Speciation in Little: the Role of Range and Body Size in the Diversification of Malagasy Mantellid Frogs Wollenberg Et Al
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Speciation in little: the role of range and body size in the diversification of Malagasy mantellid frogs Wollenberg et al. Wollenberg et al. BMC Evolutionary Biology 2011, 11:217 http://www.biomedcentral.com/1471-2148/11/217 (21 July 2011) Wollenberg et al. BMC Evolutionary Biology 2011, 11:217 http://www.biomedcentral.com/1471-2148/11/217 RESEARCH ARTICLE Open Access Speciation in little: the role of range and body size in the diversification of Malagasy mantellid frogs Katharina C Wollenberg1*, David R Vieites2, Frank Glaw3 and Miguel Vences4 Abstract Background: The rate and mode of lineage diversification might be shaped by clade-specific traits. In Madagascar, many groups of organisms are characterized by tiny distribution ranges and small body sizes, and this high degree of microendemism and miniaturization parallels a high species diversity in some of these groups. We here investigate the geographic patterns characterizing the radiation of the frog family Mantellidae that is virtually endemic to Madagascar. We integrate a newly reconstructed near-complete species-level timetree of the Mantellidae with georeferenced distribution records and maximum male body size data to infer the influence of these life-history traits on each other and on mantellid diversification. Results: We reconstructed a molecular phylogeny based on nuclear and mitochondrial DNA for 257 species and candidate species of the mantellid frog radiation. Based on this phylogeny we identified 53 well-supported pairs of sister species that we used for phylogenetic comparative analyses, along with whole tree-based phylogenetic comparative methods. Sister species within the Mantellidae diverged at 0.2-14.4 million years ago and more recently diverged sister species had geographical range centroids more proximate to each other, independently of their current sympatric or allopatric occurrence. The largest number of sister species pairs had non-overlapping ranges, but several examples of young microendemic sister species occurring in full sympatry suggest the possibility of non-allopatric speciation. Range sizes of species included in the sister species comparisons increased with evolutionary age, as did range size differences between sister species, which rejects peripatric speciation. For the majority of mantellid sister species and the whole mantellid radiation, range and body sizes were associated with each other and small body sizes were linked to higher mitochondrial nucleotide substitution rates and higher clade diversity. In contrast, small range sizes were unexpectedly associated with a slow-down of mitochondrial substitution rates. Conclusions: Based on these results we define a testable hypothesis under which small body sizes result in limited dispersal capabilities and low physiological tolerances, causing smaller and more strongly fragmented ranges. This can be thought to facilitate reproductive isolation and thus favor speciation. Contrary to the expectation of the faster speciation of such microendemic phenotype species, we only found small body sizes of mantellid frogs to be linked to higher diversification and substitution rates, but not small range sizes. A joint analysis of various species-rich regional anuran radiations might provide enough species with all combinations of range and body sizes for a more conclusive test of this hypothesis. * Correspondence: [email protected] 1Department of Organismic and Evolutionary Biology & Museum of Comparative Zoology, Harvard University, 26 Oxford St., Cambridge, MA 02134, USA Full list of author information is available at the end of the article © 2011 Wollenberg et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Wollenberg et al. BMC Evolutionary Biology 2011, 11:217 Page 2 of 15 http://www.biomedcentral.com/1471-2148/11/217 Background Van Bocxlaer and colleagues [21] found that speciose Inferring the processes generating large-scale patterns of clades were composed mainly of large species with large biodiversity, especially those shaping adaptive radiations, range sizes. They used a combination of various traits to are fascinating areas of past and current biological define an optimal expansion phenotype (OEP) which research [e.g., [1-3]]. Current debates on diversification they invoked to explain the diversification and success of focus on speciation modes in the temporal dimension these amphibians. In fact, much of the evolutionary suc- (gradual vs. instantaneous speciation, e.g. through hybrid cess of bufonids is related to the fact that their radiation speciation), the spatial dimension (allopatric, including was intimately connected to colonization of vast new dichopatric and peripatric, versus parapatric and sympa- areas, i.e., continents on which these toads were pre- tric speciation), and increasingly on the general mechan- viously absent [21]. This pattern is opposite than would isms driving divergence (ecological adaptation, sexual be expected from a classical explanation for adaptive selection, or non-adaptive factors such as genetic drift radiation, where small ranges and specialization by occu- [4,5]). Life history traits have equally long been dis- pation of specific ecological niches are thought to pro- cussed as drivers of speciation. Changes in these traits mote diversification within a given area [3]. might act as key innovations, promoting ecological To test correlation among traits (like body and range opportunity for the emergence of adaptive radiations in size) or influences of such traits on diversification rates, the absence of habitat changes [[6-8]; reviewed in [2,3]]. phylogenetic comparative methods (PCM) are applied, Body size of animal species is such a life history trait ensuring phylogenetic independence of data [22]. Usually, that has numerous ecological consequences [9]. For PCM rely on ancestral character state reconstruction instance, in many taxa body size seems to determine the along a phylogenetic tree [e.g., [23]]. A generally more size of distribution ranges, with smaller taxa having robust alternative is the direct comparison of sister spe- more restricted ranges [10]. This might, at least in some cies, with each pair of sister species being a phylogeneti- taxa, be simply related to lower dispersal capabilities of cally independent data point (tip contrasts), but in most smaller species; for instance, a phylogenetically indepen- data sets there are not sufficient sister species pairs for dent positive correlation between home range and body adequate statistical analysis [22]. size has been found in ferungulate mammals [11], a Frogs of the family Mantellidae are a highly diverse clade result later refined using optimization methods [12]. restricted to Madagascar and the Comoroan island of Adaptive radiation likely evolves in stages [13], with Mayotte, with 100% species-level endemism and over 250 body size being one of the first proposed axes of morpho- species and candidate species [24]. A large number of logical change leading to new ecological opportunity [[14] mantellid species are small sized, i.e., below 30 mm snout- as reviewed in [3]]. Body size frequency distributions of vent length (SVL), and many are microendemic, i.e., animals are generally right-skewed, i.e., most species are restricted to a very small geographic area. Mantellids have generally small [15-17], as for example in 99% of the evolved a variety of ecological adaptations, including world’s major lizard groups [18] and often the right- arboreal, semiaquatic and fully terrestrial species. Given skewed shape is kept after log-transformation. In lizards, that they diversified within a single geographic setting, often there is a strong negative correlation between body these frogs can serve as an excellent model group to test size within families and species richness: groups contain- mechanisms of diversification and correlates of diversity, ing mainly small species are more species rich [18]. Similar and their large species diversity allows to base PCM calcu- unimodal body size distributions have been found in other lations on pairs of sister species additional to tree-based poikilotherm vertebrates (frogs, deep-sea and freshwater methods. Because most amphibian lineages at the genus fishes), with right-skewedness of the distributions increas- or family level are restricted to major biogeographic areas ing towards the equator [19]. Other researchers found that [25], and show a strong pattern of regional diversification large radiations of species tend to be small bodied [20], similar to the mantellids [e.g., [26-28]], we assume that which is in agreement with the hypothesis that the num- observations on these frogs in Madagascar are more repre- ber of ecological niches is potentially greater for small- sentative for overall amphibian diversification patterns bodied taxa (although this pattern was not statistically sig- than are bufonid toads with their high dispersal capacity. nificant), which suggests a correlation between small body In this paper, we reconstruct a near-complete species- size and species richness. level phylogeny of the Mantellidae from nuclear and mito- Despite the intuitive nature of the hypothesis that small chondrial markers (257 nominal and candidate species). organisms should have elevated net