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The intersection of specialization and speciation

The title of his most famous book notwithstanding, Darwin dealt little with how new species come into being (Mayr, 1942, p. 147). His work is a marvel of brilliant induction and insightful examples of adaptation and anagensis, but cladogenesis was largely ignored. It was into this void that Ernst Mayr stepped. He provided the first lucid explanation of how speciation occurred, an explanation grounded chiefly in geography. The sweeping success of biology’s Modern Synthesis assured that the explanation proffered by Mayr (1942), a chief architect of that synthesis, became entrenched for years to come. Indeed, it is a rare biogeographer who does not consider allopatry to be a prerequisite for speciation. Recent decades have, however, seen a slow and quiet revolution, as more and more researchers studying the phenomenon of speciation have embraced ecological rather than geographical explanations for how new species come into being. A key feature of this ‘ecological speciation’ revolution is the full invocation of Darwinian natural selection as a means of generating biodiversity and, specifically, reproductive isolation (Rundle & Nosil, 2005). Any classical view of ecological factors, from environment to predator avoidance to competition, falls under this rubric. Despite the recency of this revolution (some would say ‘revival’), over the years various mechanisms by which microevolutionary forces could result in speciation have been formulated. A chief example of such a postulated mechanism is character displacement. When Brown & Wilson (1956) first formalized the concept, it swept through ecological circles like wildfire as it gained near-universal acceptance. Its logic – that competition shaped the extent of morphological, ecological, behavioural, and physiological diversity – seemed beyond reproach. After all, embryonic forms of the argument dated back at least to Darwin (Schluter 2000). Together with Hutchinson’s (1959) concept of limiting similarity, a bedrock ecological principle seemed to be born: interspecific competition for resources leads to a restriction on the number of ways that resource can be acquired, and, consequently, phenotypes (in the broadest Dawkinsian sense) diverge to an extent that no two are similar enough to allow further competition. The concept soon had its critics, however, with much of the debate focusing on two key points: (1) the provision of alternative explanations for observed patterns, phenotypic plasticity and species sorting, and (2) the lack of clear quantitative results as opposed to the existence of merely qualitative comparisons. These problems were exacerbated by selective publication of positive results and because, in studies of character release or displacement, ‘choice of morphological characters determines pattern’ (Dayan & Simberloff, 1998). Nonetheless, over time criticism was quelled enough that the notion of character displacement regained its former prominence (Dayan & Simberloff, 1998; Schluter, 2000). Indeed, the detection of character displacement has now achieved a rigorous standard by which six criteria need to be met before the claim can be made confidently (Schluter, 2000): 1. the observed pattern cannot be explained by chance, 2. phenotypic differences between populations under study must have a genetic basis, 3. phenotypic differences must be the result of microevolutionary change (not species sorting), 4. character shifts must reflect acquisition of a primary resource, 5. environments between sites of sympatry and allopatry should be similar, and 6. competition among extant phenotypes must be demonstrated. Two recent publications in the Journal of Biogeography meet, in part, these rigorous standards, and extend our thinking on this subject. Mammalogists have contributed considerably to the literature on character displacement, but most of this work has focused on the order Carnivora and on morphometric variation, especially cranial or dental characters (Dayan & Simberloff, 1998). By contrast, Russo et al. (2007) not only explored patterns in the speciose order Chiroptera, but also focused on divergence in echolocation. They found that call frequency among three species of horseshoe bats (Rhinolophus spp.) shifted upwards or downwards when the species occurred in sympatry, the shifts being unrelated to body size. Brown & Wilson (1956) contended from the start that the displaced characters could be behavioural, so work in this area is welcome. Moreover, it emphasizes the role that acoustic and other signals may play in generating biodiversity (see Patten et al., 2004). A study by Meiri et al. (2007) used a more ‘standard’ data set: canine and skull variation among weasels (Carnivora: Mustelinae) of the Holarctic. The interesting wrinkle is not so much their solid evidence for character displacement as their equally solid lack of evidence for character release. When a species was freed from interspecific competition, it did not revert to some intermediate morphology but instead retained the prevailing characters of its species in sympatric settings. What should be made of this finding? Is it merely a consequence of similar ecological conditions? A solution may lie in the phylogenetic constraints resulting from evolution towards ecological specialization. Character displacement is, in effect, a type of specialization: a species narrows its niche breadth to avoid competition in a zone of contact with a congener. Just what a species loses or gains by becoming specialized has been a subject of considerable theory and disagreement. Nevertheless, there is a reasonable consensus that, whatever a specialist gains in efficiency of resource use, minimization of competition, or fixation of beneficial alleles (see Whitlock 1996), it loses in future evolutionary potential, meaning a potentially irreversible loss in niche breadth. Or, Journal of Biogeography (J. Biogeogr.) (2008) 35, 193–194