Island Diversification research by curator Rafe Brown and a large group of students and colleagues in the U.S. and the Philippines focuses on land vertebrate biogeography across a dynamic and complex island archipelago.
Island life has fascinated biologists for centuries. In addition to many bizarre and enigmatic species like giant flightless birds, miniaturized pygmy mammoths, and colossal tortoises and monitor lizards, island forms of plants and animals have inspired philosophers and evolutionary biologists to conceive of many of history’s most contemplative theories and insightful paradigm shifts. It is no wonder that islands played a central role in Alfred Russell Wallace’s and Charles Darwin’s conception of the fundamental mechanisms of evolution by natural selection and that the archipelagos of the western Pacific served as the backdrop for the formulation of several highly influential macroevolutionary hypotheses: equilibrium theory biogeography, the taxon cycle, the great speciator, supertramps, assembly rules, the biological species concept, etc.
The Philippine archipelago is an ideal geographical system for testing predictions regarding the production, partitioning, and maintenance of biodiversity in a dynamic archipelago setting with a well-studied geological history. Given our detailed knowledge of variable patterns of land connectivity, sea level fluctuations, landmass subduction, and accretion, and paleo-transport along the Philippine “Mobile Belt,” researchers can readily formulate predictive models and test classic evolutionary hypotheses from earlier works, with genetic/genomic data, population genetic methods, phylogenies, and information about species distributions.
Built on a foundation of earlier undergraduate and graduate work, Brown’s long-term interests in biodiversity and conservation of Philippine vertebrates has become focused on testing biogeographical hypotheses with phylogenies, species distribution information, population genetic approaches, most recently with genomic data and robust geographical and multi-taxon comparative frameworks. With data from numerous co-distributed Philippine radiations of amphibians, reptiles, birds, and mammals now available, we can test general synthetic predictions relevant to all faunal groups: are species distributions predominately governed by ecological or historical factors? Did vertebrate diversification accelerate immediately following the emergence of the major volcanic components of the archipelago? Did the species-pump mechanism of Pleistocene sea-level oscillations contribute substantially to diversity accumulation? How have ancient landmass movements and connectivity facilitated or limited colonization and subsequent diversification? What role do elevational, ecological and atmospheric gradients played in accumulation of land biodiversity? Have the sensationally diverse and endemic Philippine clades been produced by adaptive, non-adaptive, or a combination of processes?
The approaches have been numerous and continue to develop, but common themes in our works include well-sampled empirical studies, phylogeny estimation, statistical approaches to hypothesis testing, and collaborations with many students and museum-based researchers on both sides of the Pacific.
Recent review of this work
Brown, R. M., C D. Siler, C. H. Oliveros, J. A. Esselstyn, A. C. Diesmos, P. A. Hosner, C. W. Linkem, A. J. Barley, J. R. Oaks, M. B. Sanguila, L. J. Welton, D. S. Blackburn, R. G. Moyle, A. T. Peterson, and A. C. Alcala. 2013. Evolutionary processes of diversification in a model island archipelago. Annual Review of Ecology, Evolution, and Systematics 44:411–435.
Barley, A., J. White, A. C. Diesmos, and R. M. Brown. 2013. The challenge of species delimitation at the extremes: diversification without morphological change in Philippine sun skinks. Evolution 67: 3556–3572.
Blackburn, D. C, C. D. Siler, A. C. Diesmos, J. A. McGuire, D. C. Cannatella, and R. M. Brown. 2013. An adaptive radiation of frogs in a Southeast Asian island archipelago. Evolution 67:2631–2646.
Brown RM, Diesmos AC. 2009. Philippines, biology. In Gillespie & Clague 2009, Encyclopedia of Islands, pp. 723–32.
Brown, R. M., and C. D. Siler. 2013. Spotted stream frog diversification at the Australasian faunal zone interface, mainland versus island comparisons, and a test of the Philippine ‘dual-umbilicus’ hypothesis. Journal of Biogeography 41:182–195.
Esselstyn JA, Maher SP, BrownRM. 2011. Species interactions during diversification and community assembly in an island radiation of shrews. PLoS One 6:e21885
Esselstyn JA, Brown RM. 2009. The role of repeated sea-level fluctuations in the generation of shrew (Soricidae: Crocidura) diversity in the Philippine Archipelago. Molecular Phylogenetics and Evolution 58:171–81.
Esselstyn JA, Oliveros CH. 2010. Colonization of the Philippines from Taiwan: a multi-locus test of the biogeographic and phylogenetic relationships of isolated populations of shrews. Journal of Biogeography 37:1504–14.
