Dispersal predicts hybrid zone widths across animal diversity: Implications for species borders under incomplete reproductive isolation

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.