Studies bolster empirical evidence for speciation without geographical barriers, but some experts remain unconvinced
By Stuart Blackman | February 9, 2006
Geographical barriers can cause populations to differentiate genetically and morphologically, but real world evidence of sympatric speciation - the emergence of two species from a single, mixed-parent population, without the aid of geography - has proved elusive. Two studies published this week in Nature claim to provide solid examples.
One study found that two species of palm tree endemic to Lord Howe Island in western Australia met the criteria for sympatric speciation - specifically, they were sister species that diverged long after the island was formed, 6.9 million years ago. "We've ticked all the boxes," co-author William Baker at the Royal Botanic Gardens, Kew, UK, told The Scientist.
Previous research has attempted to provide examples of sympatric speciation, but appear less than watertight. To convincingly demonstrate sympatric speciation, the species must be true sister taxa that are reproductively isolated while occupying overlapping ranges. However, there must also be little opportunity for allopatric speciation, or speciation as a result of geographic isolation -- a condition that has proven hard to rule out completely.
"Empirical studies have been lagging behind the theoretical work," said Axel Meyer at the University of Konstanz, Germany, who led one of the new studies. "There's a handful of examples - probably less than ten - where the majority would concur that the data support a sympatric [scenario]."
Baker and his colleague Vincent Savolainen produced dated phylogenetic trees based on multiple nuclear and mitochondrial genes for Howea forsteriana and H. belmoreana along with their closest relatives, and found the two species met the criteria for sympatric speciation. "The chances of geographical isolation between differentiating populations on such a tiny island is highly unlikely," said Baker, "especially as they're wind pollinated and produce vast amounts of pollen."
The two species are also segregated ecologically, with H. forsteriana preferring calcareous soils, and H. belmoreana predominating on neutral and acidic soils. Intriguingly, the timing of flowering in each species depends to some extent on the type of soil a plant grows on, raising the possibility that this kick-started their reproductive isolation.
Meyer's work focused on a different sort of island - an isolated crater lake in Nicaragua that is home to two species of cichlid fish. Again, molecular phylogenies revealed them to be sister taxa, and Meyer's team showed that they were distinguishable on the basis of genetic, morphological, and ecological (stomach contents) criteria.
"Both these cases are most parsimoniously interpreted as sympatric speciation," said Jerry Coyne at the University of Chicago. Still, he questioned whether the species are truly sister taxa, and didn't reach the island or crater lake from separate invasions from a source population. "If there's a little bit of hybridization between the species, they're going to become genetically more similar to each other than either is to a mainland species," Coyne told The Scientist. "They'd look like sister species when they're not." He noted that the two palm species do indeed form hybrids, albeit infrequently. In the crater lake, an overlap in the respective morphologies of the two cichlids suggests they may also hybridize.
Nick Barton, a population geneticist at the University of Edinburgh, UK, told The Scientist that both systems rank among the best candidates for sympatric speciation identified to date. But he and Coyne, neither of whom contributed to either study, remain unconvinced that they are much more robust than a 1994 study of cichlids in Cameroonian crater lakes, to which they also did not contribute. The strength of that study, said Coyne, is that it involved a monophyletic group of several species from a single lake, which is harder to explain by multiple independent colonizations from a source population.
Meanwhile, "the real issue is knowing how widespread sympatric speciation is," said Barton. "That's really hard to get at, given that it's so hard to even demonstrate one example." Coyne noted that the palms represent the first botanical example of sympatric speciation. "Maybe people need to start looking for it in plants," he said.
Links within this article
V. Savolainen et al, "Sympatric speciation in palms on an oceanic island," Nature, published online, February 8, 2006.
Royal Botanic Gardens, Kew
M. Barluenga et al, "Sympatric speciation in Nicaraguan crater lake cichlid fish," Nature, February 9, 2006.
U.K. Schliewen et al, "Sympatric speciation suggested by crater lake cichlids," Nature, 368:629-632, 1994.
J.A. Coyne and T.D. Price, "Little evidence for sympatric speciation in island birds," Evolution, 54: 2166-2171, 2000.