Fish speciate to have food to their taste
it's an old debate. Does it take specimens of the same species separated by time and space to speciate out. In a small west African lake, one fish species seems to be proving it does not. It is evolving into two species more because of different tastes. This fission supports the idea that species can divide without being separated in space or time.
At first inspection, Lake Ejagham in Cameroon has five species of Tilapia milling around in its waters. But Diethard Tautz, researcher at the University of Munich, and colleagues noticed that in one of these species, which has been given no common or scientific name, small fish tended to breed with other small ones while the big ones stayed with the other big ones. Research showed that the smaller cichlid fish spend most of their time in the lake's open water, probably feeding on plankton. The big fish prefer to stay at the edge of the lake, eating insects. Big ones lay their eggs in holes in fallen logs; small nest on the bare lakebed.
Genetic analysis and measurements of the fishes' body parts indicate that there is little genetic exchange between the two groups. The fish are nonetheless more closely related to each other than to any other species, a fact that is often hard to prove, says Tautz.
This pattern of ecological specialisation going hand-in-hand with differing taste in mates, fits with recent theories of how one population might split into two species. It also adds to a growing body of evidence, particularly from cichlid fish, that speciation can occur even in the absence of physical barriers that prevent populations from interbreeding. This 'sympatric' speciation seems occur rapidly, within just a a few thousand years.
Comparison of the dna of fish from the lake with others from nearby rivers suggests that Lake Ejagham was colonised about 10,000 years ago. No streams flow into it now and the only water outlet is a waterfall. The two proto-species must have split on the spot, believes Tautz ( Nature , June 19, 2001).
The most likely scenario, he says, is that the ancestor of today's big and small forms was intermediate between the two, but that the environment of the lake favoured extreme sizes, driving the two types apart.
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