A community genomics approach to natural hybridization

Hybridization and introgression (admixture) play a complicated role in ecology and evolution. While once deemed unnatural and uncommon, admixture is now recognized as remarkably ubiquitous between species. However, few studies have quantified its prevalence at the community level, where hybridization may modulate species interactions, change ecological processes, or alter evolutionary trajectories. We quantified rates of admixture across 75 communities of freshwater fish within the Ozarks (USA) by genotyping 33 species (N=2,865 individuals) across thousands of loci using restriction-site associated DNA sequencing (ddRAD). In contrast to more conventional single species-pair evaluations, our approach is less biased because it does not require a priori assumptions of putative admixture and thus tests a wider breadth of parental species pairs. We found extensive admixture (N=70 individuals; 2.4%) across 18 parental species pairs involving 73% of the species analyzed (N=24). Most admixed individuals were minnows (Leuciscidae; N=15 species; N=66 individuals). Introgression was evident in 24 backcrossed individuals providing evidence for genetic exchange among fish lineages. Our community-level assessment suggested hybridization is even more common than now-contemporary considerations and suggests this approach is more robust for accurately surveying hybridization in nature. By contrast, literature-based per-individual hybridization rates predicted only 2-3 hybrids within our dataset.