>
> Subject: Fwd: [evol-psych] Biologists uncover Darwin's 'missing evidence'
> for divergence of species in a warbler's song
>
> Hey, Wishpush, this is too cool!
> best--krazykoyote
> Tweets,
>
> FYI.
>
> Gene Hunn, enhunn at Home.com, Seattle
>
> =======
> FOR RELEASE: 17 JANUARY 2001 AT 14:00 ET US
> University of California, San Diego
>
> http://www.ucsd.edu/
>
> Biologists uncover Darwin's missing evidence for divergence of species in
> a warbler's song
>
> Biologists at the University of California, San Diego have demonstrated,
> in a study of the songs and genetics of a series of interbreeding
> populations of warblers in central Asia, how one species can diverge into
> two.
>
> Their description of the intermediate forms of two reproductively isolated
> populations of songbirds that no longer interbreed is the "missing
> evidence" that Darwin had hoped to use to support his theory of natural
> selection, but was never able to find.
>
> "One of the largest mysteries remaining in evolutionary biology is exactly
> how one species can gradually diverge into two," says Darren E. Irwin, a
> biologist at UCSD who headed the study, detailed in the January 18 issue
> of the journal Nature. "This process, known as speciation, is very
> difficult to study because it can take a great deal of time to occur."
>
> Biologists have generally learned about the divergence of species by
> comparing many different species at various stages of speciation. But in
> their study of the greenish warbler, a songbird that breeds in forests
> throughout much of temperate Asia, Irwin and his colleagues' Trevor D.
> Price, a biology professor at UCSD, and Staffan Bensch, a former
> postdoctoral student at UCSD now at Sweden's Lund
>
> University discovered a rare situation known to biologists as a "ring
> species.""Ring species are unique because they present all levels of
> variation, from small differences between neighboring populations to
> species-level differences, in a single group of organisms," says Irwin, a
> former student of Price who is in the process of beginning his
> postdoctoral work with Bensch at Lund University.
>
> In the case of the greenish warbler, Phylloscopus trochiloides, the
> scientists discovered a continuous ring of populations with gradually
> changing behavioral and genetic characteristics encircling the Tibetan
> Plateau, which is treeless and uninhabitable. This ring is broken by a
> species boundary at only one place, in central Siberia, where two forms of
> the songbird coexist without interbreeding.
>
> "This creates a paradox in which the two co-existing forms can be
> considered as two species and as a single species at the same time," says
> Irwin. "Such ring species are extremely rare, but they are valuable
> because they can show all of the intermediate steps that occurred during
> the divergence of one species into two."
>
> In their paper, the scientists show how they discovered a gradual
> variation in the song patterns, morphology and genetic markers of 15
> populations of the greenish warbler. At each end of the ring of
> interbreeding populations, which extend around each side of the Tibetan
> plateau and through the Himalayas, the scientists found that the two
> distinct, non-interbreeding forms of the bird do not recognize each
> other's songs, which are critical in the selection of their mates. They
> determined this from experiments in which they played recordings of male
> greenish warbler songs and judged the response of other birds in the
> trees."In the greenish warbler, as in most songbirds, males sing to
> attract mates and to defend territories," says Irwin. "The greenish
> warblers living in the Himalayas sing songs that are simple, short and
> repetitive. As you go north along the western side of Tibet, moving
> through central Asia, the songs gradually become longer and more complex.
> On the eastern side of the ring, moving northwards through China, songs
> also become longer and more complex, but the structure is different than
> on the western side. Where the birds meet in Siberia, their songs are so
> different that they do not recognize each other as mates or competitors.
> They act like separate species, and the genetic evidence supports that
> conclusion.
>
> "Apparently, as the birds moved north along two pathways into the forests
> of Siberia, their songs became longer and more complex, perhaps because
> females in the north rely more strongly on song when choosing a mate. But
> the forms of complexity differed between west and east Siberia, because
> there are more ways to be complex than simple.
>
> "The greenish warbler is the first case in which we can see all the steps
> that occurred in the behavioral divergence of two species from their
> common ancestor. These results demonstrate how small evolutionary changes
> can lead to the differences that cause reproductive isolation between
> species, just as Darwin envisioned."
>
> The study was financed in part by the National Science Foundation and
> National Geographic Society.
> http://www.eurekalert.org/releases/ucsd-bud011601.html
>
>
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