Selective neutrality of mitochondrial ND2 sequences, phylogeography and species limits in Sitta europaea

Variation and geographic differentiation in mitochondrial DNA (mtDNA) was studied in the widespread and phenotypically variable Eurasian nuthatch (Sitta europaea). To assess whether sequences were evolving in a selectively neutral fashion, we used McDonald-Kreitman [Nature 351 (1991) 652] tests and a tree-based method, which suggested that although ND2 sequences are affected by natural selection against slightly deleterious alleles, the effects do not compromise phylogeographic inferences. Three phylogenetic species-level clades of nuthatches were discovered, corresponding to the Caucasus, southern Europe, and northern Europe plus Asia. Unimodal mismatch distributions within each clade suggest that populations have undergone recent growth. A westward range expansion was inferred from the geographic pattern in nucleotide diversity. Although samples were insufficient, it is possible that nuthatches in England and Japan are recently differentiated. Two specimens of the subspecies S. e. arctica formed a sister group to all other S. europaea, differing by ca. 10% uncorrected sequence divergence, pointing the need for additional study of this phenotypically distinct taxon. As with other species, mtDNA data support major phenotypic distinctions, but not subspecies.