Darwinian sapience takes a beating as the notion of 'punctuated equilibrium' - which postulates that evolution occurred in sudden fits and starts - gathers shape

-- THE mystery of origin and evolution of life has boggled many minds and provided inspiration to several theories. But none - excepting perhaps the biblical account - has succeeded as famously as Charles Darwin's masterly enunciation of the logic of life. The central idea of Darwin's story of life is that organisms evolve by a combination of random genetic change and natural selection. Our intellectual tradition is so steeped in this idea that Darwinism has been exalted to a dogma. Hence, when respectable biologists produce evidence which flies in the face of Darwinian wisdom, it gives some cause for excitement.

But could Darwin really have been wrong? In recent years, Darwin's cherished theory has been put to much embarrassment by fossil and molecular testimonies, flaring into what Niles Eldredge, Harvard palaeontologist, calls the "great evolutionary debate". The protagonists of this drama are the diehard apologists of Darwin (the ultra-Darwinians) and the naturalists (those who want to reinterpret Darwin in the light of new fossil and molecular information).
Duelling over a dilemma Many do not see eye to eye with Darwin over the 'how' of evolution. Darwin acknowledged that evolution speeded up when conditions changed, but he also supposed that animals and plants must continue to evolve uninterruptedly (ultra-Darwinian viewpoint). But some modern biologists argue that new species appear suddenly in the geologic record after millions of years of evolutionary stupor. This controversial idea is rooted in the notion of 'punctuated equilibrium', championed by Eldredge and the celebrated science writer and Harvard palaeontologist Stephen Gould in the 1970s (naturalist viewpoint).

This year, the naturalists went one up over ultra- Darwinians. Alan Cheetham of the Washington -based Smithsonian Institution authored a study of filigreed remains of coral-like animals known as bryozoa, hoping to determine the pace at which new species had appeared during the past 15 million years. He discovered individual species persisting virtually unchanged for millions of years and then, in a geologic moment lasting only about 100,000 years, the rise of a new species.

Recognising new species on the evidence of fossils alone can be problematic. Cheetharn and his colleague Jeremy Jackson of the Smithsonian Tropical Research Institute in Panama have defused that criticism by testing their methods for distinguishing fossil species on living bryozoa. Their study has persuaded some palaeontologists that punctuated equilibrium has been the dominant mode of evolution.

Cheetharn meticulously classified the bryozoan fossils into 17 species using 46 microscopic characteristics of their skeletons, such as the length of the individual zooids (the animals that make up bryozoan colonies). But contrary to received wisdom, he found that through 15 million years of the geologic record, these species would persist unchanged for 2-6 million years and then, in less than 160,000 years, split off a new species that would continue to coexist with its ancestor species.

Cheetham and Jackson subjected their approach to two more rigorous tests. For, they argued, the strength of any discovery of punctuated equilibrium depends on the ability to recognise species. They first examined whether the morphological measurements used for fossils could also be applied to living species and whether the immediate environment alters their skeletal structure, making populations of the same species living in different environments look like separate species. A "yes" to the former and a "no" to the latter query confirmed their suspicions.

Pulse prognosis
Cheetham and Jackson have some competition. Timothy Collins (University of Michigan, US) and his colleagues, taking the same approach, found punctuated equilibrium in the evolution - over 20 million years - of a genus of snails called Nucella, studied along the Californian coast. Another supporter of Gould's hypothesis is Elisabeth Vrba of Yale University. She postulates that the big evolutionary changes - migrations, extinctions and the creation of new species - are triggered by climatic changes. To the extent that climate changes tend to happen in bursts and at regular intervals, major evolutionary changes also occur in bursts, or pulses. Vrba calls this idea the "turnover pulse hypothesis".

Vrba, who has spent much of her professional life in Africa studying the fossil record, is intrigued by the changes in evolution and climate which occurred some 2.5 million years ago. At that time, there were huge turnovers in fauna worldwide, and the earliest ancestors of true humans appeared. What is more, says Vrba, this burst of evolutionary activity is only one of several that occurred in Africa and elsewhere. If Vrba is right, humanity may owe its existence to a relatively short period of change in the world's climate.

