MITOCHONDRIAL GENE TREES AND THE EVOLUTIONARY RELATIONSHIP OF MALLARD AND BLACK DUCKS

We assayed restriction site differences in mitochondrial DNA (mtDNA) within and among allopatric populations of the Mallard (Anas platyrhynchos) and the American Black Duck (A. rubripes). The observed mtDNA clones grouped into two phylogenetically distinct arrays that we estimate differ by about 0.8% in nucleotide sequence. Genotypes in one clonal array were present in both species, while genotypes in the other array were seen only in Mallards. In terms of the mtDNA "gene tree," the assayed Mallards exhibit a paraphyletic relationship with respect to Black Ducks, meaning that genealogical separations among some extant haplotypes in the Mallard predate the species separation. Evidence is advanced that this pattern probably resulted from demographically based processes of lineage sorting, rather than recent, secondary introgressive hybridization. However, haplotype frequencies were most similar among conspecific populations, so the Mallard and Black Ducks cluster separately in terms of a population phenogram. The results provide a clear example of the distinction between a gene tree and a population tree, and of the distinction between data analyses that view individuals versus populations as operational taxonomic units (OTUs). Overall, the mtDNA data indicate an extremely close evolutionary relationship between Mallards and Black Ducks, and in conjunction with the geographic distributions suggest that the Black Duck is a recent evolutionary derivative of a more broadly distributed Mallard-Black ancestor.