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William Donald Hamilton, one of the towering figures of modern biology and the man who helped to unify Darwin's principles of natural selection with a rigorous understanding of Mendelian genetics, died on Tuesday in Oxford, England. He was 63.
Dr. Hamilton was best known for a theory offering a genetic basis for altruism, the inspiration for Richard Dawkins's best-selling popularization, "The Selfish Gene," and for such well-known work by E. O. Wilson as "Sociobiology."
Middlesex Hospital in London, where Dr. Hamilton had spent the previous six weeks, said the cause of death was malaria contracted on a recent expedition to Congo. He was there seeking evidence to bolster a radical hypothesis that the AIDS epidemic can be traced to contaminated polio vaccines.
Those who knew Dr. Hamilton said his zealous pursuit of the theory was characteristic of his unslakable intellectual curiosity and his interest in ideas that others might initially dismiss as half-baked.
"He believed that if you have a weird idea that sounds wrong at first, you should give it a chance before you throw it out," said Nancy Moran, a biologist at the University of Arizona who studied with Dr. Hamilton.
Dr. Hamilton, a professor at Oxford University since 1984, burst into the field of evolution while still a graduate student at Cambridge University.
In 1963 and 1964, he published two papers based on his doctoral work that have proved so seminal to evolutionary biology that it is virtually impossible to read a contemporary study in the discipline without encountering his name and the term he coined, inclusive fitness, also known as kin selection.
Through the model of inclusive fitness, Dr. Hamilton proposed an elegant and mathematically sophisticated way of understanding altruistic behavior, a problem that had baffled naturalists from Darwin onward.
If organisms are inherently selfish, and supposedly devoted to personal survival and reproduction, why, scientists wondered, do so many species display seemingly self-sacrificial behavior? Why, for example, do worker bees forsake the opportunity to breed in favor of caring for the queen's young? And why will those infertile auntie bees commit suicide in defense of the hive?
Dr. Hamilton realized that the unusual genetic structure of the bees resulted in the workers being so closely related to one another that, in slaving for the hive, they were essentially slaving for the persistence of their own gene pool. In other words, although they appeared altruistic, they were, from a gene's-eye view, behaving with characteristic selfishness.
Dr. Hamilton thus recast the concept of fitness, that is, an individual's success in reproducing, to incorporate the survival and reproductive success of the creature's close relatives -- hence the term inclusive fitness.
In so doing, he merged Darwin's focus on individual animals competing for the privilege of siring the next generation with Mendel's studies of how distinct genetic traits are transmitted over time.
The idea can be roughly understood by one biologist's remark in a pub that he would "gladly die for two brothers, four cousins or eight second cousins," each of them carrying the requisite percentage of the individual's genes to compensate for the mortal deed.
Though human altruism is more complicated than that, and people are capable of behaving with profound self-sacrifice for nonrelatives, research has shown that the general principles of inclusive fitness and kin selection apply throughout the natural world, and in many human transactions as well.
Dr. Hamilton also published important papers explaining what he called "extraordinary" sex ratios, which occur when an organism produces far more offspring of one sex than the other.
More recently, he sought to understand the existence of sex, and why most species propagate sexually rather than asexually.
He suggested that sexual reproduction, by continually shaking up the genome, helps to keep organisms one step ahead of their parasites, which might be capable of annihilating a population of asexually spawned clones.
The parasite-avoidance hypothesis of sex remains debatable, but many studies of how animals choose their mates at least partly support it.
Dr. Hamilton, who made insects the focus of his work, was as renowned as a natural historian as he was as a theorist.
"He was the best field biologist I've ever met," said Marlene Zuk, a biologist at the University of California at Riverside who studied with him in the 1980's at the University of Michigan. "If you went out with him in England, he could identify every bird, every plant, every insect, practically every microorganism he encountered."
He had feelings of fraternity with the objects he studied, even when they were, say, wingless male fig wasps in combat.
In a 1996 collection of his papers called "Narrow Roads of Gene Land," he wrote, "A male's fighting movements can be summarized thus: touch, freeze, approach slowly, strike, and recoil."
Although the wasp's fighting style might look cowardly, he continued, it becomes understandable if one likens it "in human terms to a darkened room of full of jostling people" among whom are "a dozen or so maniacal homicides armed with knives."
William Donald Hamilton was born on Aug. 1, 1936, in Cairo, but moved to England as a young child. He was educated at Tonbridge School and St. John's College, Cambridge.
He was a large, vigorous man with a full head of white hair, who could be seen in the early morning doing pull-ups from a tree. Friends said he was reclusive, almost shy, but that he was also a risk-taker, physically as well as intellectually. As a teenager, he blew off parts of two fingers in an experiment with explosive chemicals.
Dr. Hamilton's close observations of the natural world made him rethink phenomena that others took for granted.
Recently, he suggested that autumn leaves turn brilliant colors not simply as a result of their loss of green chlorophyll, as is commonly believed, but as a way to warn off insect pests that might lay eggs on the tree, red and orange being common warning colors in nature.
He has also argued that cloud formations are actually bacterial dispersal vehicles, for microbes have been found in abundance in clouds.
Dr. Hamilton was a member of the Royal Society of London and the winner of many prizes, including the Darwin Medal, the Linnean Medal and the Crafoord Prize, awarded by the Swedish Academy of Sciences.
He was separated from his wife, Christine Friess. He is survived by their three daughters and by his partner, Luisa Bozzi.