Liam J Revell

University of California, Davis, Davis, CA, United States

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Publications (31)163.14 Total impact

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    ABSTRACT: Snakes in the families Boidae and Pythonidae constitute some of the most spectacular reptiles and comprise an enormous diversity of morphology, behavior, and ecology. While many species of boas and pythons are familiar, taxonomy and evolutionary relationships within these families remain contentious and fluid. A major effort in evolutionary and conservation biology is to assemble a comprehensive Tree-of-Life, or a macro-scale phylogenetic hypothesis, for all known life on Earth. No previously published study has produced a species-level molecular phylogeny for more than 61% of boa species or 65% of python species. Using both novel and previously published sequence data, we have produced a species-level phylogeny for 84.5% of boid species and 82.5% of pythonid species, contextualized within a larger phylogeny of henophidian snakes. We obtained new sequence data for three boid, one pythonid, and two tropidophiid taxa which have never previously been included in a molecular study, in addition to generating novel sequences for seven genes across an additional 12 taxa. We compiled an 11-gene dataset for 127 taxa, consisting of the mitochondrial genes CYTB, 12S, and 16S, and the nuclear genes bdnf, bmp2, c-mos, gpr35, rag1, ntf3, odc, and slc30a1, totaling up to 7561 base pairs per taxon. We analyzed this dataset using both maximum likelihood and Bayesian inference and recovered a well-supported phylogeny for these species. We found significant evidence of discordance between taxonomy and evolutionary relationships in the genera Tropidophis, Morelia, Liasis, and Leiopython, and we found support for elevating two previously suggested boid species. We suggest a revised taxonomy for the boas (13 genera, 58 species) and pythons (8 genera, 40 species), review relationships between our study and the many other molecular phylogenetic studies of henophidian snakes, and present a taxonomic database and alignment which may be easily used and built upon by other researchers.
    Molecular Phylogenetics and Evolution 12/2013; · 4.07 Impact Factor
  • Liam J Revell
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    ABSTRACT: Evolutionary biology is a study of life's history on Earth. In researching this history, biologists are often interested in attempting to reconstruct phenotypes for the long extinct ancestors of living species. Various methods have been developed to do this on a phylogeny from the data for extant taxa. In the present article I introduce a new approach for ancestral character estimation for discretely valued traits. This approach is based on the threshold model from evolutionary quantitative genetics. Under the threshold model, the value exhibited by an individual or species for a discrete character is determined by an underlying, unobserved continuous trait called 'liability.' In this new method for ancestral state reconstruction, I use Bayesian MCMC to sample the liabilities of ancestral and tip species, and the relative positions of two or more thresholds, from their joint posterior probability distribution. Using data simulated under the model, I find that the method has very good performance in ancestral character estimation. Use of the threshold model for ancestral state reconstruction relies on a priori specification of the order of the discrete character states along the liability axis. I test the use of a Bayesian MCMC information theoretic criterion (DIC) based approach to choose among different hypothesized orderings for the discrete character. Finally, I apply the method to the evolution of feeding mode in centrarchid fishes. This article is protected by copyright. All rights reserved.
    Evolution 10/2013; · 4.86 Impact Factor
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    ABSTRACT: G. G. Simpson, one of the chief architects of evolutionary biology's modern synthesis, proposed that diversification occurs on a macroevolutionary adaptive landscape, but landscape models are seldom used to study adaptive divergence in large radiations. We show that for Caribbean Anolis lizards, diversification on similar Simpsonian landscapes leads to striking convergence of entire faunas on four islands. Parallel radiations unfolding at large temporal scales shed light on the process of adaptive diversification, indicating that the adaptive landscape may give rise to predictable evolutionary patterns in nature, that adaptive peaks may be stable over macroevolutionary time, and that available geographic area influences the ability of lineages to discover new adaptive peaks.
    Science 07/2013; 341(6143):292-5. · 31.20 Impact Factor
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    ABSTRACT: The evolutionary and biogeographic history of West Indian boid snakes (Epicrates), a group of nine species and 14 subspecies, was once thought to be well understood; however, new research has indicated that we are missing a clear understanding of the evolutionary relationships of this group. Here, we present the first multilocus, species-tree based analyses of the evolutionary relationships, divergence times, and historical biogeography of this clade with data from 10 genes and 6,256 bp. We find evidence for a single colonization of the Caribbean from mainland South America in the Oligocene or early Miocene, followed by a radiation throughout the Greater Antilles and Bahamas. These findings support the previous suggestion that Epicrates sensu lato Wagler is paraphyletic with respect to the anacondas (Eunectes Wagler), and hence we restrict Epicrates to the mainland clade and use the available name Chilabothrus Duméril and Bibron for the West Indian clade. Our results suggest some diversification occurred within island banks, though most species divergence events seem to have occurred in allopatry. We also find evidence for a remarkable diversification within the Bahamian archipelago suggesting that the recognition of another Bahamian endemic species C. strigilatus is warranted.
