Lake Victoria is Africa's largest lake (420 kilometers long, but only 69 meters at its deepest). It is also the home of more than 300 species of cichlid fishes. Ordinarily, that number of different species would pose no problem for the biologists -- look at the 400 or so species of hummingbirds in Central and South America! Lake Victoria, however, is a very young lake, and all of these cichlid fishes are endemic. Therefore, they must have evolved rather rapidly.
Recent seismic surveys of Lake Victoria and piston cores from its deepest parts by T.C. Johnson et al have surprised everyone: Lake Victoria was completely dry 12,400 years ago. Nor were there deeper "satellite" lakes that could have served as refuges for Lake Victoria's biota during extreme droughts. It is difficult to avoid the conclusion that the present-day 300+ species of cichlid fishes all evolved in less than 12,400 years.
This being so, can random mutations -- the accepted source of evolutionary novelty -- have generated so many new species in such a short time? That would be one new species every 40 years or so on the average.
(Johnson, Thomas C., et al; "Late Pleistocene Desiccation of Lake Victoria and Rapid Evolution of Cichlid Fishes," Science, 273:1091, 1996)
Comments. Of course, hybridization may have accelerated the evolution of the 300+ species. Perhaps "adaptive" or "purposeful" evolution might have sped up the process, but this latter concept -- assuming it exists at all -- is not at all understood and highly controversial.
As Lake Victoria began filling up again after the Pleistocene drought, the many open niches must have resembled the situation on the Galapagos when the "pioneer" finches first arrived, took advantage of the many new opportunities for making a living and, as the story goes, evolved into the several species known as Darwin's finches.
