Speciation: Birds of a feather...

Date:  March 26, 2019

Source:  Ludwig-Maximilians-Universität München

Carrion crows and hooded crows are almost indistinguishable genetically, and hybrid offspring are fertile. Ludwig-Maximilians-Universitaet (LMU) in Munich biologists now show that the two forms have remained distinct largely owing to the dominant role of plumage color in mate choice.

Crows have divided Europe between them. Western Europe is the realm of the soot-black carrion crow, while the eastern half of the continent is home to the hooded crow with its grayish black plumage. The boundary between the two populations -- or more precisely, the hybrid zone where the two meet -- is only 20-50 km wide, and in Germany it essentially follows the course of the River Elbe. This is the only stretch of territory in which both of these species are found and sucessfully mate with each other. The plumage of the fertile offspring of these pairings is intermediate in color between those of their parents. The sharp demarcation between the two populations, however, clearly indicates that gene flow across the hybrid zone is restricted, which implies that hybrids are at a selective disadvantage. "Defining speciation as the buildup of reproductive isolation, carrion crows and hooded crows are in the process of speciation," says LMU evolutionary biologist Jochen Wolf. He and his research team have now analyzed the genetic basis for the division of European crows into two populations. Indeed, the results of the study demonstrate that the old saying "birds of a feather flock together" really does apply in this instance: The only genes that differ significantly between the two variants are those involved in determining the color of the plumage. This suggests that each form preferentially mates with partners of the same color as themselves. The new findings appear in the journal Nature Ecology and Evolution.

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