| OCR Text |
Show Vastly different rates of evolution are seen in streams and lakes. Allopatric speciation in isolated basins and post- pluvial environments begins rapidly ( as shown by Hubbs and Miller), but often fails to produce new species. One possible exception involves extreme environmental change, small populations, and sexual selection, as hypothesized for pupfishes. Ecologically- driven divergence in long- lived lakes may have rapidly produced ecologically and morphologically divergent species. Fluctuating pluvial- interpluvial cycles resulted in extinction of all but generalist survivors and a few relict endemics, not rapid proliferation of species. This observed extinction and infrequent origin of new species seem to be the consequence of two factors- extreme loss of habitat during interpluvials and rapidity of Pleistocene fluctuations ( assumed to be on an orbitally- driven cycle of about 100,000 years). The resulting ecological circumstances are probably not equatable with random variations in selective pressures; they were probably relentless and forceful reversals of directional selection, caused by nearly two dozen climatic cycles. The result was apparently large- scale stasis, notwithstanding rapid but superficial changes in morphology. Great Basin fishes exhibit the low diversity and high endemism characteristic of technically active areas. The low diversity suggests high extinction rates caused by high elevations, high gradients, and climatic instability. The action of these factors may be more important than volcanism, which is usually suggested as the cause of low diversity in tectonically active regions. It is surprising that the speciation rates are not as high as theory would predict for tectonically active areas with severe isolation. |
