EXPLAINING THE GEOGRAPHIC DISTRIBUTIONS OF SEXUAL AND ASEXUAL POPULATION

Examination of the geographic distributions of sexual organisms and th eir asexual, or parthenogenetic, competitors reveals certain consisten t patterns. These patterns are called geographic parthenogenesis(1-8). For example, if we compare sexual organisms with closely related asex uals, we find that, in the Northern Hemisphere, there is a strong tend ency for the asexuals to occur further to the north. One researcher to document this pattern is Bierzychudek, who examined 43 cases (drawn f rom 10 genera) where the geographic distributions of a sexual plant an d a closely related asexual are known(4). In 76% of these cases, the a sexual plant's range was more northerly than the range of the sexual. Some of the remaining cases probably fit with this pattern, but more d ata must be obtained before this suggestion can be confirmed Asexuals also tend to occur at high altitudes, and in marginal, resource-poor e nvironments(1-8). We have constructed a mathematical model of a habita t that stretches from south to north in the Northern Hemisphere. Our c omputer 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 fro m areas in which they are well adapted to areas where they are poorly adapted.