Findings: Birds confound founder theory

New species emerge through gradual evolution and not through sudden jumps, according to a new study of sea birds, writes Steve Farrar.

Computer modelling and analysis of DNA from populations of silvereyes, a species of bird that has colonised islands across the southwest Pacific, indicated that slow genetic drift and repeated colonisation of new habitats are more likely to shape evolution.

The results deal a blow to the founder effect theory, which holds that the emergence of new species can occur abruptly when a small group forms a new population isolated from its neighbours. The idea is that this tiny group can create many new combinations of genes and hence ultimately could become a new species.

Research by Sonya Clegg, a biologist at Imperial College, London at Silwood Park, and colleagues from France, Australia and the US, suggest that in the case of the silvereye, this is not the case.

"The results are exciting because this is the first time the theory has been tested using natural populations - previous tests have used artificially introduced ones that do not tell you much about how real biodiversity evolves," Dr Clegg said.

The scientists were able to find the dates when silvereyes colonised a series of Pacific islands from Australia over the past 200 years.

DNA from these populations were compared with genetic material taken from silvereye colonies on the Australian mainland and island colonies known to have been founded more than 3,000 years ago.

Genetic differences between these groups were fed into computer models of evolution.

The analysis suggested that genetic changes built up gradually and the founder effect did not have a strong impact.

The scientists estimated that the average successful founding flock would number more than 100 birds while laboratory studies on the founder effect have dealt with far smaller populations.

The findings are published in the Proceedings of the National Academy of Science .