What is the genetic architecture of adaptation?
Although the evidence for adaptation is overwhelming, few studies have described the genetic basis of adaptive traits (Orr and Coyne 1992). Fishers infinitesimal model of evolution proposed that adaptation is due to the fixation of many genes with small individual effects, and is based on the assumption that large effect mutations will have negative pleiotropic effects, and will therefore move a population away from its phenotypic optimum (Fisher 1930). Similarly, Lande (1983) demonstrated that adaptation would usually result from micromutation unless selection is strong and persistent. This micromutationist view of adaptive geometry (Barton 1998) has had widespread support, but was challenged recently by Orr (1998) who suggested that mutations of large effect can often be beneficial during the early stages of adaptation as populations move towards their optimum phenotype. With this assumption, he found that the distribution of factors fixed during the approach to a stationary phenotyp
