Genic Identity and Geographic Differentiation of Trophically Dichotomous Ilyodon (Teleostei: Goodeidae)

Allozyme survey of 15 polymorphic loci in four Ilyodon populations from a tributary of the Río Coahuayana, Jalisco, Mexico, revealed marked interlocality populational differences, but striking within-locality similarities between two trophic morphotypes which once were considered different species. The observed level of geographic variation was unusually high for a continuously distributed population. The differentiation may be the result of genetic drift due to periodic bottleneck effects in small tributaries, but natural selection cannot be ruled out. Regardless of the basis of the geographic differentiation, it is doubtful that two independent gene pools could display genic concordance at all polymorphic loci. Thus, gene flow seems to be maintaining allozymic identity of the two morphs at each locality. Previous suggestions that the morphs are conspecific are substantially supported.