There is now unequivocal evidence for global climate change; however, its potential impacts on evolutionary processes remain unclear. Many species have responded to contemporary climate change through shifts in their geographic range. This could lead to increased sympatry between recently diverged species; likely increasing the potential for hybridization. Recently, following a series of warm winters, southern flying squirrels (Glaucomys volans) in Ontario, Canada rapidly expanded their northern range limit resulting in increased sympatry with the closely related northern flying squirrel (Glaucomys sabrinus). This provided the opportunity to test the prediction that contemporary climate change can act as a catalyst creating conditions for the formation of hybrid zones. Following extensive sampling and molecular analyses (nuclear and mitochondrial DNA), we identified the occurrence of hybridization between sympatric G. sabrinus and G. volans. There was evidence of backcrossing but not of extensive introgession, consistent with the hypothesis of recent rather than historic hybridization. To our knowledge, this is the first report of hybrid zone formation following a range expansion induced by contemporary climate change. This is also the first report of hybridization between North American flying squirrel species.
We undertook a large-scale survey of the distribution of northern, Glaucomys sabrinus (Shaw, 1801), and southern, Glaucomys volans (L., 1758), flying squirrels in Ontario, Canada. Livetrapping was conducted along a northsouth transect spanning about 500 km, from 42.5°N to 47.2°N. During 2002-2004, we conducted 42 971 trap-nights at 26 sites and captured 232 northern and 538 southern flying squirrels. During 2002 and 2003, southern flying squirrels occurred >200 km farther north than we expected. However, the range of this species appeared to contract to the south by about 240 km after the winter of 2004. Weather and food data suggested that cold temperatures during January and February 2004 combined with a failed mast crop in the autumn of 2003 resulted in an energetic bottleneck and subsequent population crash. We speculate that prior to 2004 southern flying squirrels had expanded their geographic range in response to recent climate warming. In particular, the nine winters between 1994 and 2004 were relatively warm. By measuring the range expansion over this warm interval, we were able to estimate a rate of spread of 22 km per year, and a diffusion coefficient of 6.9 × 10 7 m 2 per generation.
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