Literature Review: Bird Species Boundaries

by Carl Strang

I’ll begin my review of the 2014 scientific literature with my notes on three interesting studies of birds. These focus on two kinds of boundaries between species: range boundaries that sometimes result in hybrid zones where the adjacent species are closely related; and the biological species definition, in which species are separated by their inability to hybridize freely. When hybrids reproduce less effectively than the parent species, the parent species are legitimate.

Black-capped Chickadee

Black-capped Chickadee

Taylor, Scott A., et al. 2014. Climate-mediated movement of an avian hybrid zone. Current Biology, DOI: 10.1016/j.cub.2014.01.069

They studied the northward advance of Carolina chickadees, the southern boundary of the black-capped chickadee range, and the hybrid zone between them. The average northward advance is 0.7 mile per year. The mapped boundary extends across northern Indiana in the third or fourth tier of counties, but westward plunges south so that in Illinois, it angles down roughly from Kankakee County to St. Louis. The hybrid zone in Pennsylvania, where the most intensive part of the study was done, averages 21 miles wide. The narrowness and evanescence of that zone results from hybrids being less successful than either parent species. Female Carolina chickadees are dispersing northward faster than males, and disperse about 0.6 mile per year, so they are the focus of the expansion. The average winter low temperature has advanced at the same speed as the chickadee shift, so by that measure climate likewise is moving north 0.7 mile per year.

Tobias, Joseph A., et al. 2013. Species coexistence and the dynamics of phenotypic evolution in adaptive radiation. Nature, DOI: 10.1038/nature12874

From a ScienceDaily article. They looked at evolutionary relationships in the large family of tropical ovenbirds (over 300 species, unrelated to the North American warbler of that name). They were interested in testing the degree to which competition among closely related species pushes their evolutionary divergence. Estimated ages of divergence revealed that competition did not play a role in speciation, as the species had diverged millions of years before they were brought together in the same places. Though instances are known in which young species diverge through competition, the overall pattern found in these coexisting species did not involve that process. One result that seems contrary to the expected pattern is that coexisting species evolved convergent, rather than divergent, songs. This may allow them to space their territories not only with respect to members of their own species, but also with respect to members of closely related species, which would be their closest competitors. Though the authors seem to want to generalize this to evolution of animals broadly, this seems unjustified at least for now.

Poelstra, J.W., et al. 2014. The genomic landscape underlying phenotypic integrity in the face of gene flow in crows. Science 344:1410-1414.

De Knijff, Peter. 2014. How carrion and hooded crows defeat Linnaeus’s curse. Science 344:1345-1346.

The de Knijff article interprets the Poelstra paper. This study looked at carrion crows and hooded crows, which have a hybrid zone in central Europe and northern Britain between their otherwise separate ranges. Some have argued that they should be regarded as subspecies because of the hybridization, but this study found that a tiny portion of the genome (less than 1%, tied to an inversion of part of a chromosome), related to feather pigmentation and visual perception, is a solid separator of the species genetically. Cases like this challenge bar code species demarcation, which requires at least a 10% genomic difference. The species separation, the authors state, is based not on separate ecology but rather on sexual selection alone.

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