Biology and Conservation of North American Tortoises

15 Population and Conservation Genetics of North American Tortoises

15 One of the principal goals of conservation is to preserve the diversity of species. Integral to achieving this goal is the conservation of genetic, ecological, and morphological variation within a species and among populations . The field of conservation genetics provides tools and principles for preserving genetic diversity, which improves a species’ ability to cope with environmental change and decreases its susceptibility to extinction. In addition, genetic data can be used to estimate effective population size, migration rates, and to assess the role of landscape features and environmental conditions that contribute to the evolutionary history of a species. This information can be directly applied to conservation and management. Genetic studies contribute to our knowledge of North American tortoises (Gopherus) in a wide variety of ways, including : systematics (Lamb et al. 1989, Lamb and Lydeard 1994, Morafka et al. 1994, Spinks et al. 2004, Le et al. 2006, Thompson and Shaffer 2010), taxonomy (Murphy et al. 2011), phylogeography (Lamb et al. 1989, Ostentoski and Lamb 1995, Edwards et al. 2012, Ennen et al. 2012), population structure (Jennings 1985, Rainboth et al. 1989, Glenn et al. 1990, McLuckie et al. 1999, Edwards et al. 2004a, Schwartz and Karl 2005, Ennen et al. 2010, Fujii and Forstner 2010, Sinclair et al. 2010, Latch et al. 2011, Richter et al. 2011, Urena-Aranda and de los Monteros 2012, Clostio et al. 2012), management (Britten et al. 1997, Murphy et al. 2007, Hagerty and Tracy 2010, Ennen et al. 2011, Edwards and Berry 2013), ecological genetics (Hagerty et al. 2011), hybridization (Edwards et al. 2010), paternity (Moon et al. 2006, Davy et al. 2011), and forensics (Schwartz and Karl 2008). In this chapter, we first introduce some of the fundamental principles of population genetics as observed in Gopherus and show how they contribute to our understanding of tortoise ecology. We then review how genetic studies help to inform management and conservation efforts. PoPULation GenetiCs, GenetiC DiVeRsitY, anD PoPULation GenetiC stRUCtURe Population genetics evaluates differences in genetic variation among populations or individuals. Differences in the distribution of genetic variation can arise from a combination of processes, including natural selection, isolation, dispersal, demographic processes (e.g., changes in population size) or stochastic processes (genetic drift). Patterns in the distribution of genetic variation at different spatial and/or temporal scales can be used to test hypotheses relating to past or ongoing processes of evolutionary and ecological importance. Quantifying the amount of genetic variation within and among populations is essential for understanding several aspects of a species ecology and conservation (Frankham 1995). For example, this information can be used to determine if a species constitutes a single, continuous population or multiple , distinct populations. Informed practices for preserving genetic variation within a species are critical for effective conservation . When genetic variation of a species is partitioned nonrandomly across its geographic range, it is called population structure. In the absence of selection, the genetic structure of a population is influenced primarily by the amount of gene flow (exchange of genetic material via immigration of individuals ) among populations. The absence of structure is called panmixia, where mating between individuals is completely random. Patterns of genetic variation can be assessed using a variety of methods and inferences are dependent on the type of genetic marker being measured (table 15.1). Mitochondrial DNA (mtDNA) is a commonly used genetic marker for population studies. Unlike nuclear genes, mtDNA is maternally inherited and haploid. Because of these characteristics , mtDNA may be affected by factors such as population size reductions (bottlenecks) and isolation at a faster rate taylor Edwards J. scott Harrison Population and Conservation Genetics of North American Tortoises taylor Edwards and J. scott Harrison 128 Taylor Edwards and J. Scott Harrison served a large amount of mtDNA sequence divergence, 2.3%, with a pronounced phylogenetic break across the Apalachicola River drainage. This genetic barrier was also supported by Ennan et al. (2012), although they observed a broad zone of geographic overlap of mtDNA haplotypes on both sides of the Apalachicola River. Thus, barriers to gene flow over the evolutionary history of G. polyphemus seem to have influenced the distribution of mtDNA variation. This pattern is hardly surprising in this wide-ranging species, occupying habitats in a landscape with heterogeneous climate, vegetation, topography , and other environmental factors. In contrast to the entire species’ range, variation of mtDNA within local populations of G. polyphemus is low, a pattern which seems to be characteristic of all Gopherus species (Ostenoski and Lamb 1995). Other Gopherus species exhibit less divergence...