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Thursday, 18 January, 2001, 14:32 GMT
Songbird shows how evolution works
Greenish warbler UCSD
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.

One of the largest mysteries remaining in evolutionary biology is exactly how one species can gradually diverge into two

Darren Irwin, UCSD
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 ring-shaped 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."

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