A type of Asian bird gradually changes across its range into a separate species with its own song and colour pattern, says a B.C. researcher whose findings challenge the prevailing view that evolution only occurs in geographical isolation.

His study, published Friday in the online issue of Science magazine, implies that the world may not be taking the right approach to protect endangered species.

Darren Irwin, a zoologist at the University of British Columbia, and his team analysed genetic characteristics in the blood of greenish warblers.

They found that as the bird population spreads across Siberia's Tibetan Plateau, its genetic makeup changes and it eventually becomes an entirely different species that doesn't interbreed with the first.

Irwin's research calls into question the predominant theory that evolution only takes place when a species is isolated – by huge glaciers or on an island, for example – and adapts to new surroundings.

"Until now, no one has been able to show continuous gene flow between reproductively isolated species via geographically connected populations – a process of evolution called 'speciation by distance,'" Irwin said in a statement.

The songbird's gradual evolution adds up to some of the most convincing evidence yet found to support evolutionary theory, the researchers said.

Irwin, who studied the birds for 10 years, warned their findings could mean that current conservation methods won't work.

"Much of endangered species law relies on identifying distinct groups that are reproductively isolated from other groups, and only those distinct groups are targeted for protection," he said.

"Our findings show that in some cases there are not well-defined groups, but rather a gradient of forms. In such cases, the whole gradient of forms needs to be conserved."

He also noted that humans are destroying the songbirds' habitat so quickly that he doubts they could have done the study a decade later.

"Ten years from now, I'm not sure I would be able to find this same evidence."

Irwin conducted the research with colleagues from his university, Lund University in Sweden, and the University of Chicago.