Spatial genetic structuring in a vagile species, the European wood mouse W. Booth à , W. I. Montgomery & P. A. Prod ¨ ohl School of Biological Sciences, Medical Biology Centre, Queen’s University Belfast, Belfast, Northern Ireland Keywords population structure; dispersal; isolation by distance; habitat fragmentation; European wood mouse; small mammals; Apodemus sylvaticus. Correspondence P. A. Prod ¨ ohl, School of Biological Sciences, Queen’s University Belfast, Medical Biology Centre, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland. Email: p.prodohl@qub.ac.uk à Current address: W. Booth. Department of Entomology and W. M. Keck Center for Behavioral Biology, North Carolina State University, Campus Box 7613, Raleigh, NC 27695–7613, USA. Editor: Jean-Nicolas Volff Received 5 December 2008; revised 1 June 2009; accepted 2 June 2009 doi:10.1111/j.1469-7998.2009.00608.x Abstract We examined the genetic structure of natural populations of the European wood mouse Apodemus sylvaticus at the microgeographic (o3 km) and macrogeo- graphic (430 km) scales. Ecological and behavioural studies indicate that this species exhibits considerable dispersal relative to its home-range size. Thus, there is potential for high gene flow over larger geographic areas. As levels of population genetic structure are related to gene flow, we hypothesized that population genetic structuring at the microgeographic level should be negligible, increasing only with geographic distance. To test this, four sites were sampled within a microgeographic scale with two additional samples at the macrogeographic level. Individuals (n = 415) were screened and analysed for seven polymorphic microsatellite loci. Contrary to our hypothesis, significant levels of population structuring were detected at both scales. Comparing genetic differentiation with geographic distance suggests increasing genetic isolation with distance. However, this distance effect was non-significant being confounded by surprisingly high levels of differ- entiation among microgeographic samples. We attribute this pattern of genetic differentiation to the effect of habitat fragmentation, splitting large populations into components with small effective population sizes resulting in enhanced genetic drift. Our results indicate that it is incorrect to assume genetic homogeneity among populations even where there is no evidence of physical barriers and dispersal can occur freely. In the case of A. sylvaticus, it is not clear whether dispersal does not occur across habitat barriers or behavioural dispersal occurs without consequent gene flow. Introduction Most mammal species exhibit distinct levels of population subdivision associated with small effective population sizes and low dispersal rates, which are a consequence of both social and mating behaviour (Chepko-Sade & Halpin, 1987). However, unlike larger mammals, the frequency and distance of dispersal in small rodents, relative to their home-range size, are significant (Gaines & McClenaghan, 1980; Gliwicz, 1988, 1992; Krebs, 1991; Zhang & Usher, 1991; Halle, 1993). Assuming such dispersal results in gene flow, species with a high propensity for dispersal should display low levels of population genetic substructuring (i.e. close to panmixia) due to the homogenizing effect of gene flow on gene frequencies, which opposes the diversifying effects of genetic drift and natural selection (Slatkin, 1987). Dispersal in small mammal species, however, may be limited by geographic barriers that would not influence larger species, including wide stretches of open land, roads, water bodies or sparse hedgerows (Clarke & Johnson, 1990; Gerlach & Musolf, 2000; Arthur, Pech & Dickman, 2004; Berthier et al., 2005). The degree of genetic differences among populations is considerably influenced by recurrent levels of gene flow and, hence, geographic distance. Several studies on small rodent species demonstrated population genetic differentia- tion despite of the potential for dispersal (Calhoun & Greenbaum, 1991; Mossman & Waser, 2001; Berthier et al., 2005). However, inconsistencies observed between species indicate that patterns of population subdivision are highly heterogeneous, and assumptions of dispersal and population structure, at present, cannot be easily general- ized. Species with low rates of gene flow may also consist of many small subpopulations, each following more or less independent evolutionary pathways mediated by genetic drift and/or selection (Dallas et al., 1995). Small isolated populations are potentially more susceptible to genetic drift and inbreeding, which can increase genetic erosion and thus reduce their evolutionary potential to changing envir- onments. The European wood mouse Apodemus sylvaticus is widely distributed in the western Palaearctic, and is com- mon across a variety of habitats from arable land to Journal of Zoology Journal of Zoology 279 (2009) 219–228 c  2009 The Authors. Journal compilation c  2009 The Zoological Society of London 219 Journal of Zoology. Print ISSN 0952-8369