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Science Frontiers ONLINE No. 110: Mar-Apr 1997 |
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Cichlids Punctuate Equilibrium
In those pesky cichlid fish of African lakes we may be seeing punctuated evolution during an actual punctuation.
Responding to the article in SF#108 on the Lake Victoria cichlids, A. Mebane called our attention to Lake Malawi. While the Lake Victoria cichlids seem to have evolved a profusion of species in a space of about 12,500 years, those cichlids in Lake Malawi may have done the same in only a century or two.
T. Goldschmidt advances this evenmore-abbreviated time scale in his book Darwin's Dreampond. In it, he discusses how the water level of Lake Malawi fell more than 120 meters during the 1800s -- an exceptionally dry period in Africa. Today, the Lake is again high and once more host to isolated rocky islands, each with its own unique complement of cichlid fish; each island has species found nowhere else in the lake. Where did all these species come from, considering that their little islands were bone dry just a century ago? Goldschmidt writes:
"Cichlids that inhabited these exposed rocks would have suffocated, unless they had already left for wetter climes. Yet today, species that do not exist anywhere else can be found near almost every rocky island. From an orthodox point of view, the most plausible explanation for this is quite surprising: many color forms as well as biological species developed over a period of less than two hundred years."
This is certainly explosive speciation -- real biological punctuation! But, perhaps as water levels fell, the original cichlids found refuges in surviving pools and then repopulated the lake when the waters rose. But is it reasonable to believe that they all sorted themselves out so perfectly that many species are found nowhere else in the lake? The probability seems high that cichlid speciation has been very rapid -- too rapid, one would think for random mutation and the slow feedback of natural selection to accomplish this daunting task in just a century or two. Might there not be some additional nonsupernatural factor at work?
(Goldschmidt, Tijs; Darwin's Dreampond, Cambridge, 1996, p. 125.)