"Odd fly uncovers evolution secret" (2005)

A unique fly from the Canary Islands has helped shed light on one driving force behind the birth of new species, Nature magazine reports this week.

The type of robber fly is found nowhere else, and scientists speculate that the rich biodiversity on the islands may actually have led to its emergence.

The researchers think sharing an island with a myriad of other lifeforms may push one species to spawn another.

This new theory adds fresh insight into how biodiversity arises.

"Why some areas contain greater species diversity than others has been a fundamental question in evolutionary ecology and conservation biology," said co-author Brent Emerson, of the University of East Anglia, UK.

Genetic drift

It is thought "speciation" - the evolution of a new species - can occur when two populations of the same species become isolated, allowing them to "grow apart" genetically over the course of many generations.

Eventually, the two populations become so different that if they were to meet again they would no longer be able to breed, meaning they had become separate species.

One species can also evolve into another if strong selective forces are placed upon it (where certain genes or genetic traits are favoured by natural selection), or if its population is small enough to allow for "genetic drift", which happens when certain traits are lost - or become proportionately more common - simply because the gene pool has shrunk.

But exactly what drives speciation is still not fully understood by scientists, and it is an area of intense research.

By carefully studying animals and plants in the Canary and Hawaiian Islands, Dr Emerson and his colleague Niclas Kolm were able to show an apparent link between biodiversity and the evolution of new species.

They found that endemic species, such as the predatory robber fly ( Promachus vexator ), are more common in places that are bustling with many different species. Therefore, they speculate, new species are more likely to evolve if they are surrounded by an already rich biodiversity.

Species competition

"Imagine you have an island colonised by 100 species and a similar island colonised by 10 species," explained Dr Emerson. "If you leave that for a period of evolutionary time, the percentage of entirely new forms will be higher on the island with 100 species on it."

The researchers can think of three reasons why this might be the case. First, species that are forced to share a space with a lot of other species usually have smaller population sizes. That means they are more susceptible to genetic drift, which can speed up speciation.

Secondly, islands with a rich biodiversity have more habitat complexity. In other words, instead of just one habitat - say, grass - there is, for example, grass, shrubs and trees. That means species are more likely to evolve new adaptations and, eventually, become different species.

Thirdly and, the researchers believe, most importantly, competition between species can encourage speciation.

"We think the islands with more species have an increased interaction effect - and that is the most significant thing," said Dr Emerson. "So the more species you have, the more competitors and predators you face as an individual.

"And that puts pressure on you that can lead to your extinction or you can adapt to that pressure and survive and that would result in a new species forming."

Tropical diversity

This new research could help explain why islands in warm areas (which tend to start off with a richer biodiversity than colder areas), like Hawaii and the Canary Islands, tend to have a high proportion of totally unique species.

Professor Axel Meyer, of Konstanz University in Germany, who is eminent in the field of speciation, says the research is very interesting - if it stands further scrutiny.

"It is very thought provoking," he told the BBC News website. "I'm sure it will have people rushing to their computers to see whether this pattern holds up and it will be interesting to see if it does hold up in other systems."

He also stressed that a rich biodiversity could not entirely explain a rich biodiversity because, of course, you had to start somewhere.

"They are saying that if you have biodiversity it will create more biodiversity - I can buy that. But it still doesn't explain the initial step: how do you get more biodiversity in the first place?"