Rapid radiation and hybridization contribute to weak differentiation and hinder phylogentic inferences in the New World mallard complex (Anas spp.) / Philp Lavretsky, Blanca E. Hernandez-Banos, and Jeffrey L. Peters.

Includes bibliographical references (pages 535-538).

Of the 13 taxa composing the Mallard complex, 4 occur in North America: the sexually monochromatic American Black
Duck (A. rubripes), Mexican Duck (A. [platyrhynchos] diazi), and Mottled Duck (A. fulvigula), and the dichromatic Mallard
(Anas platyrhynchos). Although morphologically distinct, inferring the evolutionary relationships of this group is
confounded by extensive genic sharing due to incomplete lineage sorting and ongoing hybridization. The objective of
this study was to examine the underlying cause (i.e. incomplete lineage sorting vs. contemporary gene flow) of
phylogenetic uncertainty. Whereas most taxa were fairly structured at mitochondrial DNA, a ‘‘starburst’’ pattern of
divergence consistent with a rapid radiation was recovered with 17 nuclear introns. Furthermore, nuclear-based
divergence estimates and tests of population structure recovered Florida and West Gulf Coast Mottled Ducks as welldifferentiated
and genetically diagnosable from each other and the remaining taxa, whereas Mallards, American Black
Ducks, and Mexican Ducks were indistinguishable. In general, neither population structure analyses nor coalescentbased
gene flow estimates conclusively identified the presence of hybrids or significant gene flow, suggesting that
genetic similarity within the group is largely influenced by incomplete lineage sorting. However, we also cannot reject
potentially high levels of gene flow. Moreover, inconsistent relationships among species trees indicated that
phylogenetic results were sensitive to which individuals were included. Taxa within the New World group are
phenotypically distinguishable, yet genetically similar and seemingly lack the apparent reproductive isolation that is
consistent with early stages of (incomplete) speciation. Future work should focus on genomic regions under selection
to better understand the stage of speciation among the various incipient forms.