Silvereye birds help scientists to settle disputed evolutionary theory

A study of birds on Pacific islands shows conclusively that the formation of new species is a gradual and not a sudden process, according to biologists from the UK, France, Australia and the USA writing in the Proceedings of the National Academy of Sciences today (28 May issue).

The `founder effect` theory, a controversial idea among biologists, says that speciation occurs suddenly due to a small influx of colonists founding new populations, in the process creating many new gene combinations and losing many others, in what is known as a `genetic revolution`.

But according to the team`s new evidence from fieldwork and computer modelling, the theory doesn`t apply to island birds, and the way in which populations change their genetic diversity is a result of successive colonisation events and long-term genetic drift.

“Our results indicate that speciation in island birds occurs gradually, not suddenly as a result of island colonisation through founder effects,” said Dr Sonya Clegg of Imperial College London.

In order to carry out the work, Dr Clegg and her collaborators visited a series of islands through the south west Pacific.

Dr Clegg said: “The result is exciting because this is the first time the theory has been tested using natural populations. Previous tests have used artificially introduced ones, which don`t tell you much about how real biodiversity evolves. It`s obvious that genetic changes can occur if a single pair of individuals founds a population, but the question is whether that really happens. Our results suggest that it doesn`t.”

The scientists tested the founder effect model in an unusual island bird species, the Silvereye (Zosterops lateralis), which has colonised a series of islands in the south west Pacific from the Australian mainland during the last 200 years.

The Silvereye`s pattern of colonisation there was originally used to support the founder effect model, first proposed by Ernst Mayr in 1954. Though subsequently tested in lab studies and artificial populations, up until now the theory had been untested in a natural population because of the need to know the colonisation date for a series of island populations.

The south west Pacific islands benefit from a well-documented history of such events, and DNA samples were taken from birds captured on the islands to be compared with samples from Silvereye colonies on the Australian mainland and with samples from island populations of Silvereyes known to be founded over 3,000 years ago. This allowed the scientists to make the crucial contrast between genetic changes in recent and old island populations.

Their computer models of evolution based on these data showed that single founder effects do not lead to strong genetic changes in the Silvereye population, contrary to the founder effect model. Instead, genetic changes build up gradually – either as a result of long-term genetic drift or though multiple bottlenecks. The authors estimate that the average successful founding flock would number more than 100 birds.

Laboratory-based studies have shown that founder effect models can work, but only under very restrictive circumstances, for example, with a very small number of colonists. A natural population with known colonisation dates is very rare and most studies have been done on artificially introduced populations, which limit the value of subsequent results because humans have set the size of the colonising group.

The Australian Research Council and the Stuart Leslie Fund (Birds Australia) funded the work.

For further information please contact:

Dr Sonya Clegg
Department of Biological Sciences
Imperial College at Silwood Park
Ascot, Berkshire SL5 7PY, UK
Tel: +44 (0)20 7594 2359
Email: sonya.clegg@ic.ac.uk

Tom Miller
Press Office, Imperial College
Tel: +44 (0)20 7594 2624
Mob: +44 (0)7803 886248
Email: t.miller@ic.ac.uk