TOPOGRAPHIC DIVERSITY, ZONE WIDTH, AND THE STRENGTH OF REPRODUCTIVE ISOLATION IN A ZONE OF OVERLAP AND HYBRIDIZATION

Two closely related species of ground crickets, Allonemobius fasciatus and A. socius, overlap and hybridize in a contact zone in the eastern United States. In earlier work, Howard (1986) described geographic variation in the width of the zone and in the strength of reproductive isolation between the two ground crickets. The zone was wider in the hills and mountains of southeastern Ohio and West Virginia than along the eastern coastal plain, and reproductive isolation appeared to be stronger where the zone was wider. Howard attributed the greater width in the mountains to the wide intermingling and patchy distribution of habitats appropriate for a species adapted to a northern climate and for a species adapted to a southern climate. He also pointed out that the mosaicism and the increased breadth of the zone in the mountains enhanced the probability of occurrence of reinforcement. We tested three predictions that emerged from Howard's hypothesized links among topographic diversity, zone width, and the strength of reproductive isolation. The first two predictions were fulfilled. The northern cricket, A. fasciatus, occurred in the high mountains south of its previously known distributional limit; and the zone narrowed considerably in Illinois, an area of low topographic diversity. These results provide further evidence for the importance of the environment in determining the structure of the zone. The third prediction was falsified. Contrary to the prediction, the strength of reproductive isolation between the two species was as strong in Illinois as in the Appalachian Mountains. This result suggests that if reinforcement has occurred in the zone, the width of the zone has not been a major factor in the process.