Abstract

Hybridization is accepted as a common and important evolutionary force in plants and animals. It can occur randomly or in spatially and temporarily dynamic hybrid zones where the geographic ranges of closely related species meet. Here, I propose that poleward increases in the seasonality of day length promote the maintenance of circumpolar hybrid zones at higher latitudes. Hybridization at higher latitudes should be most prevalent among species that use seasonal changes in day length to coordinate their reproductive phenology but may apply to other species as well if their fitness is dependent on species that are adapted to photoperiodic cues. Given that climate change is expanding species ranges across latitudes and creating opportunities for reproductive interactions between formerly isolated and genetically distinct populations and species, a better understanding of the evolutionary role of hybridization and its consequences for biodiversity, ecosystem functions and services is needed. At the global scale, I predict that day length-promoted circumpolar hybrid zones and consequent hybrid speciation are play significant roles in maintaining genetic admixture and species diversity on a geological time scale because species' range shifts across latitudes during the cycles of Earth's cooler and warmer periods cause recurrent isolation of and contact among species.