OUR impact on the environment may be forcing evolution into reverse, collapsing recently formed species back into a single one. This “reverse speciation” may be robbing evolution of the raw material for future biodiversity.
For a species to split into two, something must block the flow of genes between the two incipient species. This happens when a physical barrier prevents gene flow, as when two populations end up on different islands, or become separated by inhospitable habitat. Another driver is behavioural differences: different breeding seasons, say, or insects choosing different food plants to live on can also set two lineages on a divergent path.
Eventually, the two fledgling species can become different enough genetically that they can no longer hybridise effectively, but if the barriers to gene flow come down too soon the two may hybridise and merge again. Just such a collapse appears to be happening today among Darwin’s finches in the Galapagos Islands, says Andrew Hendry, an evolutionary ecologist at the Redpath Museum of McGill University in Montreal, Canada.
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Records from the early 1960s show that on Santa Cruz Island, members of one finch species had either large or small bills but rarely medium-sized ones (see above left). The feature likely reflects two populations specialising in eating two different sizes of seeds, but when Hendry and colleagues measured bills of the same finch species four decades later they found that only birds living in sparsely settled parts of the island still showed two different bill sizes. Near the island’s only town, birds with middle-sized bills had become more common: the two formerly distinct groups of birds had collapsed into one (Proceedings of the Royal Society B, DOI: 10.1098/rspb.2006.3534).
“The two formerly distinct groups of birds had collapsed into one. It was no longer a disadvantage to have an intermediate beak”
The reason for the change, Hendry thinks, is that people are providing bird feeders filled with rice. “Everybody can eat this rice,” he says. “It’s no longer a disadvantage to be an intermediate beak.”
The same sort of collapse was observed earlier this year among fish in lakes on Vancouver Island, Canada, where there used to be one species of stickleback that fed near the lake bottom and another that fed closer to the surface. After an introduced crayfish began muddying the water, however, the two species began to feed in the same zone (Molecular Ecology, vol 15, p 343). A similar reverse speciation may be behind the collapsing diversity of cichlid fish of Lake Victoria in east Africa (91av, 28 February 2004, p 40).
Collapses such as these may be growing more common as human activity blurs the difference between habitats and introduces species into new regions. “There is good reason to believe that we are accelerating the rate of those reversals,” says Eric Taylor, a zoologist at the University of British Columbia in Vancouver, who led the stickleback research.
The species lost through this reverse speciation could play a crucial role in the long run. “They are the foundation of future biodiversity,” says Hendry.
