The myriad of colorful cichlid fishes of the African Great Lakes is a classic example of explosive evolution, with thousands of species having appeared in the geological equivalent of a blink of an eye. Now, in a paper in this month's issue of Molecular Ecology, biologists report a close-up look at one spark in that burst of evolution.
Lake Malawi, shared by Malawi, Mozambique, and Tanzania, is home to at least 500 species of cichlids, all of which probably took less than a million years to evolve from a common ancestor. Still, evolutionary biologist J. Todd Streelman of the University of New Hampshire, Durham, and his colleagues were not prepared for the speed of evolution they discovered at the lake's Thumbi West Island.
Here, at a 100-meter-long promontory called Mitande Point, a fish dealer in the 1960s released Cynotilapia afra, a species restricted to the other end of Lake Malawi. Twenty years later, the fish hadn't ventured beyond Mitande Point. But in 2001, when the team dipped nets into the water at six spots along the island's 5-km-long coastline, the scientists found that it had spread everywhere. At each station, they netted some 40 individuals and recorded the color pattern for each. They also took a so-called microsatellite DNA fingerprint.
As it turned out, C. afra had evolved into two distinct varieties in less than 20 years. The ones along the northern coast of the island had developed about four vertical blue bars on the black dorsal fin, whereas the ones along the southern coast had only two or three. (The original stock had no blue bars on the dorsal fin.) Also, DNA fingerprints of fish from the north coast sites were significantly different from those of fish in the south, making it likely that the two are well on their way toward becoming separate species. Fish evolution this hasty has been recorded so far only in salmon and sticklebacks. "We were not expecting [this]," Streelman says.
Evolutionary ecologist Jacques van Alphen of Leiden University in the Netherlands is amazed at the speed with which C. afra has split in two. "It's a very nice finding," he says. But he hopes the researchers will continue their work to find out what is different about the two sides of the island to have caused the two forms to evolve.