A comparison of the historical demography of birds from Hispaniola: An analysis using mitochondrial and nuclear DNA sequences
Because of rapidly changing environmental conditions (e.g., climate change and habitat destruction), conservation of Earth's biosystems has become an important issue for society at large and conservation biologists in particular. Tropical ecosystems contain some of the greatest biodiversity on the planet, yet the evolutionary history (and the evolutionary potential) of most species is unknown. A thorough understanding of a populationes history can shed light on the types of changes a species can withstand and the selection and adaption a population has experienced. The conservation of island biodiversity is particularly important as islands often harbor endemic species that do not occur elsewhere. The West Indies are an especially interesting place to study the evolution of island biodiversity because the islands are isolated enough that endemic species have arisen, yet close enough to the continents to have migration to and from the mainland. In a global survey of biodiversity, Hispaniola has been ranked as the highest priority for protection. The critical ranking of the island is due to the large amount of endemic species with limited ranges and habitat destruction to >90% of the forests. Hispaniola also has had a noteworthy climate history, much of which has been recorded in the genes of its residents. ^ Here we use a broad assemblage of birds to determine if general demographic patterns emerge. If so, these patterns may be informative with regard to the evolutionary pressures that have regulated populations across the millennia. Homogeneous demographic histories across a species assemblage would imply that environmental impacts (such as climate or geological events) shaped them similarly. Alternatively, species-specific influences related to diet, habitat distribution, social organization, pathogens, or other factors could result in idiosyncratic demographic histories. Demographic histories also might vary according to migratory tendencies. Hispaniolan bird species can be classified into one of three categories: species that are endemic to the island, species we term \regional residents. that inhabit the island year-round (and whose conspecifics also occur on other islands or on the mainland), and migratory species that breed on the North American continent and spend only a part of the year on Hispaniola. Do these geographic distributions, determined in part by migration tendencies, shape historical demographies similarly across different species? We reasoned that migrant species are most likely to exhibit population expansions due to the increase in available breeding habitat following the retreat of the glaciers in North America. If so, then regional residents and endemics should exhibit more static population demographies relative to migrants. ^ To answer these questions we employed both mitochondrial (mtDNA) and nuclear (nDNA) genes in evolutionary demography analyses. MtDNA has been the marker of choice for the past two decades because of the ease of use and the potential for it to be informative over shorter periods of time. However, mtDNA paints an incomplete picture because of its haploid nature and strict maternal (i.e., asexual) inheritance. Multiple nDNA genes have the potential to provide a more complete representation of evolutionary history. Because of their diploid nature and sexual inheritance, nDNA gene trees are expected to coalesce in the more distant past than are mtDNA gene trees. If sufficient time has passed such that both mtDNA and nDNA loci are informative then they may corroborate each other or highlight violations to our general assumptions (e.g., selective neutrality, independent inheritance, etc). The work described herein uses both mtDNA (Chapter 1) and nDNA (Chapter 2) markers to characterize standing levels of genetic variation in birds from the West Indies. Chapter 3 addresses the use of mtDNA vs. nDNA in some detail, and attempts to interpret our bird data in light of other published studies. Overall, this body of work raises questions about the relative utility of nDNA markers for the study of recent (e.g., Pleistocene) geologic events while highlighting the dynamic evolutionary history of biotic lineages.^
J. A. DeWoody, Purdue University.
Biology, Genetics|Biology, Evolution and Development