Two theories of sex and variation

Abstract

For almost the whole history of evolutionary biology, sex has been thought of as creating preadaptation to an uncertain future, either permitting species to adapt more quickly or enabling individual females to produce a few unexpectedly fit offspring. The demise of this powerful idea dates from theoretical difficulties noted by Maynard Smith (1971 a) and was completed by the overwhelming hostility of the comparative evidence: it is parthenogenesis, and not outcrossed sexuality, that prevails in harsh, uncertain, disturbed and novel conditions (Bell, 1982). The comparative evidence points instead to a quite different role for sex, concerned with the efficient exploitation of the full range of possibilities presented by a diverse environment. Two theories of this sort have been especially prominent. The first is the Tangled Bank (Bell, 1982), which descends from the economic analogies of Ghiselin (1974) and is related to the sib-competition models introduced by Williams (1975) and elaborated by Maynard Smith (1976 a) and Price and Waser (1982). In its simplest form, the Tangled Bank holds that the state of the environment varies widely from place to place on a very local scale, such that different genotypes are optimal at different sites. Since each site can support only a few individuals, the uniform progeny of an asexual female will compete intensely with one another for the same set of resources, while the progeny of a sexual female, which by virtue of their diversity will be able to exploit a much wider range of sites, will compete amongst themselves less intensely and thus achieve greater overall success. In this way, a narrowly specialized asexual clone cannot replace a diverse sexual lineage, despite its greater reproductive efficiency.