"The cichlids of Africa's Lake Malawi are estimated to have formed 1,000 new species in only 500,000 years, lightning speed in evolutionary terms.
In the 1960s a fish exporter may have unwittingly set the stage for an evolutionary explosion when he introduced individuals of the species Cynotilapia afra to Mitande Point on the lake's Thumbi West Island. As of 1983, the species hadn't budged from Mitande Point."
When Streelman, then at the University of New Hampshire, Durham, and colleagues went to the island in 2001, they found the fish had evolved into two genetically distinct and differently coloured populations in less than 20 years - one on the north side of the island, the other on the south. Cichlid color patterns are important in mate selection, so these distinct markings may promote the evolution of new species."
Science Blog reporting Streelman et al., 2004. Hybridization and contemporary evolution in an introduced cichlid fish from Lake Malawi National Park J. Molecular Ecology, 13 (8): 2471-. (Requires NUS Digital Library access).
See also Smith, P. F., A. Konings & I. Kornfield, 2003. Hybrid origin of a cichlid population in Lake Malawi: implications for genetic variation and species diversity. Molecular Ecology, 12 (9): 2497 - 2504 (free access).