Plants and Animals

New Zealand Herald, Volume LXXV, Issue 23121, 20 August 1938, Page 10 (Supplement)

Plants and Animals

By R. A. FALLA, M.A.

■ t OT/T, OWING tho appearance Ll 1 some months ago of an article JL on the development of the science of genetics, a reader wiote to ask how far the modern study of heredity has confirmed or weakened the various theories that have been advanced to explain organic evolution of plants and animals. It is difficult to attempt an answer to this in a short article, but it is possible to say that genetical science has thrown light on the raw materials of evolution, a field in which the older theories relied on speculative guesswork. Natural Selection When Darwin postulated natural selection as the principal cause of evolution, he did not altogether overlook tho possibility that characters acquired in tho lifetime of an individual might be transmitted to offspring. But the difficulty in proving this experimentally led him to give more credit to natural selection. He considered that the raw material on which it worked consisted of variations, large and small, some inheritable, and some not. Those that were inheritable and at the same time gave their possessor some sort of advantage were naturally selected, and thus the fittest typos tended to survive. Breeders' Experiments J. B. S. Haldane, a capable exponent of the present position of Darwinism, has pointed out that artificial selection is practised experimentally by breeders, and that even natural selection may be sometimes observed at work. As an example, he quotes a

mixed wood of pine aiicV birch in Yorkshire which was divided into two about the year 1800 by a stretch of heath. By 1885 tho pines in one half were replaced by birches, and in the other half tho pines took charge. Both woods were inhabited by a species of moth, of which light and dark varieties occur, but the proportion of dark to light specimens was found to be as high as 96 per cent in the pine wood, and as low as 15 per cent in the birch wood. Environment's Influence This looked like a case of direct influence of environment, but experiment showed otherwise. Three generations of the dark race' were fed in captivity on birch leaves, but no light moths were produced as a result. That the difference had been brought about selectively was discovered by collecting the wings of moths left by owls that had discarded them while feeding. In the pine forest, where so few living lightcoloured moths could be found, nearly all the wings of owls' victims were light.. The paler moths, conspicuous against the dark vegetation, were gradually being exterminated in the pine forest. A Fact Overlooked In the same article, written more than ten years ago, Haldane states that there is another and far rarer kind of variation, known as mutation, which consists in the origin of a new gene. A gene is a 'divisible structure in the cell carrying heritary characters. Haldane

.The Mechanism of Evolution

elsewhere reminds his readers that any statement lie writes is likely to be out of date before it is printed. The f" rc ® of this reminder is shown by the fact that another geneticist—and probably Haldane also —writing in 1938, says or mutations, that "it is justifiable to conclude that mutations arc so frequent in nature that not only every individual but probably every chromosome carries one or more niutiuit llio reason whv this fact has been overlooked so long is that mutations are concealed in the hereditary mateiials of the organism due to a majority or them being recessive " Units of Measurement The last quotation is from Professor Theodosius Dobzhansky, of the California Institute of Technology, who points out that the study of the mechanism of evolution has now devolved upon geneticists. This comparatively new science has the advantage of working with definite units of measurement. These units, the genes, are arranged within the chromosome in definite number, and in fixed linear Order. According to their kind and to the manner in which they are distributed in tho chromosome they determine all tho external and internal characteristics of tho organism, their influence being only modified by environment. Theoretical Work Having established their unit of measurement, geneticists _ carry out their experiments 011 artificially-breu populations of animals or plants. lor convenience and ready comparison vinegar flies of the genus Drosoplnla are widely used for theoretical work. The practical geneticist, of course, works on some particular animal or plant, for example, wheat, that # it is desired to develop in several varieties. In the theoretical work on Drosopbila it has been found possible to observe mutations as they occur in laboratories and also in wild stock. Some of them affect the external appearance of tho insect, the colour of its eye or the shapo

of its wings. Other mutations' produce 110 noticeable change in the structure of the body, but cause a change in .viability of the insect. Avenue of Escape A disconcerting fact is that most mutations are deleterious to the organism. Tlicy would seem to presage eventual catastrophe, a very different picture from the progressive evolution that might be imagined as resulting from the action of natural selection on favourable variations. Dobzhansky points out that one possible avenue of escape from this apparent impasse is to take into account the equilibrium betweenan organism and its environment. It is precarious because environment can and does change, and if an organism is to survive it niuat adapt itself. Unless we allow that it can do so bv producing only favourable mutations in a given set of conditions, the only alternative is to admit that natural selection which can pick out mutations useful at a given time cannot prepare the organism for environmental change But the carrying of a lethal mutation, with all its risks, may do so. Strength and Weakness As a general 'picture of the mechanism of evolution this is admitted by Dobzhansky to be far from pleasing. But in reminding us that the words ''good" and "bad" have no meaning in a scientific context he also emphasises that the mechanical explanations which are tho greatest source of strength in science also constitute its greatest weakness.