Models of symbiosis.

A tentative outline of concepts is proposed for the evolutionary genetics of symbiosis. There are three main topics. The first concerns the tension between the integrative and disruptive forces of kin selection. Kin selection can be disruptive because competition among close relatives favors dispersal and a reduction in relatedness among neighbors. Kin selection acts independently within each species of a symbiotic community but has important consequences for the integration of the community into a cooperative unit. The second topic describes the evolution of beneficial, synergistic effects between species. The evolution of mutual effects depends on various correlations between species. Genetic correlations are analogous to linkage disequilibrium in standard Mendelian genetics. Correlations in reproductive success between symbiotic partners arise from codispersal and reproductive synchrony. The third topic concerns the evolution of asymmetrical symbioses in which one species can dominate its partner. Dominance may explain the evolution of uniparental inheritance among cytoplasmic symbionts and a peculiar form of germ-soma separation in the symbionts of insects.