Phylogeographic studies that are based on only one or a few closely related species predominate in the literature, but the progressive accumulation of data means that we can now also start looking for geographical trends by comparing the genetic distributions of multiple species across a common geographical area. The advantages of this comparative approach are twofold. First, it may improve our understanding of the ways in which historical events have directly influenced the evolution of populations and species. If, for example, we suspect that a geological process such as the formation of a river was a vicariant event that separated multiple terrestrial populations, then we would expect to find similar levels of divergence between sister species from either side of the river. In this case, a consistent pattern could strengthen our argument that vicariance and not dispersal has promoted population divergence and speciation. Second, identifying common threads in the history of multiple taxa can be important from the perspective of conservation biology; to continue with our river example, we may find that the populations of many species on opposing banks are genetically distinct, and therefore a greater proportion of existing biodiversity would be preserved if some habitat was retained on both sides of the river as opposed to a longer stretch on just one side of the river. But how much agreement are we likely to find among a group of ecologically diverse taxa? A river will obviously have different impacts on species that can float, swim, fly or be wind-borne compared with those with a very low probability of crossing the water. Patterns of concordance may also depend on the spatial scales across which they are expected to occur, which is why we shall look first at some examples of comparative phylogeography at regional scales, and then extend this to see if any patterns remain at the continental level.
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