Abstract
BackgroundIdentifying causal relationships in phylogeographic and landscape genetic investigations is notoriously difficult, but can be facilitated by use of multispecies comparisons.Methodology/Principal FindingsWe used data visualizations to identify common spatial patterns within single lineages of four taxa inhabiting Pacific Northwest forests (northern spotted owl: Strix occidentalis caurina; red tree vole: Arborimus longicaudus; southern torrent salamander: Rhyacotriton variegatus; and western white pine: Pinus monticola). Visualizations suggested that, despite occupying the same geographical region and habitats, species responded differently to prevailing historical processes. S. o. caurina and P. monticola demonstrated directional patterns of spatial genetic structure where genetic distances and diversity were greater in southern versus northern locales. A. longicaudus and R. variegatus displayed opposite patterns where genetic distances were greater in northern versus southern regions. Statistical analyses of directional patterns subsequently confirmed observations from visualizations. Based upon regional climatological history, we hypothesized that observed latitudinal patterns may have been produced by range expansions. Subsequent computer simulations confirmed that directional patterns can be produced by expansion events.Conclusions/SignificanceWe discuss phylogeographic hypotheses regarding historical processes that may have produced observed patterns. Inferential methods used here may become increasingly powerful as detailed simulations of organisms and historical scenarios become plausible. We further suggest that inter-specific comparisons of historical patterns take place prior to drawing conclusions regarding effects of current anthropogenic change within landscapes.
Highlights
Landscape genetic and phylogeographic investigations generally rely on a three-step process: data generation, pattern identification, and pattern explanation
Both analyses suggested that no spatial genetic structure was present for S. o. caurina and P. monticola (Fig. 2)
Our analyses suggested that dominant patterns could be explained by historical processes that are not explicitly tied to landscape features
Summary
Landscape genetic and phylogeographic investigations generally rely on a three-step process: data generation, pattern identification, and pattern explanation. Though data generation is generally a straightforward process, the latter two stages can be difficult in many scenarios. Most genetic studies of natural populations produce large, complex data sets (i.e., data are obtained from relatively large numbers of organisms at numerous distinct spatial locations). The set of putative causal mechanisms that may explain said patterns must likewise be discerned. Identification of causal mechanisms becomes notoriously difficult in most cases due to the nonexperimental (non-manipulative) nature of investigations and the fact that only a single realization of the evolutionary process can be observed for a given species in a specific geographical region. Identifying causal relationships in phylogeographic and landscape genetic investigations is notoriously difficult, but can be facilitated by use of multispecies comparisons
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