Divergent selection and plasticity shape reproductive isolation at the onset of speciation

Reproductive isolation is a key process during speciation, but the factors that shape its evolution during the early stages of speciation remain largely unknown. Using a meta-analysis of 34 experimental speciation studies, we show that populations that experience divergent selection evolved stronger reproductive isolation compared to populations that evolved in similar environments, consistent with ecological speciation theory. However, and contrary to predictions, reproductive isolation did not increase with the number of generations. We present evidence that phenotypic plasticity could play a role in explaining these results, as divergent environments induce an increase in reproductive isolation in the first few generations. Our results highlight that adaptation to different environments in conjunction with plasticity can lead to a rapid increase in reproductive isolation at the beginning of speciation.