Coevolution, local adaptation and ecological speciation

Coevolution is one of the major processes organizing the earth's biodiversity, but it remains unclear when and how it may generate species diversity. The study by Parchman et al. (2016) in this issue of Molecular Ecology provides the clearest evidence to date that divergent local adaptation in a coevolving interaction may lead to speciation on one side of an interaction but not necessarily on the other side. Red crossbills in North America have diversified into ecotypes that specialize on different conifer species, use different calls and vary in the extent to which they are nomadic or sedentary. This new study evaluated genomic divergence among nine crossbill ecotypes. The authors found low overall genomic divergence among many of the ecotypes, but the sedentary South Hills crossbills, which are specialized to eat the seeds of a unique population of lodgepole pines, showed substantial divergence from other crossbills at a small number of genomic regions. These results corroborate past studies showing local coadaptation of the morphological traits of South Hills crossbills and lodgepole pines, and premating isolation of the South Hills crossbills from other populations. Together, the past and new results suggest that local coevolution with lodgepole pines has led to reduced gene flow between South Hills crossbills and other crossbills.