    Molecular Phylogenetics and Evolution 05/2013; · 4.07 Impact Factor
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    ABSTRACT: Adaptive divergence in coloration is expected to produce reproductive isolation in species that use colourful signals in mate choice and species recognition. Indeed, many adaptive radiations are characterized by differentiation in colourful signals, suggesting that divergent selection acting on coloration may be an important component of speciation. Populations in the Anolis marmoratus species complex from the Caribbean island of Guadeloupe display striking divergence in the colour and pattern of adult males that occurs over small geographic distances, suggesting strong divergent selection. Here we test the hypothesis that divergence in coloration results in reduced gene flow among populations. We quantify variation in adult male coloration across a habitat gradient between mesic and xeric habitats, use a multilocus coalescent approach to infer historical demographic parameters of divergence, and examine gene flow and population structure using microsatellite variation. We find that colour variation evolved without geographic isolation and in the face of gene flow, consistent with strong divergent selection and that both ecological and sexual selection are implicated. However, we find no significant differentiation at microsatellite loci across populations, suggesting little reproductive isolation and high levels of contemporary gene exchange. Strong divergent selection on loci affecting coloration probably maintains clinal phenotypic variation despite high gene flow at neutral loci, supporting the notion of a porous genome in which adaptive portions of the genome remain fixed whereas neutral portions are homogenized by gene flow and recombination. We discuss the impact of these findings for studies of colour evolution and ecological speciation.
    Molecular Ecology 05/2013; 22(10):2668-82. · 6.28 Impact Factor
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    ABSTRACT: The endemic Puerto Rican boa (Epicrates inornatus) has spent 42 years on the Endangered Species List with little evidence for recovery. One significant impediment to effective conservation planning has been a lack of knowledge of the distribution of genetic variability in the species. It has previously been suggested that boas might best be protected around caves that harbor large populations of bats. Prior study has found Puerto Rican boas at relatively high densities in and around bat caves, which they use both to feed and seek shelter. However, it is unknown whether these behaviorally distinctive populations represent a distinct evolutionary lineage, or (conversely) whether caves harbor representative genetic diversity for the species across the island. We provide the first genetic study of the Puerto Rican boa, and we examine and compare genetic diversity and divergence among two cave populations and two surface populations of boas. We find three haplogroups and an apparent lack of phylogeographic structure across the island. In addition, we find that the two cave populations appear no less diverse than the two surface populations, and harbor multiple mtDNA lineages. We discuss the conservation implications of these findings, including a call for the immediate protection of the remaining cave-associated populations of boas.
    PLoS ONE 01/2013; 8(5):e63899. · 3.73 Impact Factor
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    Liam J Revell, R Graham Reynolds
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    ABSTRACT: Phylogenetic comparative methods that incorporate intraspecific variability are relatively new and, so far, not especially widely used in empirical studies. In the present short article we will describe a new Bayesian method for fitting evolutionary models to comparative data that incorporates intraspecific variability. This method differs from an existing likelihood-based approach in that it requires no a priori inference about species means and variances; rather it takes phenotypic values from individuals and a phylogenetic tree as input, and then samples species means and variances, along with the parameters of the evolutionary model, from their joint posterior probability distribution. One of the most novel and intriguing attributes of this approach is that jointly sampling the species means with the evolutionary model parameters means that the model and tree can influence our estimates of species mean trait values, not just the reverse. In the present implementation, we first apply this method to the most widely used evolutionary model for continuously valued phenotypic trait data (Brownian motion). However, the general approach has broad applicability, which we illustrate by also fitting the λ model, another simple model for quantitative trait evolution on a phylogeny. We test our approach via simulation and by analyzing two empirical datasets obtained from the literature. Finally, we have implemented the methods described herein in a new function for the R statistical computing environment, and this function will be distributed as part of the 'phytools' R library.
