Partitioning the effects of isolation by distance, environment, and physical barriers on genomic divergence between parapatric threespine stickleback.
Genetic divergence between populations is shaped by a combination of drift, migration, and selection, yielding patterns of isolation-by-distance (IBD) and isolation-by-environment (IBE). Unfortunately, IBD and IBE may be confounded when comparing divergence across habitat boundaries. For instance, parapatric lake and stream threespine stickleback (Gasterosteus aculeatus) may have diverged due to selection against migrants (IBE), or mere spatial separation (IBD). To quantitatively partition the strength of IBE and IBD, we used recently developed population genetic software (BEDASSLE) to analyze partial genomic data from three lake-stream clines on Vancouver Island. We find support for IBD within each of three outlet streams (unlike prior studies of lake-stream stickleback). In addition, we find evidence for IBE (controlling for geographic distance): the genetic effect of habitat is equivalent to geographic separation of ∼1.9 km of IBD. Remarkably, of our three lake-stream pairs, IBE is strongest where migration between habitats is easiest. Such microgeographic genetic divergence would require exceptionally strong divergent selection, which multiple experiments have failed to detect. Instead, we propose that nonrandom dispersal (e.g., habitat choice) contributes to IBE. Supporting this conclusion, we show that the few migrants between habitats are a nonrandom subset of the phenotype distribution of the source population.
Department of Integrative Biology, University of Texas at Austin, Austin, Texas, 78712.|Division of Biological Sciences, University of Montana, Missoula, Montana, 59801.
Jesse N
JN
Weber
Department of Integrative Biology, Michigan State University, East Lansing, Michigan, 48824.
Gideon S
GS
Bradburd
Department of Integrative Biology, University of Texas at Austin, Austin, Texas, 78712.
Yoel E
YE
Stuart
Department of Integrative Biology, University of Texas at Austin, Austin, Texas, 78712.|Department of Ecology and Evolutionary Biology, University of Colorado at Boulder, Boulder, Colorado, 80309.
William E
WE
Stutz
Department of Integrative Biology, University of Texas at Austin, Austin, Texas, 78712.
Daniel I
DI
Bolnick
D063147:Animal Distribution; D000818:Animals; D001955:British Columbia; D004777:Environment; D051456:Gene Flow; D016678:Genome; D060047:Reproductive Isolation; D023701:Smegmamorpha / Q000235:genetics*
Cline; Gasterosteus; gene flow; isolation by distance; isolation by environment; microgeographic divergence; parapatry; resistance