Molecular Ecology (2007) 16, 4028 – 4038 doi: 10.1111/j.1365-294X.2007.03423.x © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd Blackwell Publishing Ltd Gene flow in Dubautia arborea and D. ciliolata: the roles of ecology and isolation by distance in maintaining species boundaries despite ongoing hybridization E. A. FRIAR,* J. M. CRUSE-SANDERS † and M. E. McGLAUGHLIN ‡ *Rancho Santa Ana Botanic Garden, 1500 N. College Ave. Claremont, CA 91711, USA, †Biology Department, Salem College, 601 S. Church Street, Winston-Salem, NC 27108, USA, ‡Department of Biology, University of South Dakota, 414 East Clark Street, Vermillion, SD 57069, USA Abstract The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time. The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness. Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow. In this study, we examine gene flow and population differentiation among populations of two species of the Hawaiian silversword alliance, Dubautia arborea and D. ciliolata. We compare divergence in putatively neutral microsatellite markers with divergence in leaf morphometric traits, which may be selectively important or physiologically linked to selectively important traits. Gene flow between populations was found to be significant in only one of the two species, D. arborea. Leaf morphometric differentiation between species was significant, though not among populations within species. No evidence of effective genetic introgression was observed between apparently ‘pure’ populations of these species. Gene flow as measured by microsatellites was not correlated with geographic distance between populations, but was correlated with the linear placement of the widest part of the leaf. Because these two species are interfertile, as demonstrated by the presence of active hybrid zone, the lack of genetic introgression and the maintenance of species boundaries may be associated with natural selection on differential habitat. Keywords: Dubautia, gene flow, Hawaiian silversword alliance, hybridization, microsatellites Received 19 November 2006; revision accepted 10 May 2007 Introduction Gene flow is an important factor that connects evolutionary processes across a landscape. Particularly among diverged lineages, the level and direction of gene exchange dictates the zone of contact for hybridizing species. Some models of gene flow predict an isolation-by-distance effect from a source area (Wright 1943; Friedman & Adams 1985; Nurminiemi et al. 1998). These models predict that gene flow is most common over short geographic distances, and decreases as geographic distance increases. If, in contrast, gene flow fits a landscape model (Handel 1983; Campbell & Dooley 1992; Kaufman et al. 1998; Sork et al. 1999), patterns of gene flow would be explained more by ecological, demographic, or morphologic qualities of the population, such as high pollen movement within particular ecological types, regardless of the distance from the source. The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time (Haldane 1948; Ehrlich & Raven 1969; Endler 1977). The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness (Haldane 1948; Hendry et al. 2001, 2002; Lenormand 2002; Saint-Laurent et al. 2003; Hendry & Taylor 2004). Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow (Schluter 2000; McKinnon et al. 2004). One can compare these two hypotheses by comparing patterns of genetic divergence and gene flow with a pattern of divergence as a proxy for natural selection, typically a Correspondence: Elizabeth Friar, Fax: 909-626-7670, E-mail: elizabeth.friar@cgu.edu