    Evolution 09/2012; 66(9):2697-707. · 4.86 Impact Factor
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    ABSTRACT: In recent years, a suite of methods has been developed to fit multiple rate models to phylogenetic comparative data. However, most methods have limited utility at broad phylogenetic scales because they typically require complete sampling of both the tree and the associated phenotypic data. Here, we develop and implement a new, tree-based method called MECCA (Modeling Evolution of Continuous Characters using ABC) that uses a hybrid likelihood/approximate Bayesian computation (ABC)-Markov-Chain Monte Carlo approach to simultaneously infer rates of diversification and trait evolution from incompletely sampled phylogenies and trait data. We demonstrate via simulation that MECCA has considerable power to choose among single versus multiple evolutionary rate models, and thus can be used to test hypotheses about changes in the rate of trait evolution across an incomplete tree of life. We finally apply MECCA to an empirical example of body size evolution in carnivores, and show that there is no evidence for an elevated rate of body size evolution in the pinnipeds relative to terrestrial carnivores. ABC approaches can provide a useful alternative set of tools for future macroevolutionary studies where likelihood-dependent approaches are lacking.
    Evolution 03/2012; 66(3):752-62. · 4.86 Impact Factor
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    ABSTRACT: Currently available phylogenetic methods for studying the rate of evolution in a continuously valued character assume that the rate is constant throughout the tree or that it changes along specific branches according to an a priori hypothesis of rate variation provided by the user. Herein, we describe a new method for studying evolutionary rate variation in continuously valued characters given an estimate of the phylogenetic history of the species in our study. According to this method, we propose no specific prior hypothesis for how the variation in evolutionary rate is structured throughout the history of the species in our study. Instead, we use a bayesian Markov Chain Monte Carlo approach to estimate evolutionary rates and the shift point between rates on the tree. We do this by simultaneously sampling rates and shift points in proportion to their posterior probability, and then collapsing the posterior sample into an estimate of the parameters of interest. We use simulation to show that the method is quite successful at identifying the phylogenetic position of a shift in the rate of evolution, and that estimated rates are asymptotically unbiased. We also provide an empirical example of the method using data for Anolis lizards.
    Evolution 01/2012; 66(1):135-46. · 4.86 Impact Factor
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    ABSTRACT: The adaptive landscape and the G-matrix are keys concepts for understanding how quantitative characters evolve during adaptive radiation. In particular, whether the adaptive landscape can drive convergence of phenotypic integration (i.e., the pattern of phenotypic variation and covariation summarized in the P-matrix) is not well studied. We estimated and compared P for 19 morphological traits in eight species of Caribbean Anolis lizards, finding that similarity in P among species was not correlated with phylogenetic distance. However, greater similarity in P among ecologically similar Anolis species (i.e., the trunk-ground ecomorph) suggests the role of convergent natural selection. Despite this convergence and relatively deep phylogenetic divergence, a large portion of eigenstructure of P is retained among our eight focal species. We also analyzed P as an approximation of G to test for correspondence with the pattern of phenotypic divergence in 21 Caribbean Anolis species. These patterns of covariation were coincident, suggesting that either genetic constraint has influenced the pattern of among-species divergence or, alternatively, that the adaptive landscape has influenced both G and the pattern of phenotypic divergence among species. We provide evidence for convergent evolution of phenotypic integration for one class of Anolis ecomorph, revealing yet another important dimension of evolutionary convergence in this group.
    Evolution 11/2011; 65(12):3608 - 3624. · 4.86 Impact Factor
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    ABSTRACT: The independent evolution of similar morphologies has long been a subject of considerable interest to biologists. Does phenotypic convergence reflect the primacy of natural selection, or does development set the course of evolution by channelling variation in certain directions? Here, we examine the ontogenetic origins of relative limb length variation among Anolis lizard habitat specialists to address whether convergent phenotypes have arisen through convergent developmental trajectories. Despite the numerous developmental processes that could potentially contribute to variation in adult limb length, our analyses reveal that, in Anolis lizards, such variation is repeatedly the result of changes occurring very early in development, prior to formation of the cartilaginous long bone anlagen.
