In Memory of Bill Hamilton

A Tribute to W. D. Hamilton; The Times, London, 9 March 2000

Professor W. D. Hamilton, FRS, biologist, was born on August 1, 1936. He died on March 7 aged 63

The evolutionary biologist W. D. Hamilton was one of the leaders of what has been called "the second Darwinian revolution", and Richard Dawkins, author of The Selfish Gene, has called him one of the most important Darwinians of the 20th century.

Hamilton, who taught at Oxford for his last 15 years, proposed the theory of kin selection as an explanation of why animals are sometimes altruistic towards each other, despite the struggle for the survival of the fittest. He then moved on to the difficult problem of the evolution of sex from asexual reproduction, and proposed the now widely discussed parasite theory.

He began the now widespread effort among evolutionary biologists to make sense of a set of phenomena - in particular social behavior, sex andsenescence - which at first sight are hard to explain by a simple Darwinian model of the survival of the fittest. This work was initiated by Hamilton's two 1964 papers on The Genetical Evolution of Social Behavior, and he was a leader in the field ever afterwards. The papers were the result of his work as a doctoral student, when he was supervised by an economist at the London School of Economics and a statistical geneticist at University College London. Probably neither of them had the remotest idea what their student was up to, for he was always something of a loner.

The problem Hamilton set is easy to pose but hard to answer. If natural selection favours the survival and reproduction of the individual, how are we to explain behaviors such as the bee dance, in which a sterile worker informs an equally sterile sister about the nature and direction of a source of food?

By the time it reached the editor of the Journal of Theoretical Biology, Hamilton's 1964 paper had baffled two academic referees, though they admitted that it might be important. The third referee, John Maynard Smith, was at first equally baffled - Hamilton's mathematics are hard to follow now, and were harder then - but when he read the main argument, he pressed strongly for the paper to be published.

The argument began with ants, bees and wasps. These have a peculiar genetic system which results in a female having more genes in common with her sisters than with her daughters, a state of affairs which, Hamilton noted, "is very favourable to the evolution of reproductive altruism". This means that a gene which causes an individual to act so as to favour the survival of a relative will increase in frequency if, on average, more copies of the gene are transmitted to future generations by the individual or its relatives.

Hamilton had formulated the idea in a brief paper in 1963, where he argued that if there is a gene G which causes its carrier to act altruistically, "the ultimate criterion which determines whether G will spread is not whether the behavior is to the benefit of the behaver but whether it ithe benefit of the gene G". The key is to see the matter from the point of view of the genes, rather than the creatures.

It is reported that when T. H. Huxley first saw Darwin's theory of evolution by natural selection, he remarked "how stupid not to have thought of that", and many biologists had the same reaction when they read Hamilton's paper, offering as it did a whole new way of thinking about evolutionary questions.

This work became the basis of a continuing attempt to explain altruistic behaviour in terms of relatedness, not only in social insects, but at all levels of the living world, from the origin of life to the origin of human beings. Although first developed to explain self-sacrificing behaviour in animals, Hamilton's ideas became the basis of sociobiology, an attempt to reduce the social sciences to a branch of biology, and the later and less naive evolutionary psychology. Hamilton was sympathetic to these attempts to apply Darwinian ideas to human society, but his own work remainedgrounded in natural history.

If he had published nothing since 1964, he would still be one of the great scientific minds. But he went on generating ideas, essentially by tackling phenomena that at first sight do not make Darwinian sense. He wrote on the evolution of ageing, and of cases in which the numbers of males and females in a species are grossly unequal (foreshadowing the application of game theory to evolution).

His main preoccupation in later years, however, was the evolution of sex. Again, the problem is easy to formulate but hard to solve. The problem is as follows. Natural selection favours entities that reproduce rapidly. At the cellular level, reproduction is the process whereby one cell splits into two. In the sexual process, two cells fuse to form one. Why bother to do this?

Two classes of solution have been suggested. First, populations that reproduce sexually can evolve faster: secondly, such populations are better at getting rid of harmful mutations. Hamilton preferred the first class of answer, but he was unhappy with any explanation that depends on an advantage to the population rather than to the individual.

He saw that if sex is to be explained by an advantage to the sexual individual, the rate at which the environment is changing, and hence the requirement for an evolutionary response, must be very great. He then suggested that there is one feature of the environment that may change rapidly enough to give sexual individuals an advantage over females reproducing by virgin birth. There is a continuing evolutionary race between us and our parasites, and sexual individuals have an advantage in such a race. The matter is still controversial, but Hamilton's ideas look very likely to be part of the answer.

Hamilton's ability to continue producing original ideas depended on a rare combination of two characteristics: he was a brilliant and clear-thinkinventor of models, and, like Darwin, he was also a passionate and knowledgeable naturalist. The stimuli for his theoretical ideas came from details of natural history. For example, his ideas concerning the evolution of the sex ratio were stimulated by facts about parasitic wasps that few biologists would have known. Analogously, Darwin tells us that his ideas about evolution originated in observations on the finches of the Galapagos.

Hamilton was not a charismatic lecturer - he once stopped in the middle of a talk, staring into space for some two minutes while he tried to think out the answer to a question he had just raised - but at the individual level he was a stimulating teacher of young research workers, provided they did not expect to be told what to do.

He died at an age when he still had so much to offer, from malaria contracted on a trip to Africa in search of new challenges from the natural world that he loved. William Donald Hamilton was educated at Tonbridge School and St John's College, Cambridge, before becoming a lecturer in genetics at Imperial College, London, in 1964. He moved to the University of Michigan in 1978,

and since 1984 he had been Royal Society Research Professor in the zoology department at Oxford. He also held numerous visiting professorships abroad. He was elected to the Royal Society in 1980, received the Darwin Medal in 1988, the Linnean Medal in 1989 and the Frink Medal in 1991, as well as many other awards and honours.

In 1967 he married Christine Friess, who survives him, along with their three daughters. He is also survived by his partner of recent years, Luisa Bozzi.