EVIDENCE FOR SELECTION AGAINST HYBRIDS IN THE FAMILY CYPRINIDAE (GENUS NOTROPIS)

Notropis cornutus and N. chrysocephalus are two cyprinid fishes which hybridize extensively in the midwest. In this paper, statistical analysis was used to determine if the deficiency of Notropis cornutus × N. chrysocephalus hybrids can be attributed partly to selection against hybrids. Principal component analysis was used to construct a folded hybrid index (FHI) using morphological and electrophoretic characters which distinguish N. cornutus, N. chrysocephalus, and their hybrids. A Wilcoxon signed-rank test comparing mean FHI between successive age classes within cohorts and regression analysis of FHI on age supported the conclusion that hybrids are less fit than parentals in nature; however, this analysis did not include a test of selection at zygotic and larval life history stages or partial infertility of hybrids. Selection eliminated hybrids at a rate of 9.2% per year. Considering the intensity of selection and the age of hybridization, it is surprising that premating reproductive isolating mechanisms have not been perfected by selection against gametic wastage as predicted by classical speciation theory. It appears that the reinforcement of premating isolating mechanisms is an extremely slow process, if it occurs at all, despite apparently intense selection against hybrids.