    Proceedings of the Royal Society B: Biological Sciences 08/2011; 279(1729):739-48. · 5.68 Impact Factor
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    Liam J. Revell
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    ABSTRACT: Summary1. A common procedure in the regression analysis of interspecies data is to first test the independent and dependent variables X and Y for phylogenetic signal, and then use the presence of signal in one or both traits to justify regression analysis using phylogenetic methods such as independent contrasts or phylogenetic generalized least squares.2. This is incorrect, because phylogenetic regression assumes that the residual error in the regression model (not in the original traits) is distributed according to a multivariate normal distribution with variances and covariances proportional to the historical relations of the species in the sample.3. Here, I examine the consequences of justifying and applying the phylogenetic regression incorrectly. I find that when used improperly the phylogenetic regression can have poor statistical performance, even under some circumstances in which the type I error rate of the method is not inflated over its nominal level.4. I also find, however, that when tests of phylogenetic signal in phylogenetic regression are applied properly, and in particular when phylogenetic signal in the residual error is simultaneously estimated with the regression parameters, the phylogenetic regression outperforms equivalent non-phylogenetic procedures.
    Methods in Ecology and Evolution 11/2010; 1(4):319 - 329. · 5.92 Impact Factor
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    ABSTRACT: Male intrasexual competition should favour increased male physical prowess. This should in turn result in greater aerobic capacity in males than in females (i.e. sexual dimorphism) and a correlation between sexual dimorphism in aerobic capacity and the strength of sexual selection among species. However, physiological scaling laws predict that aerobic capacity should be lower per unit body mass in larger than in smaller animals, potentially reducing or reversing the sex difference and its association with measures of sexual selection. We used measures of haematocrit and red blood cell (RBC) counts from 45 species of primates to test four predictions related to sexual selection and body mass: (i) on average, males should have higher aerobic capacity than females, (ii) aerobic capacity should be higher in adult than juvenile males, (iii) aerobic capacity should increase with increasing sexual selection, but also that (iv) measures of aerobic capacity should co-vary negatively with body mass. For the first two predictions, we used a phylogenetic paired t-test developed for this study. We found support for predictions (i) and (ii). For prediction (iii), however, we found a negative correlation between the degree of sexual selection and aerobic capacity, which was opposite to our prediction. Prediction (iv) was generally supported. We also investigated whether substrate use, basal metabolic rate and agility influenced physiological measures of oxygen transport, but we found only weak evidence for a correlation between RBC count and agility.
    Journal of Evolutionary Biology 06/2010; 23(6):1183-94. · 3.48 Impact Factor
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    ABSTRACT: The pace of phenotypic diversification during adaptive radiation should decrease as ecological opportunity declines. We test this prediction using phylogenetic comparative analyses of a wide range of morphological traits in Greater Antillean Anolis lizards. We find that the rate of diversification along two important axes of Anolis radiation-body size and limb dimensions-decreased as opportunity declined, with opportunity quantified either as time elapsed in the radiation or as the diversity of competing anole lineages inferred to have been present on an island at different times in the past. Most previous studies of the ecological opportunity hypothesis have focused on the rate of species diversification; our results provide a complementary perspective, indicating that the rate of phenotypic diversification declines with decreasing opportunity in an adaptive radiation.
    Evolution 04/2010; 64(9):2731-45. · 4.86 Impact Factor
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    ABSTRACT: The pattern of genetic variances and covariances among characters, summarized in the additive genetic variance-covariance matrix, G, determines how a population will respond to linear natural selection. However, G itself also evolves in response to selection. In particular, we expect that, over time, G will evolve correspondence with the pattern of multivariate nonlinear natural selection. In this study, we substitute the phenotypic variance-covariance matrix (P) for G to determine if the pattern of multivariate nonlinear selection in a natural population of Anolis cristatellus, an arboreal lizard from Puerto Rico, has influenced the evolution of genetic variances and covariances in this species. Although results varied among our estimates of P and fitness, and among our analytic techniques, we find significant evidence for congruence between nonlinear selection and P, suggesting that natural selection may have influenced the evolution of genetic constraint in this species.
    Journal of Evolutionary Biology 02/2010; 23(2):407-21. · 3.48 Impact Factor
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    Karen R Lovely, D Luke Mahler, Liam J Revell
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    ABSTRACT: Background: In lizards, tail autotomy is used in defence against predators. Question: Can we infer predation regime from the frequency and pattern of tail autotomy in five lizard species? Organisms: Five species of common Puerto Rican anoles: Anolis cristatellus, A. evermanni, A. gundlachi, A. krugi, and A. pulchellus. Methods: Monte Carlo simulations. Our Monte Carlo models incorporated the probability of tail loss (as opposed to mortality) during a predatory attack, the strength of the tail over its length (tail strength modelled as a heterogeneous probability of breakage among caudal vertebrae in the tail), and age-biased sampling (young lizards are more likely to have intact tails, but less likely to be included in our sample). Results: Our models exhibited good fit to the data, with the best fitting model showing a significant lack of fit in only one species. Our parameter estimates had biologically reasonable values. Our estimated rate of mortality from predatory attacks resulting in either mortality or tail injury was quite high (> 0.4 in the best fitting model) for all species. In three of the five species, the best fitting model included heterogeneity in the strength (probability of breakage) of the tail over its length, with the tail much more likely to break towards the tip than towards the base. The remaining two species (A. krugi and A. pulchellus), for which heterogeneous tail strength was not part of the best fitting model, are known from other studies to be ecologically and morphologically similar. These two also had the most similar estimated mortality rates.
