American Society of Naturalists

Jay P. McEntee, J. Gordon Burleigh, and Sonal Singhal (July 2020)

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Why do species’ geographic ranges have limits? When you think of your favorite species, whether it be a wildflower or a songbird, you might also think about where you have to go in the world to see them. Historically, researchers have focused on how geographic limits are affected by dispersal (or how organisms move), abiotic conditions like temperature or precipitation, and biotic conditions like competition. This study explores an alternate factor: hybridization, or mating between different species. This study re-analyzes previously published data from over 130 animal hybridizing pairs in which one species meets another species in a geographically narrow zone of hybridization, or hybrid zone. These hybrid zones correspond with range edges. Based on existing theory, the authors make two predictions for what factors might explain the widths of hybrid zones. First, animals that disperse less should have narrower hybrid zones than animals that move more. Second, the more genetically different two animal species are, the narrower their hybrid zone should be.  Together, these two factors explained ~40% of the variation in the width of hybrid zones, with dispersal being a much bigger factor that genetic difference. This study emphasizes the importance of dispersal in hybrid zones and, more generally, in defining species' limits.

This study’s origins lie in a graduate student reading group of an important text on hybrid zones (John Endler’s Geographic Variation, Speciation, and Clines). Two of the authors were concurrently studying hybrid zones in the field while participating in this reading group; this analysis includes their earlier studies. Their discussions with the third author prompted the comparative approach employed in this study.

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Abstract

Hybrid zones occur as range boundaries for many animal taxa. One model for how hybrid zones form and stabilize is the tension zone model, a version of which predicts that hybrid zone widths are determined by a balance between random dispersal into hybrid zones and selection against hybrids. Here, we examine whether random dispersal and proxies for selection against hybrids (genetic distances between hybridizing pairs) can explain variation in hybrid zone widths across 131 hybridizing pairs of animals. We show that these factors alone can explain ~40% of the variation in zone width among animal hybrid zones, with dispersal explaining far more of the variation than genetic distances. Patterns within clades were idiosyncratic. Genetic distances predicted hybrid zone widths particularly well for reptiles, while this relationship was opposite tension zone predictions in birds. Lastly, the data suggest that dispersal and molecular divergence set lower bounds on hybrid zone widths in animals, indicating there are geographic restrictions on hybrid zone formation. Overall, our analyses reinforce the fundamental importance of dispersal in hybrid zone formation, and more generally in the ecology of range boundaries.