Why do asexual and self-fertilising populations tend to occur in marginal environments?

Joel R. Peck, Jonathan M. Yearsley, and David Waxman

Mathematical and computational biology. Computational morphogenesis, hierarchical complexity, and digital evolution. An international workshop, University of Aizu, Aizu-Wakamatsu City, Japan, October 21--25, 1997. AMS Lect. Math. Life Sci. 26, 121-131 (1999). Editor C. L. Nehaniv

Centre of the Study of Evolution, School of Biological Sciences, University of Sussex, Brighton BN1 9QG, Sussex UK

We present a mathematical model of a heterogeneous environment in which a sexual population and an asexual population compete. We assume that resources are most available at the centre of the environment, so that the central area can support a higher population density than can be supported on the margins. We find that, if asexuals have an intrinsic fertility advantage, then they tend to take over the entire environment, and the sexual population goes extinct. If sexuals have an intrinsic fertility advantage, then, at equilibrium, they are usually present in the environment, but the asexuals are also often present. When both sexual and asexual populations are present at equilibrium, the sexuals usually have a more narrow range than the asexuals, and the sexuals also tend to occur closer to the resource-rich centre of the environment. Similar patterns are found in many natural environments.