Exchange of genes between species through interbreeding is a potentially important force in evolution. Introgressive hybridization can facilitate evolutionary divergence and speciation by adding new genetic variation to populations. Until recently it has been thought to be rare, largely restricted to plants, and of little evolutionary significance. This view is changing. Even so its frequency may still be underestimated. Hybridization is most likely to occur in recently evolved species, yet these are the species in which it is most difficult to detect owing to strong genetic similarity deriving from their recently shared ancestry. In the July 2005 issue of The American Naturalist, Peter Grant and colleagues propose a method that teases out an effect on their genetic similarity due to hybridization, based on the fact that sympatric populations can exchange genes whereas allopatric populations cannot. Using microsatellite DNA data from Darwin's finches in the Galápagos archipelago, they compare sympatric and allopatric genetic distances in pairs of species and find a striking tendency for a species to be more similar to a sympatric relative than to allopatric populations of that relative. An important implication of this finding is that introgressive hybridization has been a persistent feature of the adaptive radiation throughout most of its relatively short history.
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Peter R. Grant, B. Rosemary Grant, and K. Petren, "Hybridization in the recent past"165:7 July 2005.
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