Evolutionists have provided what they hope is a definitive answer to the question of whether ‘sympatric speciation’ can occur in the natural world. This is where two or more species diverge from a common ancestor despite sharing the same habitat. The process has been taught as part of evolutionary theory for decades, but has been notoriously difficult to prove.

Evidence found: two species of palm on an island in Australia seem to have diverged side by side. Credit: MARK A. JOHNSON/CORBIS

There are candidate cases, such as where parasitic species seem to have diverged on the same host. But solid examples are hard to pin down, as they require showing that two or more species descended from a single ancestor, and that the lineages did not spend any time apart during the entire course of their divergence — often a span of several million years.

Two studies in disparate parts of the globe offer what their authors say are the best shots so far at such a proof. On page 719 of this issue, Axel Meyer and colleagues at the University of Konstanz in Germany describe two fish species that have apparently diverged despite sharing a relatively small lake (5 kilometres across) in Nicaragua. According to Meyer and his team, it's next to impossible that two such closely related species could have ended up in this very young (less than 23,000-year-old), isolated lake through two colonization events.

In a second study, Vincent Savolainen of Britain's Royal Botanic Gardens in Kew and his colleagues report that two species of palm tree seem to have diverged on a remote Australian island (V. Savolainen et al. Nature doi:10.1038/nature04566; 2006). In this case, the remoteness of the island and the shared genetic traits of the species make it extremely unlikely that the trees are descendants of two ancestors that arrived at the island on separate occasions.

“Almost no proposed example of sympatric speciation can be considered absolutely beyond doubt,” comments Jerry Coyne, an evolutionary biologist at the University of Chicago, Illinois. But in both of the newly reported cases, he says, sympatric speciation seems the most likely explanation. William Baker, part of the Kew team, agrees: “My first reaction is that it's hard to imagine a case being neater than ours — it sounds immodest but it's almost the ideal scenario.”

The big question now is whether sympatric speciation is widespread or rare. Although it is relatively easy to make a convincing case for a remote island or small isolated lake, things will be much tougher when biologists turn to more complex ecosystems such as East Africa's great lakes, home to hundreds of related fish species. “What would overturn our view of evolution would be to find that a fairly substantial proportion of all speciation events occur in sympatry,” Coyne says.

The concept that such speciation is widespread was largely resisted by the great evolutionist Ernst Mayr, who died last year, says Meyer. Such was Mayr's influence among biologists that he engendered a widespread bias against the idea. “But I think sympatric speciation will turn out to be more common than people thought,” Meyer adds.