SIGNIFICANT ROLE FOR HISTORICAL EFFECTS IN THE EVOLUTION OF REPRODUCTIVE ISOLATION - EVIDENCE FROM PATTERNS OF INTROGRESSION BETWEEN THE CYPRINID FISHERS, LUXILUS-CORNUTUS AND LUXILUS-CHRYSOCEPHALUS

Samples of Luxilus cornutus, Luxilus chrysocephalus, and their hybrids were collected along hypothesized routes of dispersal from Pleistocen e refugia to examine the significance of geographic variation in patte rns of introgression between these species. Patterns of allozyme and m itochondrial DNA (mtDNA) variation were generally consistent with thos e from previous studies, Tests of Hardy-Weinberg equilibrium revealed significant deficiencies of heterozygotes in all samples, indicating s ome form of reproductive isolation. Mitochondrial DNAs of each species were not equally represented in F-1 hybrids; however, this bias was e liminated when the two largest samples were excluded from the analysis . Backcross hybrids exhibited biased mtDNA introgression, as samples f rom Lake Erie (eastern) and Lake Michigan (western) drainages showed s ignificant excesses of mtDNAs from L, chrysocephalus and L. cornutus, respectively, relative to frequencies of diagnostic allozyme markers. The extent and direction of allozyme and mtDNA introgression was quant ified by calculating isolation index values from morphologically ''pur e'' individuals of each species from each locality. Analysis of varian ce of these measures identified limited introgression of allozyme vari ants with no geographic pattern, but significant differences in direct ion of mtDNA introgression between drainages (i.e., postglacial disper sal route). Association between patterns of mtDNA introgression and di spersal route across the latitudinal width of the contact zone is best explained by genetic divergence during past isolation of ancestral po pulations from these drainages. These results identify a significant r ole for historical effects in the evolution of reproductive isolation and the process of speciation.