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Orzack and Sober rightly want nothing to do with such stories. They want analytic precision. Accordingly, they define adaptationism as the claim that "natural selection is the only important cause of the evolution of most nonmolecular traits and that these traits are locally optimal." This brings us to the second key term in their title: optimality. The optimality that Orzack and Sober have in mind is not the global optimality of engineering design but the local optimality of trial-and-error tinkering.

In engineering design, the goal is to attain some prespecified function or collection of functions as efficiently and elegantly as possible. As a consequence, the engineer must frequently re-engineer things from the ground up. Darwinian evolution doesn't work that way. Instead of re-designing from scratch, evolution must take pre-existing materials and tinker with them. The optimality in this case is local. Darwinian evolution asks how much one can improve a system's function merely by tinkering with it.

Now from Orzack and Sober's perspective, the important thing about local optimality is that it admits quantification and thus enables one to assess the degree to which natural selection is responsible not just for a trait but for its optimal performance. In their essay for this volume (Adaptationism and Optimality is an edited collection of 12 essays), Orzack and Sober compare the respective contributions of natural selection and phylogenetic inertia in accounting for the stability of traits. Phylogenetic inertia refers to the influence of an ancestor on a descendent. The basic idea here is, If it ain't broke, don't fix it. Phylogenetic inertia keeps traits as they are but doesn't make them better (except by accident). How can we tell whether a stable trait owes its stability to natural selection and thus is locally optimal? Alternatively, how can we tell whether the trait's stability is merely a matter of phylogenetic inertia maintaining the status quo? Orzack and Sober lay out some techniques from Bayesian decision theory to answer these questions.

Orzack and Sober's essay is typical of the collection. Other mechanisms compete with natural selection to account for the emergence, stability, and vanishing of traits; these include genetic drift, phylogenetic inertia, and developmental constraints. The point of these essays is to provide analytic tools for teasing apart the role of natural selection from these other mechanisms, apply these tools to actual case studies, and reflect philosophically on the role of adaptationism in evolutionary biology.

The essays vary in technical sophistication. Peter Godfrey-Smith's "Three Kinds of Adaptationism" is eminently readable. It provides a sufficiently nice overview of the key philosophical issues connected with adaptationism that I would like to have seen it appear much earlier in the collection (it is essay 11 out of 12). I recommend reading it first. By contrast, Ilan Eshel and Marcus Feldman's "Optimality and Evolutionary Stability Under Short-term and Long-term Selection" is going to be tough sledding for those without the requisite technical background in mathematical game theory and evolutionary biology. This essay attempts the important task of mathematically teasing apart the effects of natural selection in cases where a population's genes are merely reshuffled (short-term evolution) from those where a population's genes are transformed, as via mutations (long-term evolution). Without an analytically tractable distinction here, one is at best justified maintaining that natural selection is a conservative force in biological evolution, not the creative force that Darwinists claim.

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