The evolutionary dynamics of direct phenotypic overdominance: Emergence possible, loss probable

An evolutionary dynamical system with explicit diploid genetics is used to investigate the likelihood of observing phenotypically overdominant heteroz ygotes versus heterozygous phenotypes that are intermediate between the hom ozygotes. In this model, body size evolves in a population with discrete de mographic episodes and with competition limiting reproduction. A genotype-p henotype map for body size is used that can generate the two qualitative ty pes of dominance interactions (overdominance versus 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 evolutionar ily stable strategy (ESS; continuously stable strategy, CSS) occur. The ESS is generated either (1) by the population ecology; or (2) by a local maxim um of the genotype-phenotype map. Overdominant heterozygotes are expected t o arise if the population evolves toward the second type of ESS, where near ly maximum body sizes are found. When other loci with partially dominant in heritance also evolve, the location of the maximum in the genotype-phenotyp e map repeatedly changes. It is unlikely that an evolving population will t rack these changes; ESSs of the second type now are at best quasi-stationar y states of the evolutionary dynamics. Considering the restrictions on its probability, a pattern of phenotypic overdominance is expected to be rare.