The bear bones: Evolutionary history and conservation genetics of the grizzly bear (Ursus arctos) in the contiguous United States.

摘要

The grizzly bear, Ursus arctos, has been extirpated from over 98% of its historical range in the contiguous United States. Among the potential consequences are reduced viability in the remnant Yellowstone population, loss of a major evolutionary lineage, and loss of information about the species' evolutionary history. These are investigated using DNA extracted from museum specimens. However, low quality DNA in historical specimens produces genotyping errors. An efficient maximum likelihood method is developed to minimize errors by estimating reliability and strategically directing replication (chapter 1). Using this method to insure data quality, diversity and allele frequencies at eight nuclear microsatellite loci are quantified in the Yellowstone population from circa 1915, 1965 and 1995 (chapter 2). Variability has declined slightly in Yellowstone, but no severe bottleneck has occurred. Using variance in allele frequencies over time, the effective population size (Ne) is estimated at ∼80 across the 20th century and greater than 100 currently. The ratio of effective to census size (Ne/N) is estimated at ∼0.3. Genetic factors are unlikely to reduce population viability in the next several generations, but gene flow will be beneficial in avoiding inbreeding and maintaining genetic variability in the long run. The appropriate amount of gene flow is assessed using an island-continent model (chapter 3). Heterozygosity in the island depends on Ne, the migration rate, and the degree of divergence between it and the continent. With recent panmixia, five migrants limit substantial inbreeding while fewer are needed when the populations are divergent and/or when the continent is more diverse. The model indicates that one to two migrants/generation from the Northern Continental Divide Ecosystem into Yellowstone will be sufficient---a value similar to estimates of historical levels of gene flow (chapter 2). Finally, 229 nucleotides of the mitochondrial DNA control region are obtained for 108 specimens from across the historical range (chapter 4). Consistent with the fossil record, genetic data indicate a colonization event by a single major clade, and expansion followed by restricted gene flow. This along with a failure to find genetic support for prior subspecies designations suggests that no major evolutionary lineage has been lost.