Hybridization and the origin of species

The phenomenon of hybridization has fascinated scientists for many decades and in various biological contexts (e.g. Darwin, 1862; Haldane, 1922; Dobzhansky, 1937; Mayr, 1946; Anderson, 1949; Hewitt, 1996; Mallet, 2005). Despite the long history of hybridization research, many open questions remain, some of which can only now be tackled due to the rapid improvement of sequencing technology. The significance of hybridization in speciation processes has long been debated, since hybridization might also promote the fusion of taxa (Seehausen, 2004). Furthermore, interspecific hybridization has often been seen as an exception rather than the rule, but it is meanwhile evident that a large number of species hybridize regularly, even though only a small fraction of a population may be involved (Mallet, 2005). The review by Abbott et al. (2013) highlights the significance and the multitude of possible impacts of hybridization on speciation processes. These vary from direct formation of new hybrid taxa to indirect effects, such as reinforcement of pre-mating barriers in hybrid zones. One important message of the review by Abbott et al. (2013) is that a high number of possible outcomes of hybridization exist, depending on the genetic constitution, demography, ecology and spatial distribution of the species involved – all of which are highly variable. The multidimensional framework in which hybridization takes place and the fact that hybridization relationships are usually asymmetric between the involved taxa and sexes (Wirtz, 1999; Groning & Hochkirch, 2008) creates an extraordinary high number of possible interactions and it will remain a challenging task to disentangle the contribution of each single factor to hybridization dynamics. Here, I want to highlight five aspects, which I consider crucial for understanding the evolutionary role of hybridization: (i) the importance to think in continua rather than in categories, (ii) the need to consider also complete reproductive isolation when discussing possible effects of hybridization, (iii) the importance to distinguish selective effects of hybridization from pure recombination, (iv) the relevance to integrate interdisciplinary information in hybridization research and (v) the potential of natural model systems for the study of hybridization and introgression processes.