Hybridization: its varied forms and consequences

Abstract

[Extract] Hybrid zones have long been recognized as 'windows on the evolutionary process' (Harrison, 1990) and their value for teasing apart core evolutionary processes has only increased with genetic advances. The review of Abbott et al. (2013) covers a lot of interesting ground in regard to the role of hybridization in speciation. The review is particularly valuable in its coverage of recent advances in our understanding of the genetics of reproductive isolation and the genetic mechanisms underlying the production of novel phenotypes in hybrids, which has not received recent synthesis. Abbott et al. (2013) also highlight that cline theory can provide an important framework for the current debate about how 'islands of genomic divergence' operate. The coupling coefficient – a measure of how selection and recombination trade-off over many loci – may be especially useful as empirical studies move away from detecting divergence of single loci of large effect, and towards the complex analysis of multiple loci of small effect. That newly evolved clines in adaptive (ecological) variation can become spatially coupled with pre-existing clines in intrinsic incompatibilities may explain how seemingly recent adaptive differentiation among populations dates to much older divergence times. An alternative explanation is that standing genetic variation in an ancestral environment becomes adaptive in a new environment, resulting in divergent selection among populations and deep genetic differentiation at these loci (e.g. Colosimo et al., 2005; reviewed in Barrett & Schluter, 2007). Distinguishing between these two quite different mechanisms will be important for understanding how rapid speciation events occur.