Genetic variation derived from natural gene flow between sympatric species in land snails (Mandarina).

The effects of introgression between two sympatric species on intraspecific genetic variation were studied in the land snail species Mandarina anijimana and M. mandarina on the island of Anijima. Allozyme analysis suggests that extensive introgression between this sympatric pair of species is occurring at several sites. In spite of this extensive gene flow, these species remain morphologically distinct. This suggests that distinctive sympatric forms can persist for long periods despite significant gene flow between them. Because there are no effective geographical barriers in the island, the geographical variation within species in allozyme frequencies shown here are mostly produced by geographical variation in the level of gene exchange between the two species. These breakdowns in reproductive isolation between species mostly occur in populations in the transitional zone between shrub and forest or at sites with poor vegetation. Mandarina anijimana is rare in the transitional zone and is never found in the forest, but alleles introduced from M. anijimana are found at high frequency in the populations of M. mandarina in these areas. This suggests ‘frequency-dependent’ hybridization in which the rare species at a site mates with individuals of the other sympatric species. [ABSTRACT FROM AUTHOR]