Explaining the geographic distributions of sexual and asexual populations

Examination of the geographic distributions of sexual organisms and their asexual, or parthenogenetic, competitors reveals certain consistent patterns. These patterns are called geographic parthenogenes is. For example, if we compare sexual organisms with closely related asexuals, we find that, in the Northern Hemisphere, there is a strong tendency for the asexuals to occur further to the north. One researcher to document this pattern is Bierzychudek, who examined 43 cases (drawn from 10 genera) where the geographic distributions of a sexual plant and a closely related asexual are known. In 76% of these cases, the asexual plant's range was more northerly than the range of the sexual. Some of the remaining cases probably fit with this pattern, but more data must be obtained before this suggestion can be confirmed. Asexuals also tend to occur at high altitudes, and in marginal, resource-poor environments. We have constructed a mathematical model of a habitat that stretches from south to north in the Northern Hemisphere. Our computer simulations based on this model support the idea that a single basic process may account for much of what is known about geographic parthenogenesis. This process involves the movement of individuals from areas in which they are well adapted to areas where they are poorly adapted.