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The spatial distribution of genetic diversity is a product of recent and
historical ecological processes, as well as anthropogenic activities. A
current challenge in population and conservation genetics is to
disentangle the relative effects of these processes, as a first step in
predicting population response to future environmental change. In this
investigation we compare the influence of contemporary population decline,
contemporary ecological marginality, and postglacial range shifts. Using
classical model comparison procedures and Bayesian methods, we have
identified postglacial range shift as the clear determinant of genetic
diversity, differentiation, and bottlenecks in 29 populations of
butternut, Juglans cinerea L., a North American outcrossing forest tree.
Although butternut has experienced dramatic 20th century decline due to an
introduced fungal pathogen, our analysis indicates that recent population
decline has had less genetic impact than postglacial recolonization
history. Location within the range edge vs. the range core also failed to
account for the observed patterns of diversity and differentiation. Our
results suggest that the genetic impact of large-scale recent population
losses in forest trees should be considered in the light of
Pleistocene-era large-scale range shifts that may have had long-term
genetic consequences. The data also suggest that the population dynamics
and life history of wind-pollinated forest trees may provide a buffer
against steep population declines of short duration, a result having
important implications for habitat management efforts, ex-situ
conservation sampling, and population viability analysis.
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