Coloration and the Genetics of Adaptation

H ow do organisms adapt to new environments? The popular conception of the adaptive process generally runs like this: A mutation arises that leads to an improved phenotype under novel environmental conditions (for example, a beak modifi cation suited to exploiting an untapped food source). Because of the enhanced reproductive success of individuals carrying the mutant allele (or gene variant), the frequency of the mutant allele increases. The mutant allele eventually becomes fi xed in the population—that is, every member of the population has two copies of the allele—and the population has increased its fi tness. The whole process can then repeat itself when the environment changes again. This is the simplest possible model of adaptation, and is enshrined in well-known examples such as the evolution of melanism in the peppered moth and of insecticide resistance in mosquitoes. However, it has long been known that there are many potential complications to this tidy scenario involving a single mutant allele at a single genetic locus, and that the situation is frequently more complex in the real world. For example, more than one locus may be involved, mutant alleles may have effects on more than one aspect of the phenotype (a phenomenon known as pleiotropy), and the effects of alleles on the phenotype may be dependent on the environment (genotype × environment interactions). The timing of the occurrence of a mutation in relation to an environmental change in which the mutation is favored—that is, whether the benefi cial mutation arose de novo in the new environment or was part of the standing genetic variation—also turns out to be critically important. Recent theoretical [1] and empirical work [2–4] has greatly increased our understanding of the adaptive process, but there are many fundamental questions remaining. In particular, we have little knowledge of the relative importance of different factors in actual cases of adaptation, and there are many details of the molecular basis of adaptation that are poorly understood. If two or more loci are involved in adaptation to a new environment, then this leads to the possibility that the phenotypic effects of the two loci are not additive, that is, they are not independent of one another. For example, the effect on the phenotype of the presence of allele A rather than allele B at one locus may depend on whether allele X or allele Y are present at a second locus. Such epistatic …