Morphological and Biochemical Evidence of Hybridization between Cave and Barn Swallows

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Abstract
Biochemical evidence of heterozygosity at 3 structural gene loci strongly supports the morphologically based interpretation of 2 juveniles as interspecific hybrids. The findings suggest that there is little variation at structural gene loci in populations of Petrochelidon fulva and Hirundo rustica in central Texas [USA]. Samples of these species show an average heterozygosity (proportion of loci in heterozygous state per individual) of only 0.3% over the 22 loci assayed. Another significant finding is the close genic similarity between P. fulva and H. rustica. On the basis of the observed allelic representation at 22 loci, a coefficient of genic similarity (S, scaled from 0-1) of 0.860 was obtained. Because of the general morphological similarity of swallows, Mayr and Bond (1943) questioned the reality of generic limits in this family and suggested that grounds for separating Petrochelidon from Hirundo were particularly weak. The occurrence of hybridization between Hirundo and Petrochelidon and the biochemical evidence of close genic similarity between H. rustica and P. fulva strongly support this viewpoint.