Genic Heterozygosity and Rate of Speciation

-The hypothesis is proposed that mean level of heterozygosity is functionally related to rate of speciation in evolutionary phylads. Under this hypothesis, phylads which speciate more rapidly do so because of increased level of within-species genetic variability which is then available to conversion to species differences under appropriate ecological or environmental conditions. An important corollary is that rate of speciation could be limited in phylads with low genetic variability, irrespective of environmental considerations. This hypothesis has been tested with respect to electrophoretically detectable variation in products of structural genes in two families of North American fishes characterized by grossly different rates of speciation. Totals of 69 species of the highly speciose Cyprinidae, and 19 species of the relatively depauperate Centrarchidae, were assayed for mean level of heterozygosity at 11-24 genetic loci. Since Cyprinidae and Centrarchidae xhibit on the average nearly identical levels of genic variation (H = 0.052 ? 0.004, and H = 0.049 + 0.009, respectively), the hypothesis that level of heterozygosity affects rate of speciation in these fishes is not supported. Nonetheless, the amount of genic variability in both Cyprinidae and Centrarchidae is large, comparable to mean levels in previously studied vertebrates. The great wealth of genome variability, reflected in the electrophoretic variation present in virtually all outcrossing organisms, apparently can accommodate considerable flexibility in rate and pattern of evolutionary response to the various environmental regimes challenging organisms. John C. Avise. Department of Zoology, University of Georgia; Athens, Georgia 30602 Accepted: June 15, 1977