A response to Justen et al. 2020: Estimating hybridization rates in the wild: Easier said than done?

We consider four key challenges related to estimating per-individual rates of hybridization in wild birds: (1) what is the meaning of the term "hybrid"?, (2) the importance of distinguishing between shared DNA sequences and on-going hybridization between populations, (3) the perils of focusing exclusively on known hybrid zones, and (4) the implications of very low rates of per individual hybridization. Because our focus is on using phenotype to recognize hybrids, we define a hybrid as an individual with a phenotype that is intermediate between two parental species. We emphasize the value of quantifying the rate of between-species mating that is ongoing in current populations and distinguish this endeavor from estimates of gene flow between populations based on genomic analysis, which can reflect both current and ancient hybridization. We restate the importance of quantifying per individual rates of hybridization among all birds without prejudging which birds are presumed to engage in hybridization. And finally, we emphasize that evidence for strong prezygotic sorting is not necessarily evidence that mate choice is a driver of speciation.