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Songbird shows how evolution works
The greenish warbler may provide the evidence Darwin lacked
By BBC News Online science editor Dr David Whitehouse
Scientists may be witnessing one of the fundamental forces of
evolution: the divergence of one species into two.
It is the evidence that the originator of the theory of evolution, Charles
Darwin, wanted to see but was never able to find.
The new data comes from the songs of the greenish warbler, a bird
that lives in the foothills of the Himalayas. Researchers have noticed
that its song changes gradually throughout its territory.
At the extreme ranges of its habitat, the greenish warbler will sing
very different songs. This means there are some birds in the territory
which, although they belong to the same species, will not mate
because they do not recognise each other's calls. Eventually, the two
singing groups will become two separate species
One becomes two
To biologists, this separation process is known as speciation.
"One of the largest mysteries remaining in evolutionary biology is
exactly how one species can gradually diverge into two," says
Darren Irwin of the University of California, San Diego, US.
The Himalayan warblers are an example of a rare condition
known as a "ring species".
"Ring species are unique because they present all levels of
variation, from small differences between neighbouring
populations to species-level differences in a single group of
organisms," says Irwin.
Defending territories
The greenish warbler (Phylloscopus trochiloides) lives in a ringshaped region around the Himalayas with gradually changing
behavioural and genetic characteristics. The ring is broken in one
place, in central Siberia, where two forms of the songbird exist.
"This creates a paradox in which two co-existing forms of the
songbird can be considered as two species and as a single
species at the same time," remarks Irwin.
"Ring species are valuable because they can show all of the
intermediate steps that occur during the divergence of one
species into two. In the greenish warbler, as in most songbirds,
males sing to attract mates and to defend territories.
"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
become longer and more complex," says Irwin.
Recorded songs
Irwin and his co-researchers publish their bird study in the
journal Nature.
In their paper, they describe how they played recorded warbler
songs to same-species birds that sang in a different way. It was
clear to the team that the listening birds did not recognise the
"music" and would therefore be unlikely to breed with the
warblers that produced it.
"The greenish warbler is the first case in which we can see all the
steps that occurred in the behavioural divergence of two species
from their common ancestor," says Irwin.
"These results demonstrate how small evolutionary changes can
lead to differences that cause reproductive isolation between
species, just as Darwin envisioned."