Evolution with full stops and commas

  • 30/01/1996

Evolution with full stops and commas 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 the fittest finches on the island - producing the maximum offspring and sending most copies of their genes into the next generation. "If the hybrids fly off to a new island, or if they back-cross to one of the parental species and their offspring are very fit, the population will be altered in a new direction, or perhaps go off on a new evolutionary trajectory," says Peter.

The Venezuelan guppies studied by John Endler (University of California, Santa Barbara, US), promise further insight into evolution. Endler observed the interplay of two evolutionary forces - natural and sexual selection - as they drive changes among populations of guppies in the streams of Venezuela"s Paria Peninsula and Trinidad"s Norton Range. In 1930, renowned geneticist Ronald Fisher, then at Pothamstead Experiment Station, UK, pointed out that sexual selection puts same amount of pressure on both the sexes. Since females that choose the most attractive males will tend to have the most attractive offspring (that will pass on more of the family genes), the female who is most discriminating of male courtships should do the best, genetically speaking. As females evolve greater powers of discrimination, they put pressure on males to refine or elaborate their displays: the better the male displays, the more selective the females become.

Endler is now interested in what triggers this change in sexual response. He is trying to implicate what biologists call the "sensory drive", which biases the animals" sensory systems to favour one sexual signal over another. These biases could help to shape the male trait that becomes the object of female desire. Endler has, therefore, selectively bred female guppies with a cultivated predilection for the colours blue and red. Now he aims to discover whether these females, with their altered colour vision, will choose males with the brightest blue or red tails. And, if they do so, whether they may force an evolutionary change in the male colour patterns.

Yet another remarkable study is that of Dolph Schluter of the University of British Columbia in Vancouver, Canada. Schluter has reported on a population of stickleback fish that began to change shape and feeding habits when a new competitor forced the fish toward a different ecological niche.

Macro problem
All these experiments confirm Darwin"s theory of evolution within a species. And this most biologists do not dispute at all. What they are uncomfortable with is macroevolution - the idea that all higher creatures, including humans, have evolved from simple creatures like amoeba and bacteria. And there is not enough evidence to suggest that we took the Darwinian route.

Some palaeontologists are, however, sticking to a middle path. A recently published survey of 58 studies since 1972 co cludes that in many of these studies, palaeontological evidence overwhelmingly supports a view that speciation is sometimes gradual and sometimes punctuated. Jackson, however, thinks that most of the studies supporting gradualism are flawed, arguing that since the researchers relied on only one characteristic to monitor evolutionary change, they could not be sure they had identified all the species.

But evolutionary biologists are still hard put to explain how punctuated evolution actually works. One much-discussed possibility is that species become caught in an "adaptive gridlock." This gridlock results when a particular species is under a lot of pressure for natural selection; the species cannot go anywhere because movement in one direction has implications for its other competing functions. For instance, if a shellfish could reduce the weight of its shell, it might have a better chance of escaping fast-moving predators. But a lighter, thinner shell would also decrease resistance to other predators who bore through their victims. So the species remains unchanged for millions of years until a part of the population, isolated in a new environment, quickly evolves into a new species.

The Cambrian outburst of life poses a formidable challenge to Darwin"s hypothesis. The popular "empty barrel" hypothesis compares the biosphere of the Cambrian era to that of the early American West - a huge expanse of wilderness that suddenly was to-let to settlers. Following the early rush to grab space, it became increasingly difficult for latecomers to establish their moorings. Some writers have compared the struggle for an explanation to the Cambrian explosion to the difficulty faced by physicists in understanding Newton"s laws in the face of peculiar behaviour of light. It has caused experts to wonder if the twin premises of Darwinian evolution are really sufficient to account for the history of life. "What Darwin described in The Origin of Species," remarks Queen"s University palaeontologist Narbonne in Time, "was the steady background kind of evolution. But there also seems to be a non-Darwinian kind of evolution that functions over extremely short time periods, and that is where all the action is."