The genetic theory of adaptation: a brief history

Key Points Early evolutionists believed that the genetic basis of adaptation was micromutational. This view was supported by Ronald Fisher's classical mathematical analysis (of 1930) of his 'geometric model' of adaptation. Beginning in the 1980s, studies of quantitative trait loci and microbial experimental evolution revealed that adaptation sometimes involves a modest number of genes, some of which have surprisingly large effects. These experimental findings pose a serious challenge to evolutionary theory. So far, phenotype-based and DNA sequence-based models of adaptation have yielded surprisingly similar results, indicating the possibility of a robust theory of adaptation. Phenotypic models of adaptation show that the genes that cause adaptation should have approximately exponentially distributed effects; that is, involve many genes that have small effect and a few genes that have large effect. DNA sequence models of adaptation indicate that adaptation should involve mutations of relatively large fitness effects and that adaptation is characterized by a pattern of diminishing returns, in which early substitutions have large fitness effects and later ones have smaller effects. Current theories of adaptation adequately explain certain qualitative patterns that characterize genetic data on adaptation; however, it is not yet clear if these theories can explain these data quantitatively. Theoretical studies of adaptation have exploded over the past decade. This work has been inspired by recent, surprising findings in the experimental study of adaptation. For example, morphological evolution sometimes involves a modest number of genetic changes, with some individual changes having a large effect on the phenotype or fitness. Here I survey the history of adaptation theory, focusing on the rise and fall of various views over the past century and the reasons for the slow development of a mature theory of adaptation. I also discuss the challenges that face contemporary theories of adaptation.