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. [doi: 10.1098/rspb.1979.0086]If you haven't read this paper by now then download it and read it carefully. It's the most important paper to read if you are interested in evolution.
Abstract: An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an organism into unitary 'traits' and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Bauplane so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin's own pluralistic approach to identifying the agents of evolutionary change.
Jerry Coyne agrees, "Read the rest; it’s certainly one of the most important papers in modern evolutionary biology." [It’s a spandrel (sort of . . .)!].
He also says,
This paper is famous because the authors were famous, because it’s very well written, but most of all because it posed a direct attack on the “Panglossian paradigm”: the view that sociobiology wants to explain all traits, particularly human behaviors, as the direct products of selection. This paper has been the subject of furious discussion and at least one book. In my view, the paper made some valid points but went overboard in its criticism of the adaptationist program, which, after all, has produced lots of insights about evolution. I knew Gould, who was on my thesis committee, and it always seemed like pulling teeth to get him to admit that natural selection was even a relatively important force in evolution. If pressed, he would, but Gould always preferred (perhaps for political reasons) to emphasize the limitations of selection. Lewontin was not nearly so extreme.It's true that the adaptationists have produced some valuable insights when the problem they are examining is actually an adaptation. However, this isn't as significant as you might imagine. Think of it like this. Everything looks like a nail when you have a large hammer in your hand. The fact that some things actually turn out to be nails is no excuse for blindly whacking at everything that sticks up.
Gould and Lewontin advocated a pluralist position where many different kinds of explanations should be considered. They note that Darwin himself was not committed to natural selection as the only possible mechanism of evolution.
Since Darwin has attained sainthood (if not divinity) among evolutionary biologists, and since all sides invoke God's allegiance, Darwin has often been depicted as a radical selectionist at heart who invoked other mechanisms only in retreat, and only as a result of his age's own lamented ignorance about the mechanisms of heredity. This view is false. Although Darwin regarded selection as the most important of evolutionary mechanisms (as do we), no argument from opponents angered him more than the common attempt to caricature and trivialize his theory by stating that it relied exclusively upon natural selection. In the last edition of the Origin, he wrote (1872, p. 395):
"As my conclusions have lately been much misrepresented, and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous position-namely at the close of the introduction-the following words: "I am convinced that natural selection has been the main, but not the exclusive means of modification." This has been of no avail. Great is the power of steady misinterpretation."
[Hat Tip: Pharyngula: Oldie moldies that are pretty darned fascinating]