Multilocus test of the absence of mtDNA phylogeographic structure in a widespread wader, the Common Sandpiper (Actitis hypoleucos)

Although geographically structured mitochondrial DNA (mtDNA) trees often present straightforward evidence for independently evolving lineages, unstructured mtDNA trees are often interpreted as inconclusive or even misleading inferences of phylogeographic history. Some authors believe that stochastic coalescent processes or natural selection can result in mtDNA-based gene trees that fail to capture existing geographic structure. In this study, we sequenced four introns and one exon to test a published mtDNA tree that showed a lack of phylogeographic structure in the Common Sandpiper (Actitis hypoleucos). Neither individual nuclear gene trees nor analyses based on the five genes combined showed geographic structure in this widespread Palearctic wader. Multilocus neutrality tests suggested that the unstructured mtDNA tree was not biased by natural selection although the variation pattern at the exon, Mc1r, was not neutral. Coalescence-based analyses showed that this species experienced a recent population expansion, which could explain the lack of population structure. Overall, the mtDNA results were consistent with those based on nuclear loci in suggesting a lack of strong population structure and a recent population increase. However, multiple genes are needed to quantify demographic history such as timing and extent of effective population size change.