![]() ![]() Here we present a synthesis of current data on DGF in a diverse group of ground squirrels, chipmunks of the genus Tamias, including previously unpublished genetic data and analyses that indicate that the many instances of mitochondrial introgression we have identified illustrate a gradual attenuation of gene flow with increasing depth of divergence ( Reid et al., 2012), which is one of the easiest to test predictions of DGF. Therefore, important questions in speciation focus on the extent and nature of gene flow that is occurring or has occurred during the process of lineage divergence and on the genomic architecture and trait associations characterizing that differentiation ( Smadja and Butlin, 2010 Wu, 2001) that is, how common is DGF (for example, Pinho and Hey, 2010) and how does it occur? This may be especially critical in instances of rapid radiations simply due to the fact that multiple radiating lineages can provide many opportunities for range fluctuations to result in temporally variable range overlap. Because range fluctuations often occur over shorter temporal scales than that of the evolution of complete reproductive isolation, speciation may frequently involve ephemeral periods of allopatric differentiation that are punctuated by intermittent periods of contact during which introgression between and among diverging lineages may take place (for example, Fitzpatrick and Turelli, 2006). However, as pointed out by Jiggins (2008) and Smadja and Butlin (2010), allopatric versus sympatric models of speciation represent extremes of a continuum, with no possibility of gene flow during divergence in the former and no (geographic) barriers to gene flow during divergence in the latter. This process differs from classical models of speciation, in which divergence is postulated to occur in allopatry or in the absence of gene flow between diverging lineages and in which hybridization on secondary contact causes homogenization of diverging lineages (for example, Taylor et al., 2006 Zitari et al., 2012). Increasing evidence has accumulated over the last two decades (for example, Rice and Hostert, 1993) that supports divergence with gene flow (DGF) as an important driver of biological diversity. Thus, the chipmunk radiation in western North America represents an excellent mammalian example of speciation in the face of recurrent gene flow among lineages and where biogeography, habitat differentiation and mating systems suggest important roles for both ecological and sexual selection. A recurrent observation is that these introgressions occur between ecologically and morphologically diverged, sometimes non-sister taxa that engage in well-documented niche partitioning. ![]() In addition, we have identified several taxa that are apparently fixed for ancient introgressions and in which there is no evidence of ongoing introgression. This includes ongoing introgression between subspecies and between both closely related and distantly related taxa. ![]() These new data (188 cytochrome b sequences) bring the total number of sequences up to 1871 roughly 16% (298) of the chipmunks we have sequenced exhibit introgressed mtDNA. quadrivittatus group in the central and southern Rocky Mountains. Here we synthesize data that have accumulated over the last dozen or so years on DGF in the chipmunk ( Tamias) radiation with new data that quantify very high rates of mitochondrial DNA (mtDNA) introgression among para- and sympatric species in the T. Although it is critical to explore the mechanisms driving divergence and preventing homogenization by hybridization, it is equally important to document cases of DGF in nature. In such cases, lineage divergence occurs on a shorter timescale than does the completion of reproductive isolation. ![]() Increasing data have supported the importance of divergence with gene flow (DGF) in the generation of biological diversity. ![]()
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