ROLE OF OVERLAPPING GENERATIONS IN MAINTAINING GENETIC-VARIATION IN AFLUCTUATING ENVIRONMENT

Population genetics theory suggests that temporally fluctuating select ion on phenotypes can act to maintain genetic variance only under very restrictive conditions. However, this conclusion is based on models w ith discrete nonoverlapping generations. We propose here that temporal ly fluctuating selection can indeed contribute significantly to the ma intenance of genetic variation when the effects of overlapping generat ions and age-specific or stage-specific selection are considered. We d evelop a simple model for a population with overlapping generations, e xperiencing stabilizing selection with a temporally fluctuating optimu m, and subject to repeated invasions by mutants with alternative pheno types. We find that an evolutionarily stable population must have posi tive genetic variance maintained by selection so long as the product ( variance of fluctuations) times (amount of generation overlap) times ( selection intensity) is sufficiently high. This result applies to hapl oid, diploid, single-locus, or multilocus inheritance, and it does not depend on any form of heterozygote advantage to maintain genetic vari ance. However, it depends on the map between genotype and phenotype be ing constrained. If a single genotype can produce an arbitrary distrib ution of phenotypes, then genetic variance is not maintained by select ion.