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.

Cross‐decades stability of an avian hybrid zone

“…This occidentalis X townsendi hybrid zone is often cited as an example of hybrid zone movement (Buggs, 2007;Price, 2007;Chunco, 2014;Curry, 2015;Grether, Peiman, Tobias, & Robinson, 2017) because of multiple lines of inference. First, mitochondrial haplotypes similar to those of occidentalis were found in coastal populations of townsendi far north of the current hybrid zone, a pattern interpreted as a 2000-km genetic wake following an original meeting of the two species much further north and movement of the hybrid zone southwards to its present location (Rohwer et al, 2001;Krosby & Rohwer, 2009).…”

Genetic admixture despite ecological segregation in a North African sparrow hybrid zone (Aves, Passeriformes, Passer domesticus × Passer hispaniolensis)

“…Generally, anthropogenic disturbances might facilitate hybridization between ecologically divergent species that under different (undisturbed) conditions do not regularly interbreed (“anthropogenic hybridization”: See Grabenstein & Taylor, ; McFerlane & Pemberton, ). In all these aspects, the admixed North African sparrow populations are in accordance with the “mosaic hybrid zone model” characterized by a spatial patchwork of secondary contact and possible “escape” of hybrids to new habitats under certain local environmental conditions (Curry, ).…”

“…That color variation does not occur elsewhere across the breeding range of P. hispaniolensis (Cramp & Perrins, 1994), but has been documented already by Rothschild and Hartert (1912) in Algeria.…”

“…Instead, additional factors can be important such as prezygotic reproductive barriers (e.g., behavior, life cycle, gamete compatibility) or environment‐dependent selection regimes acting on parental species and their hybrids. If the complex interplay between these factors varies over time and space, spatial patterns in secondary contact can be very different and range from sharply defined narrow hybrid zones to patchy distributions of parental species and hybrid populations in mosaic hybrid zones (Curry, ; Jiggins & Mallet, ). In fact, there are well‐studied examples where secondary contact between the same species resulted in different patterns of admixture at different localities, such as in marine invertebrates ( Mytilus edulis , Mytilus trossulus : Riginos & Cunningham, ; Stuckas et al, ) or tortoises ( Mauremys : Vamberger et al, 2017).…”

“…Species with large distribution ranges and long evolutionary history may have experienced several secondary contact events in time, for example, via repeated recolonization from glacial refuges (Hewitt, ), and space, for example, via spatially separated hybrid zones across the distribution range (Aliabadian, Roselaar, Nijman, Sluys, & Vences, ). If the outcome of such independent hybridization events is not identical, different patterns of neutral and/or adaptive introgression might be observed in different regions of a species range (Barton & Hewitt, ; Curry, ).…”