MULTIPLE FITNESS PEAKS AND EPISTASIS

The importance of genetic interactions in the evolutionary process has been debated for more than half a century. Genetic interactions such as under dominance and epistasis (the interaction among genetic loci i n their effects on phenotypes or fitness) can play a special role in t he evolutionary process because they can create multiple fitness optim a (adaptive peaks) separated by fitness minima (adaptive valleys). The valleys prevent deterministic evolution from one peak to another. We review the evidence that genetic interaction is a common phenomenon in natural populations. Some studies give strong circumstantial evidence for multiple fitness peaks, although the mapping of epistatic interac tions onto fitness surfaces remains incompletely explored, and absolut e proof that multiple peaks exist can be shown to be empirically impos sible. We show that there are many reasons that epistatic polymorphism is very difficult to find, even when interactions are an extremely im portant part of the genetic system. When polymorphism results in the p resence of multiple fitness peaks within a group of interbreeding popu lations, one fitness peak will quickly be nearly fixed within all inte rbreeding populations, but when epistatic or under dominant loci are n early fixed, there will be no direct evidence of genetic interaction. Thus when complex landscapes are evolutionarily most important, eviden ce for alternative high fitness genetic combinations will be most ephe meral. Genetic interactions have been most clearly demonstrated in wid e crosses within species and among closely related species. This evide nce suggests that genetic interactions may play an important role in t axonomic diversification and species-level constraints. Population gen etic analyses linked with new approaches in metabolic and molecular ge netic research are likely to provide exciting new insights into the ro le of gene interactions in the evolutionary process.