Esselstyn JA, Oliveros CH, Moyle RG, Peterson AT, McGuire JA, Brown RM. 2010. Integrating phylogenetic and taxonomic evidence illuminates complex biogeographic patterns along Huxley’s modification of Wallace’s Line. Journal of Biogeography 37:2054–66.
Esselstyn JA, Timm RM, Brown RM. 2009. Do geological or climatic processes drive speciation in dynamic archipelagos? The tempo and mode of diversification in SE Asian shrews. Evolution 63:2595–610.
Gonzales, P., Y.-C. Su, C. D. Siler, A. Barley, M. B. Sanguila, A. C. Diesmos and, R. M. Brown. 2014. Archipelago colonization by ecologically dissimilar amphibians: evaluating the expectation of common evolutionary history of geographical diffusion in co-distributed rainforest tree frogs in islands of Southeast Asia. Molecular Phylogenetics and Evolution 72: 35–41.
Hosner PA,Nyari AS, Moyle RG. 2013b. Water barriers and intra-island isolation contribute to diversification in the insular Aethopyga sunbirds (Aves: Nectariniidae). Journal of Biogeography 40:1094–106.
Linkem CW, Brown RM, Siler CD, Evans BJ, Austin CC, et al. 2012. Stochastic faunal exchanges drive diversificationin widespread Wallacean and Pacific Island lizards (Squamata: Scincidae: Lamprolepis smaragdina). Journal of Biogeography 40:507–20.
Linkem CW, Diesmos AC, Brown RM. 2011. Molecular systematics of the Philippine forest skinks (Reptilia:Scincidae: Sphenomorphus): testing morphological and biogeographic hypotheses of interspecific relationships. Zoological Journal of the Linnean Society 163:1217–43.
Linkem CA, Hesed K, Diesmos AC, Brown RM. 2010. Species boundaries and cryptic lineage diversity in a Philippine forest skink complex (Reptilia; Squamata; Scincidae: Lygosominae). Molecular Phylogenetics and Evolution 56:572–85.
Oaks JR, Sukumaran J, Esselstyn JA, Linkem CW, Siler CD, et al. 2013. Evidence for Pleistocene-driven diversification? A caution for interpreting ABC inferences of clustered historical events. Evolution 67:991–1010.
Oliveros CH, Moyle RG. 2010. Origin and diversification of Philippine bulbuls. Molecular Phylogenetics and Evolution 54:822–32.
Oliveros CH, Reddy S, Moyle RG. 2012. The phylogenetic position of some Philippine “babblers” spans the muscicapoid and sylvioid bird radiations. Molecular Phylogenetics and Evolution 65:799–804.
Sanguila MB, Siler CD, Diesmos AC, Nun˜ eza O, Brown RM. 2011. Phylogeography and conservation implications of geographic structure of genetic variation and potential species boundaries in Philippine slender toads. Molecular Phylogenetics and Evolution 61:333–50.
Siler CD, Brown RM. 2011. Evidence for repeated acquisition and loss of complex body form characters in an insular clade of Southeast Asian semi-fossorial skinks. Evolution 65:2641–63.
Siler CD, Diesmos AC, Alcala AC, Brown RM. 2011a. Phylogeny of Philippine slender skinks (Scincidae: Brachymeles) reveals underestimated species diversity, complex biogeographical relationships, and cryptic patterns of lineage diversification. Molecular Phylogenetics and Evolution 59:53–65.
Siler CD, Oaks JR, Esselstyn JA, Diesmos AC, Brown RM. 2010. Phylogeny and biogeography of Philippine bent-toed geckos (Gekkonidae: Cyrtodactylus) contradict a prevailing model of Pleistocene diversification. Molecular Phylogenetics and Evolution 55:699–710.
Siler CD, Oaks JR, Welton LJ, Linkem CW, Swab J, et al. 2012. Did geckos ride the Palawan raft to the Philippines? Journal of Biogeography 39:1217–34.
Welton LJ, Siler CD, Oaks JR, Diesmos AC, Brown RM. 2013b. Multilocus phylogeny and Bayesian estimates of species boundaries reveal hidden evolutionary relationships and cryptic diversity in Southeast Asian monitor lizards. Molecular Ecolology 22:3495–3510.
Welton, L. J., P. L. Wood Jr., J. R. Oaks, C. D. Siler, and R. M. Brown. 2014. Fossil-calibrated phylogeny and historical biogeography of Southeast Asian water monitors (Varanus salvator Complex). Molecular Phylogenetics and Evolution 74:29–37. [doi link]