There is usually no ambiguity about species delineation when distant lineages are compared. For instance, there is no doubt that dogs and cats belong to two different species. However, such distinction becomes less clear-cut when comparing recently diverged groups of individuals, between which interbreeding is still to some extent possible. This is the paradox of speciation: a gradual, continuous process that ultimately leads to distinct biological entities. New research published online on December 27, 2016 in the open-access journal PLOS Biology from French biologists Camille Roux, Christelle Fraïsse, Jonathan Romiguier, Yoann Anciaux, Nicolas Galtier and Nicolas Bierne (CNRS - University Montpellier) characterizes the ability of populations to interbreed and exchange genes as a function of the level divergence of their genomes. These authors improved existing methods, allowing them to infer the history of speciation by modeling the confounding effect of natural selection, drift, and migration rates, thereby accounting for the differing patterns of variation seen in different parts of the genome. The PLOS Biology article is titled “Shedding Light on the Grey Zone of Speciation along a Continuum of Genomic Divergence.” The new method was applied to a large genomic dataset consisting of 61 pairs of populations or species of animals. Their analysis uncovered a zone of intermediate molecular divergence, between 0.5% and 2% of differences between genomes, in which the transition from one to two species proceeds - the so-called "gray zone of speciation." Pairs of populations/species falling in this zone are typically characterized by a semi-permeable genome: some genes are freely exchanged between populations, but some are blocked and contribute to isolation - the so-called species barriers.
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