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Reproductive isolation in a contact zone between divergent forms of winter wren (Troglodytes troglodytes) Toews, David P. L.

Abstract

Geographic variation in vocalizations and genetics of winter wrens (Troglodytes troglodytes) in North America have led to speculation that the eastern (Troglodytes troglodytes hiemalis) and western (T. t. pacificus) subspecies are in fact distinct biological species. To determine whether two regional forms are separate species it is crucial to gather data from an area of overlap between the groups, if such an area exists. To address whether these forms are reproductively isolated, I quantified song characteristics and two types of molecular markers, mitochondrial (ND2) and nuclear (amplified fragment length polymorphisms), in individuals in a recently described overlap area and compared them to those individuals from further east and west. In this overlap area, near Tumbler Ridge, BC, both forms can be found inhabiting neighboring territories and each male wren sings either an eastern or western song, with the differences between these types of song being as distinct as they are in allopatry. The two taxa differ distinctly in mitochondrial DNA and in every case singing type perfectly predicts mitochondrial DNA clade, strongly supporting the hypothesis that the two forms are reproductively isolated where they co-occur and are therefore separate species. Analysis of multilocus nuclear markers supports this result, with only one first generation hybrid individual identified and no evidence of genetic introgression. A n estimate of the initial split between these two species, based on mitochondrial DNA sequence variation, dates to just prior to the Pleistocene, roughly 2.3 million years ago. It is therefore suggested that T.t. pacificus and related western taxa be elevated Troglodytes pacificus with the common name "Pacific wren". I speculate that behavioural isolation due to song evolution is likely an important premating barrier to gene flow in this system. These findings raise the possibility that there are other such morphologically cryptic species pairs in North America.

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