Applying hidden-state speciation-and-extinction models to a 469-species salamander phylogeny, Patton and colleagues found that lineages engaged in contemporaneous hybridization have significantly higher net diversification rates, driven by increased speciation and decreased extinction. The authors explicitly left open whether this pattern holds beyond salamanders — other animal groups with homoploid hybridization — and could not quantify the contribution of reinforcement (strengthening of prezygotic isolation driven by selection against hybrids) or test whether ancient rather than contemporary hybridization fueled radiations. Subsequent macroevolutionary studies give a mixed picture: seed plants show only very small hybridization–diversification effects, while lemurs and corals show correlational links between introgression and diversification. Whether hybridization is a general, mechanistically understood driver of animal diversification remains unresolved.

Background

Hybridization — interbreeding between genetically distinct lineages — has long been debated as either a destructive force that erodes species boundaries (the classical view for animals, going back to Mayr) or a generative one that injects new genetic variation and fuels speciation (the classical view for plants). When hybrids backcross, genes move between lineages (introgression); if a hybrid lineage becomes reproductively isolated from both parents without a change in chromosome number, this is called homoploid hybrid speciation, which must succeed despite continued gene flow. One mechanism by which hybridization may promote speciation is reinforcement: selection on populations in geographic contact (sympatry) to strengthen prezygotic reproductive isolation (mate choice or other barriers acting before fertilization) because poorly adapted hybrids waste reproductive effort. Most evidence about hybridization and speciation has come from species pairs or small clades; whether hybridization changes diversification rates — the balance of speciation and extinction — at large taxonomic scale was rarely tested. Patton, Margres, Epstein, Eastman, Harmon and Storfer (Scientific Reports, 2020) scored contemporary hybridization from the literature for the salamander order Caudata (469 species; Plethodontidae, the lungless salamanders, make up roughly two-thirds of the order) and used HiSSE — a Hidden State Speciation & Extinction model, i.e., a phylogenetic birth-death model that estimates speciation and extinction rates for a binary character state while allowing for unmeasured 'hidden' traits that also affect diversification — plus character-independent null models. Hybridizing lineages showed elevated speciation and net diversification rates. The authors flagged six unresolved issues: reinforcement's contribution could not be quantified; the signal was not robustly supported in the non-plethodontid subset (167 species), where such state-dependent models are known to have low statistical power below roughly 300 taxa (Davis, Midford and Maddison 2013 showed BiSSE, the binary-state predecessor, has under 15% power at 100 taxa); extinction-rate estimates were unreliable, some being effectively zero; the design used only contemporary hybridization and so could not test the 'hybrid swarm' hypothesis that ancient hybridization ignited present-day radiations; generality to other homoploid-hybridizing animal groups was untested; and climate-change-mediated hybridization in montane salamanders was proposed as a future direction. Independent methodological critiques (Rabosky and Goldberg 2015) warn that trait-dependent diversification models can mistake model inadequacy for trait effects, making character-independent null models and power analyses essential to any follow-up.

Problem Statement

Determine whether the positive association between hybridization and diversification demonstrated for salamanders is a general macroevolutionary pattern in animals, and disentangle its mechanism. Specifically: (1) for one or more additional animal clades in which homoploid hybridization occurs, test with adequately powered trait-dependent diversification analyses (e.g., hidden-state speciation-and-extinction models with character-independent nulls) on time-calibrated species-level phylogenies, with hybridization states scored from documented contemporary hybridization, whether hybridizing lineages exhibit elevated speciation and/or net diversification rates relative to non-hybridizing lineages, with explicit attention to statistical power at the clade's tree size and to the reliability of extinction-rate estimates; (2) quantify, across taxa, the degree of association between contemporaneous hybridization and reinforcement (strengthening of prezygotic reproductive isolation in sympatry), to measure how much of the hybridization–diversification association reinforcement can explain; and (3) where evidence permits, distinguish contemporary from ancient hybridization (the hybrid swarm hypothesis) as the timescale on which hybridization affects diversification. These deliverables jointly answer one question: is hybridization a general generative force in animal macroevolution, and through what mechanism and timescale?

References

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