Range-wide distribution of genetic diversity in the North American tree Juglans cinerea: a product of range shifts, not ecological marginality or recent population decline SEAN M. HOBAN,* DANIEL S. BORKOWSKI,* SUNSHINE L. BROSI,† TIM S. M C CLEARY,* LAURA M. THOMPSON,‡ JASON S. M C LACHLAN,* MARIE A. PEREIRA,* SCOTT E. SCHLARBAUM‡ and JEANNE ROMERO-SEVERSON* *Department of Biological Sciences, University of Notre Dame, 327 Galvin Life Sciences, Notre Dame, IN 46556, USA, †Department of Biology, Frostburg State University, 201 Compton Science Center, 101 Braddock Road, Frostburg, MD 21532- 2303, USA, ‡Department of Forestry, Wildlife and Fisheries, The University of Tennessee, 282 Ellington Plant Sciences, Knoxville, TN 37996, USA Abstract 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 because of 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. Keywords: bottlenecks, butternut, climate change, genetic diversity, range shifts Received 13 March 2010; revision received 5 August 2010; accepted 9 August 2010 Introduction A central aim of evolutionary biology is to identify and describe the processes that determine the spatial distri- bution and abundance of genetic diversity. Many dis- tinct processes, acting at different time and spatial scales, shape this distribution, including variation in habitat quality at the range margins, demographic fluc- tuations and range shifts because of long-term climate changes (Loveless & Hamrick 1984; Foll & Gaggiotti 2006). A pressing challenge in population genetics is to disentangle the effects of each process (Grivet et al. 2008; Gaggiotti et al. 2009; Zellmer & Knowles 2009). Identifying those having the greatest impact is a logical first step in developing predictive models for popula- tion responses to future environmental change (Ham- rick 2004; Petit et al. 2008; Jaramillo-Correa et al. 2009). Correspondence: Jeanne Romero-Severson, Fax: (574) 631 7413; E-mail: jromeros@nd.edu Ó 2010 Blackwell Publishing Ltd Molecular Ecology (2010) 19, 4876–4891 doi: 10.1111/j.1365-294X.2010.04834.x