Gene Frequency Comparisons between Sunfish (Centrarchidae) Populations at Various Stages of Evolutionary Divergence

Abstract

A total of 11–15 genetic loci encoding enzymes and other proteins has been assayed in populations belonging to 20 taxa of the North American sunfish in the family Centrarchidae. Four stages of evolutionary divergence are recognized among these taxa by systematists. We have determined mean levels of genetic differentiation, D, between populations at each of these stages: geographic populations, D = 0.024 ± 0.003; subspecies, D = 0.171 ± 0.004; species, D = 0.626 ± 0.028; genera, D = 1.340 ± 0.064. D estimates the number of electrophoretically detectable codon substitutions per locus accumulated since populations separated from a common ancestor. Mean genetic distance among centrarchids increases dramatically through the various stages of evolutionary divergence. Different subspecies of <it>Lepomis macrochirus</it> exhibit about an 8-fold increase in genetic distance over geographic populations belonging to a subspecies. Species of the genus Lepomis are completely distinct in allelic composition at nearly 50 percent of loci, on the average, despite their ability to produce viable and sometimes fertile hybrids. Species belonging to different centrarchid genera have accumulated more than one allelic substitution per locus, on the average. Among the centrarchids prezygotic reproductive isolating barriers may have preceded the development of significant postzygotic isolating mechanisms. Nonetheless, mean levels of genetic divergence between centrarchid taxa are similar to those other animal taxa of corresponding rank characterized by different modes of reproductive isolation. Amounts and patterns of genetic divergence are employed to infer probable evolutionary relationships among the Centrarchidae. A great deal of evolutionary information is contained within even a relatively small number of genes.