Why biologists believe in evolution

Press, 9 May 1981, Page 15

Why biologists believe in evolution

Recent articles in these pages on evolution, and the letters that followed them, have prompted this response from Dr DAVID LLOYD, Reader in Botany at the University of Canterbury, and a specialist in evolution studies.

Of all scientific theories, that of evolution has been subjected to the most prolonged and intensive public debate. The recent exchange of views in the columns of this newspaper testify to the contuining capacity of evolutionary ideals to inflame the passions of people, more than a century after the theory came to wide attention. The interest of the public in evolution derives largely from the implications that evolutionary thinking has on man’s view of his position in the world. The theory of evolution directly affects what we think about our origins and the causes of our actions, and hence concerns the very roots of our nature and our moral responsbilities. It is not surprising then that evolutionary debates often generate • more heat than light. Modern concepts of evolution are descended from the arguments presented by Charles Darwin in his book “On the Origin of Species,” published in London in 1859. Darwin marshalled the evi-

dence for evolution and proposed a mechanism for it — natural selection. Since then, biologists have refined, modified, and extended many of the views that Darwin expounded. Evolutionary theories have changed considerably, but the great majority of comparative biologists still uphold Darwin’s twin theories of “descent with modification” and “the survival of the fittest.” After 120 years of scrutiny . and refinement, evolution . now provides a much • stronger and more compre- ; hensive set of explanations ■ than the original exposition ■ was able to. Darwin necesi sarily concentrated on com- ! parative anatomy, paleontol- • ogy, and the geographical i distribution of plants and ! animals. Modern evolution- ; ary theory also encompasses genetics and molecular bio- • logy, subjects that were only i dimly glimpsed in Darwin’s ’ time. Concepts of evolution : and selection are being ap- ' plied to an increasing range . of animal and human behavi- ■_ our.

Together with other scientific revolutions, such as the Copernican postulate that the earth revolves around the sun rather than the reverse, Darwinism has caused us to question the traditional view of the world, in which humanity rules as a divinely privileged species occupying the centre of an unchanging universe.

An acceptance of evolution implies that we see the human species as one among several millions of products of a natural process, occupying but one of even more millions of planets in the universe. The attitude of people to Darwinism is closely connected to the extent to which they are emotionally willing to abandon a static anthropocentric worldview.

Evolutionary biology contains a complex of theories of different magnitudes to explain such matters as variation between individuals, the origin of species, the patterns by which dominant groups such as the mammals arose, the causes of evolution and extinction, and the course of . evolution in particular groups. Hence

there are innumerable theories of evolution, not just one.

Nevertheless, there is one central notion underlying all the particular theories of evolution — the idea that the organisms present today are derived by a continuous line of descent from past ancestors that were different. The key postulate of Darwinism is that “descent with modification” has occurred. It is often asked whether evolution has been proven. The answer depends on what one attempts to “prove.” In. the absence of a time machine, scientists cannot actually demonstrate - that the organisms of the world are all derived from a common ancestor, and they do not attempt to do so. To consider this inability to be a failure of evolutionary theory is comparable to denying the existence of the sun because we cannot travel to ’it, or of the Ice Age because there is no human record of it.

What science does demand of a hypothesis is that it is capable of being disproved, in the sense that it is possible to make observations

which contradict or discredit the hypothesis. Evolution meets this criterion. For instance, if we were to witness the formation of even a single species from non-bio-logical origins, our belief in evolution would be shattered. No convincing evidence has yet been put forward for such a novel creation. On the other hand, many instances of evolutionary change have been demonstrated in particular species. The evolution of DDT resistance in insect species is a well-known example. The origin of antibiotic resistance in micro-organisms can be demonstrated in a laboratory any time.in a matter of days. The house sparrow was introduced into New Zealand from England between 1862 and 1871. In the intervening years, populations differing in their body measurements have evolved in various parts of New Zealand. (The differences are greater among - males than among females, for unknown reasons.) Over a roughly similar interval, Introduced plants of white, clover on the southern slopes of the Port Hills,

beside the Summit Road, have evolved statistical differences in chemical properties from those 20 yards away on the other side of the road. The plants on the two sites are adapted to different conditions — one group gets the sun,' the other the cold southerlies.

• Evolutionary concepts explain much more than observed changes and the vastly more extended fossil record. Diverse facts of comparative anatomy, development, biogeography, and ecology are brought together into one explanatory framework by the concept of evolution. Why do amphibians, reptiles, birds, and mammals have four limbs and not two or six or some other number, if it is not because they have inherited this trait (or rather, the genetic instructions for the trait) from their common ancestor?

.Even the exceptions are admirably explained on the hypothesis that resemblances reflect common ancestry. Whales lack hind limbs, but they reveal their four-legged ancestry by the presence of a tiny pelvic girdle, as the skeleton of the blue whale in

the Canterbury Museum shows dramatically. The geographical distributions of species also reflect their evolutionary history. Large flightless birds, the ratites, inhabit the continental areas of the Southern Hemisphere — or in some cases did until man eliminated them. There are (or were) kiwis and moas in New Zealand, the emu and cassowaries in Australia and New Guinea, the ostrich in South Africa, the elephant bird in the Malagasy Republic, and rheas in South America.

How did these birds, which are unable to fly, reach the isolated lands they occupy, and why are they not present in Europe, Asia, and North America? The similarities between the various ratites suggest they are more closely related to each other than to any other bird and that they obtained their flightless conditions from their common ancestor rather than derived them independently after flying to the separate lands. That is, they walked to their present distributions when the southern continents were

joined together as the supercontinent Gondwanaland, isolated by ocean from the northern continents.

A recent comparison of the genetic similarity of the living- ratites by DNA hybridisation studies has shown that emus and cassowaries, the only living groups sharing the same land mass, are the most closely related of the five surviving groups. The kiwis are next most closely related, and ostriches and rheas are the most distinct from the others. The order of distinctness of the groups matches the order in which the southern continents separated from each other. The-various degrees of divergence therefore correspond to the relative lengths of time during which the groups have evolved in isolation.

The structural and geographical relationships of the ratites and those of other living organisms can be fully and consistently explained by postulating evolutionary change. No other rational hypothesis that appeals to observable forces can explain the biological diversity of the world as adequately.