An Integrated Framework for Hybrid Zone Models

Abstract

Hybridization, where distinguishable populations meet and interbreed resulting in hybrid offspring, is useful for understanding the mechanisms involved in the evolution of reproductive isolation. Hybridization can range from interactions with near-complete reproductive isolation, to stable zones, to complete intergradation. Several models explain how stable hybrid zones are maintained, but each model emphasizes a special case and some zones fit more than one model. The models also do not emphasize moving hybrid zones or occasional hybridization events that add genetic variation to the parental populations. This essay unifies the current models along a continuum of selection pressures organized by geography. Current models differ in assumed hybrid fitness and whether selection is intrinsic or extrinsic. The type of selection is a less general category than the direction of selection (for or against hybrids); multiple types of selection can exist in each zone depending on trait. Thus, the diversity and combinations of selection types make selection direction the most general category for sorting hybrid zones, and variation in selection over geography shapes the spatial extent (in concert with dispersal), movement, and stability of a hybrid zone. This framework is useful for considering zone structure both at the whole organism level and at individual genes or traits. Unifying hybrid zones models by the geographic range of selection pressure organizes each zone into context along the continuum regardless of its fit within one of the classic models, helping us explicitly consider what assumptions are made about the zone and providing direction for further research.