Magic traits: distinguishing the important from the trivial.

Servedio et al. [1], following Gavrilets [2], define a magic trait as ‘a trait subject to divergent selection and a trait contributing to non-random mating that are pleiotropic expressions of the same gene(s)’. This clarified definition is certainly helpful, but we outline here several pivotal questions for empirical research, particularly surrounding the crucial concept of effect size. The effect size of a magic trait, defined by Servedio et al. [1] as ‘how much the trait contributed to the evolution of increased reproductive isolation’, determines whether a magic trait is actually important for speciation (an ‘important magic trait’) or is a ‘trivial magic trait’ (a magic trait of very small or zero effect size). Effect size is therefore what matters empirically, and yet it is absent from the definition of a magic trait, which instead embodies theoretical preoccupations with the genetics underlying traits. We do not propose to redefine ‘magic trait’, but instead hope to illustrate how empirical advances will require an explicit focus on effect size. Problematically, however, the definition of effect size is retrospective and not generally measurable; empirical proxies for effect size that can be used predictively are therefore needed. We here treat the strengths of divergent selection, assortative mating and pleiotropy (the three components of the magic trait definition) as the a priori expected contributors to effect size during speciation. Divergent (including disruptive) selection, the first pillar of the magic trait definition, is certainly important for speciation; however, its magnitude is more important than its mere presence [3]. Moreover, distinguishing weakly divergent selection from the absence of selection is empirically difficult [4], making it hard to determine whether a trait is magic or non-magic. Fortunately, this distinction is probably not of key importance to the process of speciation, because magic traits under such weak selection are probably trivial. The empirical focus should be on magic traits expected to be of large effect size. In addition, spatial and temporal variation in selection [5] makes it difficult to determine whether a trait is generally under divergent selection. For example, beak size in the Medium Ground Finch (Geospiza fortis) has been proposed to be magic [1,6], but selection on beak size is, at various times and places, directional, stabilizing, or divergent [7,8]. Consequently, it is hard to say whether beak size would satisfy the definition; as the selective regime changes, beak size switches frommagic to non-magic and back again. This