Adaptive Aspects Of Insular Evolution

Author : Daniel Patrick Scantlebury
Publisher :
ISBN 13 :
Total Pages : 220 pages
Book Rating : 4.:/5 (925 download)

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Download or read book Patterns of Adaptive Radiation in Insular Reptiles and Amphibians written by Daniel Patrick Scantlebury and published by . This book was released on 2014 with total page 220 pages. Available in PDF, EPUB and Kindle. Book excerpt: "Life on Earth may be characterized by many patterns. The species that surround us are not only numerous, they are often phenotypically and ecologically diverse. The fossil records shows that these species and their phenotypic diversity arose heterogeneously throughout history, and further inspection demonstrates species and phenotypes are nonrandomly distributed across the globe and environments. Ecology and evolutionary biology attempt to explain how these patterns emerge by identifying underlying processes. For instance, Charles Darwin and Alfred Russell Wallace recognized that there were similarities between the species inhabiting adjacent regions and proposed that organic evolution (common descent and modification) can explain these similarities as an alternative to special creation. My research explores three patterns that emerge from the examination of life, and how a single evolutionary process is capable of generating these patterns. That process is adaptive radiation. Adaptive radiation occurs as a response to ecological opportunity in a diversifying lineage. It is an interaction between speciation and adaptation that results in ecologically distinctive new species. If the ecological opportunities available to a diversifying lineage are limited, then rates of speciation and adaptation might decline during the course of adaptive radiation, potentially contributing to differential rates of diversification seen in both the fossil record and molecular phylogenies. Furthermore, if adaptive radiation produces the ecological diversity necessary for species to survive in a variety of climates and habitats, then it might also explain how and why species distribute themselves across landscapes. Although adaptive radiation has the potential to explain much about the diversity of life, current studies are limited to a few iconic clades making it difficult to identify the general elements of adaptive radiation because of vast historical contingencies. This thesis is a comparative effort that explores how adaptive radiation contributes to patterns of (1) species richness and ecological diversity, (2) macroevolutionary diversification rates, and (3) biogeography, by examining clades that radiated in similar regions, habitats, and times. In chapter 1 I focus on the pattern of species richness and phenotypic diversity: why are there groups of related species that differ phenotypically? In particular, I examine a group of predominately Caribbean geckos (Sphaerodactylus) and address whether or not the variation seen in the shape of their skulls has an adaptive origin. Sphaerodactylus geckos are remarkable because they are co-distributed with the wellstudied adaptive radiation of Anolis lizards and potentially provide an important system to evaluate the generalities of conclusions made from Anolis. I show that adaptive radiation probably contributed to variation seen in the shape of their skulls. I also suggest that Sphaerodactylus and Anolis both possess ecologically distinct habitat specialists. These findings show that Sphaerodactylus is an excellent clade to study adaptive radiation by revealing that adaptive radiation may be simultaneous in codistributed clades and ecological diversity may accrue via different pathways. Next, I focus on macroevolutionary patterns of diversification rates through time. Adaptive radiation is hypothesized to result in declining rates of speciation through time if ecological opportunities are limited. As adaptive radiation produces new species, ecological opportunities diminish and the rate at which new species form also declines. Many studies have recovered the signature of declining diversification rates in clades distributed around the world and with different diversification histories, though they do not explicitly prove that adaptive radiation produced these patterns. To date, no study has explored how diversification proceeds in clades that radiated in the same region and habitats during overlapping periods of history. In chapter 2, I use time-scaled phylogenies from seven reptile and amphibian clades from the island of Madagascar to compare diversification dynamics in groups that radiated in same region and through overlapping periods of history. Madagascar is an outstanding region to examine diversification dynamics because it has been isolated and geographically cohesive for the majority of its history, and its many endemic clades provide replication. Given its stability and isolation throughout history, processes general to diversification on Madagascar might be general to the diversification of life elsewhere, demonstrating what happens in the absence of paleogeography or other historical contingencies. I show that diversification rates have declined during the history of the seven clades, and that these declines are probably related to ecological limits to diversity. Although I cannot conclude that adaptive radiation produced these patterns, I note that there are ancillary observations to suggest it played a role. Regardless, these results suggest diversification declines are a general phenomenon on Madagascar and demonstrate the island is an excellent region for further investigation of this macroevolutionary pattern. In chapter 3, I explore how adaptive radiation might underlie regional biogeographic patterns and community assembly. Community assembly is the process by which species come to co-occur locally. Like others, I show that community assembly may be viewed as picking species from sets of regionally distributed species called regional species pools, and indicate that adaptive radiation makes an important prediction regarding the identity of these species pools and their geographic distribution. Several recent studies have indicated that adaptive radiation is multidimensional, with adaptation and ecological diversification occurring along multiple ecological dimensions. If one dimension confers adaptation to regionally variable environmental conditions, then we can predict that regional species pools will correspond to these environmental gradients, and local communities will be assembled from varying combinations of these species pools. I demonstrate that assembly may be modeled with a hidden Markov model. With this model, I use species distributions and community composition data to estimate the minimum number of regional species pools necessary to explain the patterns of co-occurrence in Hispaniolan Anolis lizards that have been documented through over a century of herpetological research. Consistent with my predictions, I find that the regional species pools correspond to a mesic-xeric habitat gradient and that this pattern is replicated across a paleogeographic boundary"--Pages ix-xii.