Timing and Fitness Consequences of Introgression in Speciation
Ryo Yamaguchi · 2025
During adaptation, gene flow can either accelerate or impede species divergence, sparking intense debate about how introgression shapes speciation. Although introgression is widely recognized for introducing genetic novelty, the precise conditions under which it promotes or suppresses reproductive isolation remain unclear, particularly when environmental differences and population dynamics vary. Here, using Fisher's geometric model, we examine how the timing and magnitude of migration influence adaptation and reproductive barriers in both mutation-order and ecological speciation scenarios. We show that early-phase or constant gene flow can facilitate evolutionary rescue for populations adapting to a shared optimum, whereas late-phase migration often delays final adaptation due to the introgression of large-effect mutations in initially isolated populations. In mutation-order speciation, parallel adaptation often reduces hybrid breakdown, whereas, in ecological speciation, alleles beneficial in one environment can be maladaptive in the other. Under both scenarios, early or continuous gene flow promotes allele sharing that eases adaptation but undermines reproductive isolation. We also find that intermediate levels of parental divergence can maximize the success of hybrid lineages, offering insights into when homoploid hybrid speciation may arise. Overall, this study clarifies how gene flow timing modulates the balance between adaptive introgression and isolating barriers. By integrating demography with fitness landscapes, we provide a theoretical framework to interpret genomic patterns of introgression and understand when it accelerates or hinders speciation.