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Abstract Divergence is often ephemeral, and populations that diverge in response to regional topographic and climatic factors may not remain reproductively isolated when they come into secondary contact. We investigated the geographical structure and evolutionary history of population divergence withinSceloporus occidentalis(western fence lizard), a habitat generalist with a broad distribution that spans the major biogeographical regions of Western North America. We used double digest RAD sequencing to infer population structure, phylogeny and demography. Population genetic structure is hierarchical and geographically structured with evidence for gene flow between biogeographical regions. Consistent with the isolation–expansion model of divergence during Quaternary glacial–interglacial cycles, gene flow and secondary contact are supported as important processes explaining the demographic histories of populations. Although populations may have diverged as they spread northward in a ring‐like manner around the Sierra Nevada and southern Cascade Ranges, there is strong evidence for gene flow among populations at the northern terminus of the ring. We propose the concept of an “ephemeral ring species” and contrastS. occidentaliswith the classic North American ring species,Ensatina eschscholtzii. Contrary to expectations of lower genetic diversity at northern latitudes following post‐Quaternary‐glaciation expansion, the ephemeral nature of divergence inS. occidentalishas produced centres of high genetic diversity for different reasons in the south (long‐term stability) vs. the north (secondary contact).
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Allopatric divergence and secondary contact with gene flow: a recurring theme in rattlesnake speciation
Abstract The study of recently diverged lineages whose geographical ranges come into contact can provide insight into the early stages of speciation and the potential roles of reproductive isolation in generating and maintaining species. Such insight can also be important for understanding the strategies and challenges for delimiting species within recently diverged species complexes. Here, we use mitochondrial and nuclear genetic data to study population structure, gene flow and demographic history across a geographically widespread rattlesnake clade, the western rattlesnake species complex (Crotalus cerberus, Crotalus viridis, Crotalus oreganus and relatives), which contains multiple lineages with ranges that overlap geographically or contact one another. We find evidence that the evolutionary history of this group does not conform to a bifurcating tree model and that pervasive gene flow has broadly influenced patterns of present-day genetic diversity. Our results suggest that lineage diversity has been shaped largely by drift and divergent selection in isolation, followed by secondary contact, in which reproductive isolating mechanisms appear weak and insufficient to prevent introgression, even between anciently diverged lineages. The complexity of divergence and secondary contact with gene flow among lineages also provides new context for why delimiting species within this complex has been difficult and contentious historically.
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- Award ID(s):
- 1655571
- PAR ID:
- 10114639
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Biological Journal of the Linnean Society
- Volume:
- 128
- Issue:
- 1
- ISSN:
- 0024-4066
- Page Range / eLocation ID:
- p. 149-169
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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