Ecological Versions of Fisher's Fundamental Theorem of Natural Selection

Abstract

In this paper we derive versions of Fisher's Fundamental Theorem of natural selection for the case of a single density—regulated population, and for the case of 2 species engaged in standard ecological interactions. Selection is assumed to be weak and the selective values of genotypes to depend only on the total densities of species in the ecosystems, not on the densities of specific genotypes. Our results enable us to draw some general conclusions about the way in which natural selection, of the type we consider, affects the densities of species in an ecosystem. Evolution within a single species tends to increase the density of individuals in the age—group to which density—dependent factors respond. For a pair of competing species, we conclude that often, but not invariably, the densities of both species will be increased by selection. In an exploiter—victim system, the exploiter tends to increase in density whereas the victim will increase only if the exploiter is strongly self—regulating. It will otherwise remain stationary or even decrease.