Extensive introgression at late stages of species formation: Insights from grasshopper hybrid zones

The process of species formation is characterised by the accumulation of multiple reproductive barriers. The evolution of hybrid male sterility, or Haldane’s rule, typically characterises later stages of species formation, when total reproductive isolation is strongest. Yet, understanding how quickly reproductive barriers evolve and their consequences for maintaining genetic boundaries between emerging species remains a challenging task because it requires studying taxa that hybridise in nature. Here, we address these questions using the meadow grasshopper Pseudochorthippus parallelus, where populations that show multiple reproductive barriers, including hybrid male sterility, hybridise in two natural hybrid zones. Using mitochondrial data, we infer that such populations have diverged some 100,000 years ago, at the beginning of the last glacial cycle in Europe. Nuclear data shows that contractions at multiple glacial refugia, and post-glacial expansions have facilitated genetic differen...