralist" debates of the 1970s.
Three posts from p-ter:
Do phenotypes evolve neutrally?
More on adaptation
Final Thoughts on adaptation
From Larry:
Visible Mutations and Evolution by Natural Selection
Richard Dawkins on Visible Change and Adaptationism
While RPM offers Whither Adaptation?
There are two general responses I have to these sorts of debates. First, their relevance to the "post-genomic era" is something I would question. The arguments date from a time when Richard Lewontin and John Hubby's famous allyzome papers were hot stuff. More precisely, Motoo Kimura formulated Neutral Theory in part to grapple with and explain Lewton and Hubby's findings. This was a time when molecular evolution was a nascent field, and it had to face the predictable if less than noble resistance from researchers schooled in classical Mendelian empiricism which focused on visible phenotypic markers to trace the inheritance of genetic elements. In 2007 these sorts of resentments and historical grievances seem to be shadows. You have a field like molecular ecology which serves as a branch between the lab and field tribes within the biological sciences. Human genomics is now opening up the elucidation of the DNA sequences which underlay traits whose heritable dynamics were previously analyzed via quantitative genetic methodologies (e.g., skin color). I think we should be careful about expending excessive cognitive cycles on demarcation and lexical debates about which evolutionary parameter is "more important." There are enormous data sets that need to be worked on. While skirmishes over philosophical purity have both substantive and entertainment value, they shouldn't be the main attraction.
Second, Larry seems conflate random genetic drift with stochasticity. This just isn't so, selection can be stochastic. On the fundamental level one might expect that the seemingly random walk processes which we perceive and reify as "drift" have deterministic proximate causes. Randomness simply emerges because of our ignorance of the underlying variables. In any case, if you read a paper such as The Probability of Duplicate Gene Preservation by Subfunctionalization, I think you'll agree that the old dichotomy needs to be updated to fully reflect the reality of evolutionary dynamics.
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It seems to me that first a question has to be answered. That being...
Is the trait one that can be selected for or against under local conditions?
Razib says,
Second, Larry seems conflate random genetic drift with stochasticity. This just isn't so, selection can be stochastic.
This is a very silly mischaracterization of my position. I have pointed out repeatedly that the probability of fixation of a beneficial allele is approximately 2s, where s is the selection coefficient. The reason for making this point so frequently is that there are lots of people out there who think that whenever an allele has an adaptive benefit it will sweep to fixation.
Thus, I know full well that a great deal of evolution is stochastic (or accidental). The first time I recall making this point on the internet was in talk.origins back in 1992 when I was debating Chris Colby who at the time was an adaptationist.
You are more than welcome to debate these issues with me but please don't make up things that aren't true. Give me a little credit for knowing what I'm talking about.
I find the new tactic of the adaptionists quite fascinating. They are now trying to dismiss the debate as meaningless in the post-genomic era. Meanwhile they continue to make all sorts of unsubstantiated claims about the adaptive significance of behavior (e.g., evolutionary psychology) and the function of junk DNA.
This is a very silly mischaracterization of my position.
you reap as you sow.
the adaptive significance of behavior (e.g., evolutionary psychology)
the fact that you go there so quickly implies the weakness of your contention. behavior is often (though not always) extremely intractable. but your assertions seem to apply to physical characteristics.
A morphological trait may have no adaptive significance at the phenotypic level but its underlying alleles are selected at the genic level.
Or it is adaptive in one sex but not the other. Or only under certain environmental conditions in a cyclic or fluctuating environment. Or it represents an adaptive genetically enabled polymorphism induced by environmental factors in early development but its adult manifestation is not adaptive under some conditions.
Or the morphological/behavioral trait is the adaptation not of its bearer, but of a parasite.
Or it might have no adaptive significance at any level. Who knows. The point is to try to figure it out.
The point is to try to figure it out.
bingo! it is an empirical question. one scaffolded by priors due to the inferential and contingent nature of evolutionary science; but an empirical question nonetheless. nature is one, but it is many-faced.