Does evolution happen at a snail’s pace?

Press, 28 October 1981, Page 24

Does evolution happen at a snail’s pace?

From “The Economist,” London

Evolutionists are sharply divided about how one species evolves into another: whether by sudden leaps or gradual changes. Now a discovery has been made that could help to settle the issue. What has been found is an unbroken record of how 19 species of freshwater snail evolved in an African lake during the past few million years. Snails may sound like an unlikely key to how evolution works. But*, unlike the usual fragmentary nature of the fossil evidence, there are no gaps in the record of these snails, no missing links. There are millions of intact fossils. It is "an uncensored page of fossil history,” as the magazine “Nature” put it when reporting the results this month. It could prove the most important evolutionary discovery for many years.

The theory of evolution that has held sway since the 1940 s is called the modern synthesis because it combines Darwinism with more recent discoveries, especially in genetics. This theory describes evolution as being typically a gradual process, resulting from the slow accumulation of tiny changes in genes.

In the early 19705, a new school of thought led by two American scientists, Dr Stephen Jay Gould and Dr Niles Elredge, dismissed the modern synthesis, as old hat. They argued that a species remains unchanged for long periods, abruptly disappears and is then suddenly replaced by a new species. They called this process punctuated equilibrium. Punctuationism has been gaining an increasing number of adherents, especially among those whose expertise is fossils rather than genetics. A central issue between two schools is the rarity in the fossil evidence of intermediate

stages linking distinct species. The punctuationists say the missing links never existed; the gradualists tend to say the fossil record is simply too meagre to show them. This is where the fossil snails come in. They were discovered on the shores of Lake Turkana in Kenya, at a site where Mr Richard Leakey has found some of the most important fossils of man's immediate ancestors. Mr Leakey concedes that the importance of the snails eclipses fossils of early man.

The snails have been exhaustively studied by Dr Peter Williamson. Unhappily, there is disagreement about what his analysis shows. Dr Williamson himself says his results support the punctuationist school. Certainly they provide strong evidence of species remaining unchanged for millions of years and then suddenly changing into a new species, in 500050,000 years. But the gradualists never exclude the possibility of species remaining stable for long periods. How could they? There are living “fossils” lixe coelacanth fish that have survived unchanged since the days of the dinosaurs. One problem is semantic: what constitutes sudden change? To a fossil expert, 50,000 years is an instant, well within the margin of error involved in dating his fossil. To a geneticist, who can generate significant evolutionary change within a few years by keeping two laboratory populations of fruit flies at different temperatures, 50,000 years is an aeon. Gradualists and punctuationists are also likely to argue

about whether, during the, 50,000 years when Dr Williamson’s snails evolved into new species, there were recognisable intermediate forms linking the old and the new species. There seems to have been greater variation between individuals within the snail population during this period of rapid evolution. This was a time when the water level of the lake had fallen drastically. Presumably, species that had been well adapted to high water levels suddenly found themselves at a disadvantage. So there was an opportunity for experiment, to evolve forms adapted to the new environment. But the snail record contains surprises for both evolutionary schools. One is that the rate of evolution was the same both in snail species that reproduced sexually and in species that produced asexually. This challenges the accepted wisdom that the invention of sex speeded up the rate of evolution, by providing a means to shuffle genes around between individuals.

Another surprise is that new snail species emerged from populations of millions of snails ratner than small, isolated “founder” populations. It had been widely thought that members of a species undertaking evolutionary experiments have to be isolated in order to flourish. The most sensible conclusion to draw is that both gradualists and punctuationists should rethink their theories. Researchers also need to repeat Dr Williamson’s detailed analysis on other populations of fossils that provide uncensored pages of evolutionary history. There should be plenty of similar examples, if only fossil hunters devoted less time to newsworthy species like apemen and dinosaurs and more time to invertebrates, like snails, which (because they are small and less fragile) are found in greater numbers. What might then emerge, by the late 1980 s, is a new “modern synthesis,” replacing both the present one and the rival punctuationist school. Such a synthesis would combine the knowledge of geneticists, fossil experts and molecular biologists.

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