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    ABSTRACT: Most studies of adaptive radiations focus on morphological aspects of differentiation, yet behavior is also an important component of evolutionary diversification, often mediating the relationship between animal ecology and morphology. In species within radiations that are convergent in ecology and morphology, we then also expect convergence in behavior. Here, we examined 13 Anolis lizard species to determine whether territorial strategies have evolved convergently with morphology and habitat use. We evaluated two aspects of territoriality: behavioral defense of space via territorial displays, and territory overlap within and between sexes. Controlling for the phylogenetic relationships of the taxa in our study, we found that species similar in perch height and diameter convergently evolved patterns of territory overlap, whereas species similar in habitat visibility (the proportion of space that can be seen from a perch) convergently evolved display behavior. We also found that species with greater display time have more extensive male-male territory overlap. This study provides strong evidence for the role of habitat in the evolution of territoriality and suggests that the social structure of a species ultimately evolves in concert with habitat use and morphology.
    Evolution 11/2009; 64(4):1151-9. · 4.86 Impact Factor
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    Liam J Revell
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    ABSTRACT: Phylogenetic methods for the analysis of species data are widely used in evolutionary studies. However, preliminary data transformations and data reduction procedures (such as a size-correction and principal components analysis, PCA) are often performed without first correcting for nonindependence among the observations for species. In the present short comment and attached R and MATLAB code, I provide an overview of statistically correct procedures for phylogenetic size-correction and PCA. I also show that ignoring phylogeny in preliminary transformations can result in significantly elevated variance and type I error in our statistical estimators, even if subsequent analysis of the transformed data is performed using phylogenetic methods. This means that ignoring phylogeny during preliminary data transformations can possibly lead to spurious results in phylogenetic statistical analyses of species data.
    Evolution 09/2009; 63(12):3258-68. · 4.86 Impact Factor
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    Liam J Revell, David C Collar
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    ABSTRACT: Many evolutionary processes can lead to a change in the correlation between continuous characters over time or on different branches of a phylogenetic tree. Shifts in genetic or functional constraint, in the selective regime, or in some combination thereof can influence both the evolution of continuous traits and their relation to each other. These changes can often be mapped on a phylogenetic tree to examine their influence on multivariate phenotypic diversification. We propose a new likelihood method to fit multiple evolutionary rate matrices (also called evolutionary variance-covariance matrices) to species data for two or more continuous characters and a phylogeny. The evolutionary rate matrix is a matrix containing the evolutionary rates for individual characters on its diagonal, and the covariances between characters (of which the evolutionary correlations are a function) elsewhere. To illustrate our approach, we apply the method to an empirical dataset consisting of two features of feeding morphology sampled from 28 centrarchid fish species, as well as to data generated via phylogenetic numerical simulations. We find that the method has appropriate type I error, power, and parameter estimation. The approach presented herein is the first to allow for the explicit testing of how and when the evolutionary covariances between characters have changed in the history of a group.
    Evolution 02/2009; 63(4):1090-100. · 4.86 Impact Factor

Publication Stats

463 Citations
407 Downloads
2k Views
163.14 Total Impact Points

Institutions

  • 2013
    • University of California, Davis
      • Center for Population Biology
      Davis, CA, United States
    • Interamerican University of Puerto Rico
      San Germán, San German, Puerto Rico
  • 2011–2013
    • University of Massachusetts Boston
      • Department of Biology
      Boston, MA, United States
  • 2010–2012
    • Duke University
      Durham, North Carolina, United States
  • 2010–2011
    • National Evolutionary Synthesis Center
      Durham, North Carolina, United States
  • 2007–2011
    • Harvard University
      • Department of Organismic and Evolutionary Biology
      Cambridge, MA, United States
  • 2009
    • Michigan State University
      • Department of Zoology
      East Lansing, MI, United States
  • 2006
    • Washington University in St. Louis
      • Department of Biology
      Saint Louis, MO, United States