But can her hypothesis be proved? Nobody disputes the idea that sudden environmental changes could trigger bursts of evolutionary upheaval. The contentious issue is whether such bursts could at times be global. Vrba's critics disagree with her. But does the fossil record show that animals changed dramatically en masse 2.5 million years ago? Critics say while some groups, like shrews or rodents changed, some like African pigs, did not. With other groups such as hominids, the data is simply too sparse to make any judgement.

In Vrba's view, neither pigs nor hominids can make the case. She has looked at fossil records of 20 different groups of closely related animals. The clearest data comes from the fossil of Bovidae, a group of hoofed animals which includes cattle, sheep, goats and antelopes. One particularly dramatic period of innovation occurred between seven and five million years ago when a bewildering variety of bovids came into being.

But this period of diversification seems to have given way to a wave of extinctions and replacements between three and 2.5 million years ago. A huge loss of genera occurred within the various bovid, tribes, and today's modem genera evolved. Vrba predicted that specialist creatures, like shrews, which feed on insects, slugs and worms that are very sensitive to climate, should respond to climate change more quickly than generalists such as pigs. And that is precisely what you see among African antelopes. Traditional Darwinism would predict a steady modification of the impala over three million years, even without climatic change, because it still needs to outrun cheetahs. But neither cheetahs, nor impalas have changed very much; they are both too versatile to be worried by climatic change. Competition between them does not - as Darwin supposed - provide sufficient selective pressure to cause them to alter.

An evolutionary Big Bang?
The second major shot in the arm for Gould's theory comes from recent studies of Cambrian fossils. The Cambrian period (600 million years ago) is distinguished by the abrupt appearance of an astonishing array of multicellular animals. The findings reinforce the view that biological change often occurs in fits and starts. And none of those fitful starts was more dramatic than the one that came shortly after the emergence of wormlike creatures, whose well-preserved remains were recently discovered in Namibia by Douglas Erwin, a Smithsonian palaeontologist.

Over the decades, evolutionary theorists, beginning with Darwin, have tried to argue that the appearance of multi-cellular animals during the Cambrian period merely seemed sudden, but in fact, had been preceded by a lengthy period of evolution for which the geological record was missing. But this explanation now seems unsatisfactory. Since 1987, discoveries of major fossil beds in Greenland, China, Siberia, and now in Namibia, have shown that the period of biological innovation occurred at virtually the same instant in geologic time all around the world. Biologists hitherto thought that this evolutionary Big Bang occurred over a period of 75 million years. But now, armed with latest dating techniques, scientists are finding that the incubation period was no more than 10 million years.

Advantage, Darwin
Those who doubt the importance of punctuated equilibrium can still take heart from earlier studies of fossil fresh water snails by Dana Geary, University of Wisconsin, who documented gradual change within two snail species over a period as long as two million years, along with six cases of punctuated speciation. Another verdict of gradual change came from Peter Sheldon of Open University in Milton Keynes, UK, who studied morphological change among trilobites from Wales.

That is not all. Evolution, at least within species, has become highly visible in the last two decades, something which Darwin never thought could be observed. As a result of recent advances in computer technology and molecular biology, ideas about evolution that have long been speculative are being bolstered by experimental data for the first time. The most famous of these - described in Jonathan Weiner's The Beak of the Finch - is a study on birds known as Darwin's finches. Peter and Rosemary Grant of Princeton University, US, observing the effects of seed size on the beak shapes of Galapagos finches since the early 1970s, have found that changes in the type of available seeds produce changes in beak size within a single year, from generation to generation.

Now the Grants are focusing on finches on the tiny desert islet of Daphne Major (Galapagos) for greater insight into how evolution works. A medium-beaked ground finch, Geospiza fortis is being bred with G fuliginosa, the small- beaked ground finch, and with G scandens, a cactus finch. While inter-breeding is rare among the three species, many of the hybrids are now surviving, increasing variability and creating differently shaped finches. The Daphne Major hybrids are among th

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