The selective significance of partial apomixis

Abstract

A one-locus model for the evolution of partial apomixis and the sex ratio in hermaphrodites is investigated. This model is used to find the conditions under which increased apomixis is advantageous, and to show how these conditions depend on the average sex ratio of the population. Two conceptually distinct advantages for apomictic reproduction can be discriminated: An advantage through increased rate of growth, and an advantage through non-reciprocal fertilization. A mutant that trades sperm and egg production for apomixis without changing the sperm/egg ratio will increase according to the first advantage, while a mutant that increases apomixis by decreasing egg production alone will increase according to the second advantage.It is possible to find a frequency-dependent property of phenotypes in this model that acts much like phenotypic fitness. However, the model has some qualities that are quite different from standard one-locus selection. The phenotype of the rare homozygote can influence whether a mutation can invade a population, and Hardy-Weinberg proportions are often violated. Unlike the evolution of the sex ratio in purely sexual species, there is no quantity that is maximized by evolution in this model. © 1982 The Genetical Society of Great Britain.