The Evolutionary Dynamics of Direct Phenotypic Overdominance: Emergence Possible, Loss Probable
Abstract
An evolutionary dynamical system with explicit diploid genetics is used to investigate the likelihood of observing phenotypically overdominant heterozygotes vs. heterozygous phenotypes that are intermediate between the homozygotes. In this model, body size evolves in a population ecology with discrete demographic episodes and with competition limiting reproduction. A genotype-phenotype map for body size is used that can generate the two qualitative types of dominance interactions (overdominance vs. intermediate dominance). It is written as a single-locus model with one focal locus and parameters summarizing the effects of alleles at other loci. Two types of ESS (CSS) occur The ESS is either generated (1) by the population ecology, or (2) by a local maximum of the genotype-phenotype map. Overdominant heterozygotes are expected to occur if the population evolves towards the second type of ESS, where nearly maximum body sizes occur. When other loci with partially dominant inheritance also evolve, the location of the maximum in the genotype-phenotype map repeatedly changes. It is unlikely that an evolving population will track these changes; ESS of the second type now are at best quasi-stationary states of the evolutionary dynamics. Considering the restrictions on its probability of occurrence, a pattern of phenotypic overdominance is expected to be rare.