Clines in the genetic distance between two species of island land snails: how ‘molecular leakage’ can mislead us about speciation

Abstract

Two species of land snails,Partula taeniataandPartula suturalis, occur sympatrically on the island of Moorea in French Polynesia. The genetic distance between them varies clinally from north to south. Their extreme difference in the south is attributed to an invasion from the neighbouring island of Tahiti. Their genetic closeness in the north, despite large morphological and ecological differences, is attributed to ‘ molecular leakage’, convergence of the neutral and advantageous genes in the two species through occasional hybridization. Rates of hybridization as low as 1 in 100000 can render two species nearly homogeneous in their gene frequencies over periods of time that are short on an evolutionary scale, and therefore can completely mislead us about the phylogenetic history of the taxa concerned. In such circumstances the only valid phylogenetic information may be contained in genes that are kept distinct by natural selection.