Geographic variation in the structure of oak hybrid zones provides insights into the dynamics of speciation

Studying geographic variation in the rate of hybridization between closely related species could provide a useful window to understand the evolution of reproductive isolation. Reinforcement theory predicts greater prezygotic isolation in areas of prolonged contact between recently diverged species than in areas of recent contact, which implies that old contact zones would be dominated by parental phenotypes with few hybrids (bimodal hybrid zones) whereas recent contact zones would be characterized by hybrid swarms (unimodal hybrid zones). Here we investigate how the hybrid zones of two closely related Chinese oaks, Quercus mongolica and Q. liaotungensis, are structured geographically using both nuclear and chloroplast markers. We found that populations of Q. liaotungensis located around the Changbai Mountains in Northeast China, an inferred glacial refugium, are introgressed by genes from Q. mongolica, suggesting former contact between the two species in this region. Yet, these introgre...