The two faces of secondary contact on islands: Introgressive hybridization between endemics and reproductive interference between endemics and introduced species

AimHybridization is thought to have played an important role in shaping the evolutionary history of diverse island taxa. Here, we propose an ecological and evolutionary framework for understanding the causes and consequences of heterospecific mating on islands-with and without introgressive hybridization. We use this framework to support our main contention that cases of secondary contact among endemic species should commonly result in introgressive hybridization whereas cases of contact between endemic and introduced species should commonly result in reproductive interference (RIN)-resulting in two qualitatively different faces of secondary contact on islands.LocationCanary Islands, Galapagos, New Zealand, Caribbean and Hawaii.Taxa705 vertebrate, invertebrate and plant species spanning 167 genera and 99 families.MethodsUsing a quantitative analysis of empirical research on secondary contact on islands, we weigh evidence for the causes and consequences of secondary contact and heterospecific mating on islands. In particular, we compare cases of secondary contact between endemic species versus secondary contact between endemic and introduced species.ResultsWe find that the drivers of secondary contact and heterospecific mating on islands most frequently reported in the literature are disturbance, long-distance (e.g. inter-island) dispersal and compromised assortative mating. We find support for the hypothesis that extensive introgression is a more common outcome between endemic species while RIN is a more common outcome between endemic and introduced species.Main ConclusionsWe conclude that there are biological reasons to predict secondary contact and heterospecific mating to be common on islands for all taxa, but that the consequence of secondary contact is categorically different for contact between endemic species and contact between endemic and introduced species. We conclude that the former likely explains the apparent frequency of hybridization on islands, while the latter presents a cryptic and underappreciated conservation threat.