Habitat Use Affects Morphological Diversification in Dragon Lizards
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doi:10.1111/j.1420-9101.2010.01971.x Habitat use affects morphological diversification in dragon lizards D. C. COLLAR*, J. A. SCHULTE II ,B.C.O’MEARAà &J.B.LOSOS* *Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA Department of Biology, Clarkson University, Potsdam, NY, USA àDepartment of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN, USA Keywords: Abstract Agamidae; Habitat use may lead to variation in diversity among evolutionary lineages Brownian motion; because habitats differ in the variety of ways they allow for species to make a ecomorphology; living. Here, we show that structural habitats contribute to differential Iguania; diversification of limb and body form in dragon lizards (Agamidae). Based on locomotion; phylogenetic analysis and ancestral state reconstructions for 90 species, we phylogenetic comparative method. find that multiple lineages have independently adopted each of four habitat use types: rock-dwelling, terrestriality, semi-arboreality and arboreality. Given these reconstructions, we fit models of evolution to species’ morphological trait values and find that rock-dwelling and arboreality limit diversification relative to terrestriality and semi-arboreality. Models preferred by Akaike information criterion infer slower rates of size and shape evolution in lineages inferred to occupy rocks and trees, and model-averaged rate estimates are slowest for these habitat types. These results suggest that ground-dwelling facilitates ecomorphological differentiation and that use of trees or rocks impedes diversification. tion. In this study, we test the hypothesis that diversity Introduction varies as a function of habitat. One of the great questions in evolutionary biology For many reasons, some habitats may foster greater concerns the causes of differences in diversity among diversity than others. Some habitat types may be readily clades. Ecological factors are often implicated to explain subdivided, perhaps because of spatial complexity (e.g. this pattern because the ecological circumstances avail- coral reefs [Alfaro et al., 2007]) or geographical area (e.g. able to the members of a lineage contribute to the mode arid habitats in Australia [Rabosky et al., 2007]) and may of natural selection they experience and thus shape thereby present evolutionary lineages with many alter- ecological divergence, morphological adaptation and the native means for microhabitat specialization or local evolution of new species. Although much work has adaptation. Other habitats may impose stringent func- focused on the role of biotic interactions within commu- tional constraints that lead to strong selection resisting nities (e.g. competition-driven divergent selection in ecological and phenotypic divergence away from an Anolis lizards [Williams, 1972; Losos, 2009], Hawaiian adaptive peak (Butler & King, 2004; Collar et al., 2009). silverswords [Carlquist et al., 2003] and Darwin’s finches Habitat types may also contribute differently to diversi- [Schluter, 1988; Grant & Grant, 2008]), other aspects of a fication because they vary in the number and type of lineage’s ecology may also be important for diversifica- species interactions they present, such as the presence or absence of predators (McPeek & Brown, 2000). In Correspondence: David C. Collar, Department of Organismic and addition, some habitats may provide opportunities if Evolutionary Biology and Museum of Comparative Zoology, they are variable across space in the strength of species Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA. interactions (McPeek, 1996) or in their functional Tel.: 617 496 9099; fax: 617 495 5667; e-mail: [email protected] Re-use of this article is permitted in accordance with the demands. Terms and Conditions set out at http://www3.interscience.wiley.com/ The consequences of habitat use for diversification authorresources/onlineopen.html have been investigated primarily in the context of ª 2010 THE AUTHORS. J. EVOL. BIOL. 23 (2010) 1033–1049 JOURNAL COMPILATION ª 2010 EUROPEAN SOCIETY FOR EVOLUTIONARY BIOLOGY 1033 1034 D. C. COLLAR ET AL. explaining variation in species richness among clades. Materials and methods Evolutionary transitions between habitat types that differ in the opportunities they provide for ecological diver- Reconstructing phylogeny gence are often implicated to explain shifts in rates of lineage diversification. For example, several invasions of We reconstructed phylogenetic relationships for 90 coral reefs in tetraodontiform fish lineages are temporally agamid species—representing nearly one-quarter of the coincident with increases in rates of diversification group’s recognized species diversity—as