Genomics of hybridization and its evolutionary consequences

Hybridization and its consequences have been of longstanding interest to evolutionary biologists. Darwin (1859) included a chapter on hybrids and the expression and causes of hybrid sterility in The Origin of Species, while the main proponents of the neo-Darwinian synthesis discussed the topic at varying length in the mid1900s (Haldane 1932; Dobzhansky 1937; Stebbins 1950; Mayr 1963). Subsequently, Grant (1981) underlined the importance of hybridization to plant evolution, devoting six chapters of Plant Speciation to the topic, while Arnold (1997, 2006, 2015) has written three books on hybridization and its evolutionary consequences over the last 20 years. Building upon this enduring foundation, there has been an enormous expansion in the study of hybridization over the past decade. This exciting resurgence has been fuelled by advances in DNA sequencing that now enable the rapid collection of genomic data across the tree of life and the ongoing development of population genetic theory and analytical tools that facilitate the analysis and interpretation of these powerful data. Interest in hybridization stems from its importance in understanding the origin and maintenance of breeding barriers between diverging populations, the introduction of adaptive variation to species through introgression and as a mechanism for the origin of new species through either homoploid hybrid speciation or allopolyploidy (Abbott et al. 2013). More broadly, the study of natural hybridization can help identify selected genes, and can yield estimates of selection and gene flow that would be impossible to measure directly. Insight into these fundamental evolutionary questions ultimately depends on and has been paced by the generation of genetic data in natural populations. Since the 1960s, molecular markers have been used to examine these diverse aspects of hybridization with the most recent approaches involving analyses of single nucleotide polymorphisms (SNPs) through comparison of genome sequences. By means of genomic analyses, signatures of hybridization can be detected at a fine scale, shedding light on the reticulate evolutionary history of organisms, rates of current and historical gene flow between diverging lineages and levels of selection that prevent hybridizing species from merging. Furthermore, in combination with functional gene analyses, genomic comparisons can identify the genes and gene combinations involved in maintaining hybridizing species as discrete entities, determine which genes and gene combinations have been introgressed into species and which of these are of adaptive significance. In this special issue on the Genomics of Hybridization, a collection of studies is presented which touches on these diverse, but interrelated aspects of hybridization. The issue leads off with a review by Payseur and Rieseberg of the genomic methods used to detect and characterize hybridization between diverging lineages, and is followed by studies that address the detection and adaptive significance of introgression, hybrid zones, isolating barriers, hybrid speciation and the stabilization of hybrid genomes and the challenges that the reality of hybridization presents to the conservation of biological diversity. Correspondence: Richard J. Abbott, Fax: +44 1334 463366; E-mail: rja@st-and.ac. uk