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[escepticos] La evolución es solo una teoría

SPECIES IN A WARBLER'S SONG    Ya sabeis lo que decía el gran actor y mejor
político Ronald Reagan. Pero los científicos siguen empeñados en aguar




Planeta Escéptica http://www.geocities.com/planetaesceptica

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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
other as mates or competitors. They act like separate species, and the
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.
UCSD on the we at http://ucsdnews.ucsd.edu

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