Of course, genetic mutation must ultimately fuel the genetic variation that permits adaptive evolution. However, genetic accommodation need not await mutation. There are two kinds of evidence that the standing genetic variation (Orr and Betancourt, 2001) of particular responses is probably usually sufficient to support genetic accommodation without mutation. A large accumulation of data on protein polymorphisms has shown that genetic variation is common in natural populations (Lewontin, 1974); and virtually every trait subjected to artificial selection shows a response to selection (references in West-Eberhard, 2003). A rare class of exceptions occurs under artificial selection for directional (consistently right or left) asymmetry of various traits (ommatidia number, wing folding, eye size, and thoracic bristle number) in Drosophila (Coyne, 1987; Maynard Smith and Sondhi, 1960; Purnell and Thompson, 1973; Tuinstra et al., 1990). Nonetheless, directional (consistent right or left) asymmetry has evolved repeatedly in insects (e.g., see Schuh and Slater, 1995, on genitalia and other abdominal structures) and in other organisms (Palmer, 2004), suggesting that even this category of lack of response to selection can be overcome by variation and selection during longer evolutionary time scales. The phenotypic definition of selection helps to explain some of these cases, in which lack of genetic variation initially may have blocked a response to positive selection, and yet the trait eventually evolves to fixation. Because selection (differential reproductive success) of phenotypes does not require genetic variation, directional selection can persist generation after generation, favoring either the right or left form under environmentally influenced fluctuating asymmetry (“antisymmetry”), a state known to precede the evolution of some examples of directional symmetry (Palmer, 2004). Then an evolutionary response to selection would occur as soon as favorable genetic variations arise, e.g., due to mutation. Thus, although standing genetic variation usually must be sufficient to produce a response to selection (Orr and Betancourt, 2001), genetic accommodation may in some cases, like that of directional asymmetry, await mutation (Palmer, 2004).
Biologists are inclined to doubt the evolutionary importance of environmentally induced traits because it is not immediately obvious how they can be inherited in subsequent generations. Initiation by mutation is intuitively more appealing because it solves the problem of novelty and heritability in one stroke. However, environmentally induced variants are heritable as well, insofar as the ability to respond by producing them is heritable (that is, genetically variable). The responsiveness of organ-