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Hybrid zones in fluvial fishes may be heterogeneous from drainage to drainage. The comparison of data from morphology, allozymes, and mitochondrial DNA (mtDNA) indicates variability in the causes and degree of restriction of gene flow between Notropis cornutus and Notropis chrysocephalus. Allozyme marker loci show frequency‐dependent introgression; i.e., the rarer species, whichever it is at a particular locality, tends to exhibit a higher proportion of introgressed alleles. Unlike allozymes, introgression of mtDNA haplotypes varies geographically. In westward‐flowing Michigan drainages, N. cornutus mtDNA haplotypes are more common in F1 hybrids and backcrosses, independent of parental frequencies. In eastward‐flowing Michigan drainages, N. chrysocephalus mtDNA is more common in F1 hybrids and backcrosses; this pattern may be due to local ecological effects or frequency‐dependent introgression. Morphological data alone are not sufficient to distinguish all classes of hybrids. The lack of concordance of morphological, allozymic, and mtDNA introgression patterns implies operation of one or two factors: 1) geographically variable patterns of selection against different hybrid and backcross combinations or 2) genetic differences between Michigan populations inhabiting eastward‐ and westward‐flowing drainage systems accumulated during historical isolation.
Dowling et al. (Mon,) studied this question.