Insights from genomes into the evolutionary importance and prevalence of hybridization in nature

Hybridization is an evolutionary phenomenon that has fascinated biologists for centuries. Prior to the advent of whole-genome sequencing, it was clear that hybridization had played a role in the evolutionary history of many extant taxa, particularly plants. The extent to which hybridization has contributed to the evolution of Earth's biodiversity has, however, been the topic of much debate. Analyses of whole genomes are providing further insight into this evolutionary problem. Recent studies have documented ancient hybridization in a diverse array of taxa including mammals, birds, fish, fungi, and insects. Evidence for adaptive introgression is being documented in an increasing number of systems, though demonstrating the adaptive function of introgressed genomic regions remains difficult. And finally, several new homoploid hybrid speciation events have been reported. Here we review the current state of the field and specifically evaluate the additional insights gained from having access to whole-genome data and the challenges that remain with respect to understanding the evolutionary relevance and frequency of ancient hybridization, adaptive introgression, and hybrid speciation in nature.