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Pulling together or pulling apart: hybridization in theory and practice

Abbott et al. (2013) claimed that, ‘If hybridization is defined as reproduction between members of genetically distinct populations (Barton & Hewitt, 1985), producing offspring of mixed ancestry, then it occurs in almost all proposed processes of speciation’. An anonymous reviewer noted in response to this claim, ‘It really is not hyperbole to state that this statement alone makes this an important contribution ... The conclusions of the authors are thus indicative of a major paradigm shift in evolutionary biology, away from the typological setting of the Neo-Darwinian synthesis’. The Commentaries assembled here suggest that any new paradigm recognising the near ubiquitous occurrence of hybridisation in animal and plant speciation is widely accepted. However, opinions vary much more widely as to its impact and potential role in the process of speciation. Issues emerging from these commentaries include definitions of the nature of hybridisation (and, of course, species) and the likelihood of reinforcing selection and its consequences. However, the main debate concerns the broader significance of natural hybridisation and its potential role in biodiversity. The mainly contrasting views of theoreticians versus empirical biologists are particularly striking, and thought-provoking. In a discussion on this point, Jim Mallet said, ‘The stance of theoreticians to the data on hybridization reminds me of Arthur Eddington’s famous quip, “No experiment should be believed until it has been confirmed by theory.”’ It is hard to imagine that two forms may maintain genetic differences or undergo divergence while in contact because of the two-state rule suggested by Sewall Wright, where FST 1/(1 + 4 Nm). If Nm << 1 divergence can take place. If Nm >> 1 it will not. Nick Barton used to tell us in the 1980s that host races in insects, forms that exchange genes at a reasonable rate yet nevertheless diverge in ecological adaptations, should not exist. It is now clear that they do, although why they do is still a bit of a puzzle. Species of hybrid origin, both polyploid and homoploid, appear to be common in plants whatever the theory might say (see Servedio et al., 2013). Perhaps there is another, equally valid, version of Eddington’s dictum: ‘No good theory should be believed until confirmed by adequate data’. The definition of hybridization adopted by Abbott et al. (2013) focuses attention onto the offspring resulting from mixed matings. When divergent populations are in contact, there are many factors that potentially influence the probability of the formation of hybrid zygotes (Hochkirch, 2013; Rosenthal, 2013; Svensson, 2013) including phenology, mating signals and preferences, gamete competition, gamete recognition and so on. Regardless of whether viable zygotes are produced, these interactions can be costly and so impose selection on the underlying traits. Since this is direct selection, it may be more powerful than indirect selection resulting from reduced hybrid fitness (Shaw & Mendelson, 2013). The analogy with sexual selection is an important one: direct costs and benefits of mate preferences can have much stronger effects than indirect benefits due to offspring fitness (Svensson, 2013). For these very reasons, it is important to distinguish between the two sources of selection (as argued by Shaw & Mendelson, 2013) and, therefore, helpful to define hybridization in terms of offspring production. Offspring production is necessary for gene flow, although it does not guarantee it. So, it is important to distinguish production of hybrid offspring from mating interactions. Nevertheless, mating interactions, including those between reproductively isolated species that do not result in hybrid offspring, are a potent and potentially important source of selection shaping species interactions (Hoskin & Higgie, 2013; Svensson, 2013). Perhaps calling this ‘Reproductive interference’ is the best available term (Hochkirch, 2013) to distinguish it from more typical definitions of hybridization. This distinction is very relevant to the issue of reinforcement. Abbott et al. (2013) said little about the likelihood of the process of reinforcement (it was striking how divergent were the opinions expressed at the workshop), yet the topic has attracted much attention from commentators. Hoskin & Higgie (2013) emphasise the existence of ‘solid empirical examples’ and argue that reinforcement is likely to be more common than these examples suggest because it is so difficult to demonstrate. In contrast, Barton (2013) suggests that reinforcement ‘may be so rare as to make little overall contribution’ to speciation and has been demonstrated mainly in cases with widespread overlap, indicative of strong isolation, rather than in hybrid zones. Shaw & Mendelson (2013) conclude that the ‘jury is still out’. This seems to be the safest position, especially where the question is whether reinforcement makes a common and substantial contribution to speciation and, if Correspondence: Roger Butlin, Department of Animal and Plant Sciences, The University of Sheffield, Sheffield S10 2TN, UK. Tel.: +44 114 2220097; fax:+44 114 2220002; e-mail: r.k.butlin@sheffield.ac.uk