Mark Ridley

The inherited instructions by which a body is built are carried from parent to offspring in molecular form, in the DNA. The instructions come in units, called genes, and for most purposes the DNA can he imagined as a long row of genes, arranged like books along a bookshelf. It is not known exactly how many genes are used to build a human body, but the number is between 100,000 and a million, and is probably nearer the former. Evolution (on this simple conception of the DNA) happens when the environment changes and one form of a gene rather than another builds a better body in the new conditions, or when a gene mutates to an improved form. Either way, natural selection favours the superior form of the gene, which becomes more numerous. Evolution in this sense means a change in gene frequency. It is a powerful way of thinking about evolution, and evolutionary biologists use it all the time. Taking one gene at a time, they study (or, at a more rarefied theoretical level, imagine) different forms of the gene, and see how natural selection works on them.

The ecosystems of shallow marine waters – coral reefs, for example – are the most diverse in the modern oceans, and they have probably been so throughout the history of life. And yet they are under-represented in the fossil record. For environments that are rich in life are also rich in the means of destroying it. When a shrimp or fish dies, it is rapidly devoured by scavenging crustaceans or decomposed by bacteria: every trace of the organism is destroyed in the process. In order for a fossil to be left, the dead organism must somehow be sheltered from the grave-robbing crabs, starfish and bacteria that thrive in shallow-water environments. An animal stands a much better chance of being preserved as a fossil in an anoxic deep-water environment, where there are fewer bacteria and almost no scavengers. The fossil record for oxygen-rich shallow waters is much less complete.

Sir Peter Medawar is the most distinguished biologist in Britain today. His work on immunology during the 1950s is the inspiration of all modern transplantation surgery, and was judged worthy of a Nobel Prize. In addition to his technical work, he also writes and lectures on science to a broader audience, and with more grace and wit than any one else who has tried his skill in that field. He has held many of the high offices open to a scientist in this country; he has delivered with uniform excellence the most prestigious lectures sponsored by the universities and learned societies; and, as the readers of this journal and its New York counterpart know, he is outstandingly the best reviewer of scientific books in the English-speaking world.

One of the many curious discoveries made, earlier this century, by ethologists such as Konrad Lorenz and Niko Tinbergen was that fighting in animals is restrained and, as they called it, ‘ritualised’. Animal contests, over such valuable resources as food, territory or mates, almost resemble tournaments, which pass through a regular series of harmless stages, before one animal emerges as the winner, and the other retreats unharmed as the loser. Take the cichlid fish Cichlasoma biocellatum, whose contests are described by Lorenz in his book On Aggression. A fight between two males passes through three main stages, at any of which one of the contestants may back out. They start with broadside displays, move on to tail beating, and then to harmless mouth fighting, in which the pair grip and pull each other by the mouth. The rules of the contest are, according to Lorenz, strictly obeyed. Each fish only moves on to the next stage when the other is ready. More strikingly still, if one of the fish finds itself in possession of a temporary but irregular advantage, it will not unfairly press it home. In Lorenz’s own words, ‘one of them may be inclined to go on to mouth-pulling a few seconds before the other one. He now turns from his broadside position and thrusts with open jaws at his rival who, however, continues his broadsides threatening, so that his unprotected flank is presented to the teeth of his enemy. But the aggressor never takes advantage of this; he always stops his thrust before his teeth have touched the skin of his adversary.’ Behaviour does not come much more gentlemanly than that.