Cryptic speciation in a Holarctic passerine revealed by genetic and
bioacoustic analyses
DAVID P. L. TOEWS and DARREN E. IRWIN
Molecular Ecology 2008 (June)
There has been much controversy regarding the timing of speciation events in
birds, and regarding the relative roles of natural and sexual selection in
promoting speciation. Here, we investigate these issues using winter wrens
(Troglodytes troglodytes), an unusual example of a passerine with a
Holarctic distribution. Geographical variation has led to speculation that
the western North American form Troglodytes troglodytes pacificus might be a
distinct biological species compared to those in eastern North America (e.g.
Troglodytes troglodytes hiemalis) and Eurasia. We located the first known
area in which both forms can be found, often inhabiting neighboring
territories. Each male wren in this area sings either western or eastern
song, and the differences in song are as distinct in the contact zone as
they are in allopatry. The two singing types differ distinctly in
mitochondrial DNA sequences and amplified fragment length polymorphism
profiles. These results indicate that the two forms are reproductively
isolated to a high degree where they co-occur and are therefore separate
species. DNA variation suggests that the initial split between the two
species occurred before the Pleistocene, quite long ago for sister species
in the boreal forest. Surprisingly, the two forms are similar in
morphometric traits and habitat characteristics of territories. These
findings suggest that sexual selection played a larger role than habitat
divergence in generating reproductive isolation, and raise the possibility
that there are other such morphologically cryptic species pairs in North
America.
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