well as four (Alfaro et al., 2007), the transition into arid habitats outgroup species. Our molecular data set included 1.2 kb is associated with elevated rates of diversification in of mitochondrial DNA including partial sequences for the Australian skinks (Rabosky et al., 2007), and diversifica- protein-coding genes, NADH dehydrogenase subunit 1 tion rates in damselflies vary across gradients of pond (ND1) and cytochrome c oxidase subunit I (COI), and the permanence (McPeek & Brown, 2000). complete sequence for NADH dehydrogenase subunit 2 One explanation for these associations between habitat (ND2). This analysis excluded intervening tRNA-coding use and species richness is that the process of lineage regions because they are highly variable among the splitting is mechanistically linked to niche differentiation. sampled taxa, making unambiguous alignment of these However, species richness and ecological diversity need regions difficult and potentially unreliable (Schulte et al., not be correlated during evolution (Foote, 1993; Losos & 2004a; Schulte & de Queiroz, 2008). All sequences were Miles, 2002; Adams et al., 2009), and elevated rates of obtained from GenBank (accession numbers are in lineage diversification within a habitat type do not Table S1) and aligned by eye. Base positions inferred to require increases in rates of ecological evolution. Indeed, have ambiguous homology at the ends of ND1 and ND2 the neutral theory of biodiversity emphasizes the extent were excluded from phylogenetic analyses (198 of 1281 to which species diversification may occur in the absence aligned positions). Alignment is available in TREEBASE of ecological differentiation (Hubbell, 2001). (Study accession number S2669, Matrix accession num- Using morphological variation in ecologically relevant bers M5148; to be added upon acceptance of manuscript). characters as a surrogate for ecological variation, we We used these sequences to simultaneously infer asked whether a relationship exists between habitat use phylogenetic relationships among agamid species and and ecological diversity. We focused on dragon lizards estimate branch lengths in relative time using Bayesian (Agamidae), an ecologically and morphologically diverse phylogenetic analysis and a relaxed molecular clock radiation of iguanian lizards comprising roughly 400 approach implemented in the program BEAST (Drum- species distributed throughout the Old World. Agamid mond et al., 2006; Drummond & Rambaut, 2007). We lizards vary in their structural habitat use, including partitioned mtDNA sequences by codon position and, for species that primarily use rocks, trees or terrestrial each partition, separately fit a general time reversible surfaces as well as some semi-arboreal species that model of nucleotide substitution that allows for gamma- frequently use both trees and terrestrial surfaces. Because distributed substitution rate variation among sites and the ability to move about and hold position in the invariant sites (Yang, 1994) because previous analysis of environment is partly a consequence of structural habitat these sequences for a subset of the agamid species use and because movement is important to the perfor- included in this study showed that this model provided mance of ecological tasks, such as foraging, evading the best fit relative to simpler substitution models predation and defending territory (Losos, 1990; Irschick (Schulte et al., 2004a). Variation in substitution rates & Garland, 2001), these four types of habitat use may among lineages was modelled by a lognormal distribu- contribute differently to ecomorphological diversification tion in which the mean rate was set to 1.0 (i.e. no in agamid lineages. external calibration was used to estimate divergence To evaluate this hypothesis, we applied a phylogenetic times), and no correlation was assumed between ances- approach that tests for associations between habitat and tor and descendant branches (Drummond et al., 2006; rates of morphological evolution in agamid lineages. We Drummond & Rambaut, 2007). Uninformative priors inferred phylogenetic relationships for 90 agamid species were applied for all parameter estimates. based on mitochondrial DNA sequences, reconstructed We used BEAST to sample the posterior probability ancestral habitat use and used these reconstructions as distribution of phylogenetic trees and substitution model the basis for fitting models of evolution to species values parameters given species’ sequence data according to a for morphological traits. We then compared fit and Markov chain Monte Carlo (MCMC) algorithm (Drum- parameter estimates for models that differ in the number mond & Rambaut, 2007), which we ran twice for of evolutionary rates (based on the Brownian motion 25 · 106 generations per