Genetic admixture supports an ancient hybrid origin of the endangered Hawiian duck / P. Lavretsky, A. Engilis Jr, J.M. Eadie, and J.L. Peters.

Includes bibliographical references (pages 1013-1015).

Speciation is regarded primarily as a bifurcation from an ancestral species
into two distinct taxonomic units, but gene flow can create complex signals
of phylogenetic relationships, especially among different loci. We evaluated
several hypotheses that could account for phylogenetic discord between
mitochondrial DNA (mtDNA) and nuclear DNA (nuDNA) within Hawaiian
duck (Anas wyvilliana), including stochastic lineage sorting, mtDNA capture
and widespread genomic introgression. Our results best support the hypothesis
that the contemporary Hawaiian duck is descended from an ancient
hybridization event between the mallard (Anas platyrhynchos) and Laysan
duck (Anas laysanensis). Whereas mtDNA clearly shows a sister relationship
between Hawaiian duck and mallard, nuDNA is consistent with a genetic
mosaic with nearly equal contributions from Laysan duck and mallard. In
addition, coalescent analyses suggest that gene flow from either mallard or
Laysan duck, depending on the predefined tree topology, is necessary to
explain contemporary genetic diversity in Hawaiian ducks, and these estimates
are more consistent with ancient, rather than contemporary, hybridization.
Time since divergence estimates suggest that the genetic admixture
event occurred around the Pleistocene–Holocene boundary, which is further
supported by circumstantial evidence from the Hawaiian subfossil record.
Although the extent of reproductive isolation from either putative parental
taxon is not currently known, these species are phenotypically, genetically
and ecologically different, and they meet primary criteria used in avian taxonomy
for species designation. Thus, the available data are consistent with
an admixed origin and support the hypothesis that the Hawaiian duck may
represent a young hybrid species.