Gene flow stops sooner in plants than in animals

Ever since Ernst Mayr (1) formalized species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” in 1942, zoologists have considered hybridization (creation of offspring by mating between two different species) to be rare and ultimately unproductive. Botanists were more strongly influenced by ideas such as those of Göte Turesson (2), who asserted that “the species problem is thus seen to be in a large measure an ecological problem” and believed that the exchange of genes between ecologically differentiated species through hybridization (and breeding of the hybrid and its descendants with members of one of the species) was both common and important. However, empirical evidence for either view was lacking. On page 1147 of this issue, Monnet et al. (3) report population genomic evidence that contrary to standard expectations, this exchange of genes (gene flow) between diverged plant lineages ceases at lower levels of genomic divergence than it does between animal lineages.