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Aysegul Birand |
Postdoctoral Research Associate Department of Ecology & Evolutionary Biology, University of Tennessee, 569 Dabney Hall, 1416 Circle Dr., Knoxville, TN 37996-1610 Ph: 865-974-0346; Fax: 865-974-6042, <abirand at utk.edu>, Office: 403E Austin Peay |
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My research program dwells on the interface of evolutionary and ecological theory, with a current focus on theoretical models of speciation. Speciation continues to be one of the most intriguing phenomena in evolutionary biology, and I make use of mathematical models and computer simulations to address some of the open questions in the field. I am also interested in conceptual issues on speciation, such as clarity of terminology, and importance of null models.
Ongoing research Geographical Ranges and Speciation: The exact nature of the relationship between geographical ranges and speciation is still unknown. The factors that promote larger geographical ranges, such as high dispersal abilities, broad physiological and environmental tolerances should render species insensitive to barriers, which should damp speciation. On the other hand, species with low dispersal abilities, or narrower tolerances, tend to have more fragmented populations, which should render them more prone to speciation. However, larger areas allow larger population sizes, which will in turn allow accumulation of advantageous mutations. Moreover, species with larger ranges, or high dispersal abilities, are likely to encounter different habitats that they could adapt to; which increases the probability of speciation. There is some evidence that the factors promoting broad geographical ranges also reduce speciation rates. My current research focuses on this relationship between species ranges and speciation through individual based models.
Previous research Speciation as bifurcations: My exposure to dynamical systems theory during my minor degree in Mathematical Sciences allowed me to look at speciation using bifurcation theory. Dynamical systems theory and steady state bifurcation theory form the basis of game theory and evolutionary invasion analysis, which is proven to be a useful tool to address various biological phenomena. A recent reincarnation of the game theory approach is Adaptive Dynamics (AD), where the main focus is on points on the strategy space that lack local stability but has convergent stability. These points are also known as evolutionary branching points. Even though AD has received some skepticism in the field, it continues to be attractive as disruptive selection arises through ecological interactions between individuals in the population. A recent application of AD is adaptive radiations where more than one branching events are observed. Despite this new interest, the phenomena of further branchings have been investigated only through numerical simulations, and the exact ecological conditions leading to multispecies results are not known. We investigate further branchings analytically (Birand and Barany in prep) using three different carrying capacity functions, viz. Gaussian, quartic and quadratic functions and demonstrate that the existence of evolutionary branching points, is largely model dependent.
Revisiting limiting similarity: The local stability condition in AD is closely related to another notorious topic in ecology: the limiting similarity. Surprisingly, a contradictory result between two influential papers on limiting similarity has relatively been under appreciated in literature. MacArthur and Levins (1967) suggested there is a critical distance between coexisting species that use a single continuous resource. In a following paper, May and MacArthur (1972) showed that there were no restrictions as to how similar species can be along a single continuous resource. In our study (Birand and Barany submitted), we revisit these seminal works on the theory of limiting similarity and present a new framework using dynamical systems analysis that reconciles these contradictory results. Our analysis also provides some new insights into the existence of limiting similarity and uninvadable communities in deterministic environments.
Speciation on islands – null models: Islands have contributed significantly to our understanding of how evolution works. With unique abiotic and biotic environments, islands provide novel selection pressures that allow evolutionary biologists to test various hypotheses on evolutionary mechanisms. Recently, Emerson and Kolm (2005) reported that the proportion of endemic species on the Canary and Hawaiian Islands was increasing with species diversity on those islands, which lead them to the conclusion that species diversity itself could be driving speciation. Emerson and Kolm’s suggestion has been received with skepticism from researchers in the field. Numerous responses appeared in various journals showing why Emerson and Kolm’s analysis fails to identify the agent responsible for the observed relationship between the proportion of endemics and species diversity. The majority of the responses relied on null models, showing that the same relationship reported by Emerson and Kolm can arise by chance alone. Previous null models that have been developed in response to Emerson and Kolm did not incorporate speciation, even though Emerson and Kolm suggested in situ island speciation for the observed pattern. We (Birand and Howard 2008) developed a null model that is in some ways the evolutionary counter part of presence/absence matrices used in ecology which incorporated speciation. Our results show that the relationship between the proportion of endemics and species diversity can be explained by chance alone. This study, along with the other published responses to Emerson and Kolm, reiterates the importance of null models on macro evolutionary questions.
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