George C. Williams; his ideas on natural selection widely held

The cause was Parkinson’s disease, said his wife, Doris.

Dr. Williams played a leading role in establishing the now-prevailing, though not unanimous, view among evolutionary biologists that natural selection works at the level of the gene and the individual and not for the benefit of the group or species.

He is “widely regarded by peers in his field as one of the most influential and incisive evolutionary theorists of the 20th century,’’ said Douglas Futuyma, a colleague and the author of a leading textbook on evolution.

Dr. Williams laid out his ideas in 1966 in his book “Adaptation and Natural Selection.’’ In it, he seized on and clarified an issue at the heart of evolutionary theory: whether natural selection works by favoring the survival of elements as small as a single gene or its components, or by favoring those as large as a whole species.

He did not rule out the possibility that selection could work at many levels. But he concluded that in practice, species advantage almost never happens and that selection should be understood as acting at the level of the individual gene.

In explaining an organism’s genetic adaptation to its environment, he wrote, “one should assume the adequacy of the simplest form of natural selection’’ — that of variation in the genes — “unless the evidence clearly shows that this theory does not suffice.’’

The importance of Dr. Williams’s book was immediately recognized by evolutionary biologists, and his ideas reached a wider audience when they were described by Richard Dawkins in his book “The Selfish Gene’’ (1976).

Those ideas have continued to draw attention because group selection still has influential advocates. In highly social organisms like ants and people, behaviors like altruism, morality, and even religion can be more directly explained if selection is assumed to favor the survival of groups.

Dr. Williams had a remarkably open mind, which allowed him to consider alternatives to his own ideas. David Sloan Wilson, a leading advocate of group selection, recalled in an interview that as a graduate student he once strode into Dr. Williams’s office saying he would change the professor’s mind about group selection. “His response was to offer me a postdoctoral position on the spot,’’ Wilson said.

Wilson did not take the position but remained close to Dr. Williams, though the two continued to differ. One matter of dispute was whether a human being and the microbes in the gut and the skin could together be considered a superorganism created by group selection. Dr. Williams did not believe in superorganisms. (Nonetheless, when Wilson came to visit him one day, Dr. Williams had taped to his door a hand-lettered sign saying, “Superorganisms welcome here.’’)

Dr. Williams’s interests extended to questions that evolution seemed not to answer well: Why should a woman forfeit her chance of having more babies by entering menopause? Why do people grow old and die when nature should find it far easier to maintain a body than build one?

An important article he wrote in 1957 on the nature of senescence led to a collaboration with Randolph Nesse, a psychiatrist at the University of Michigan. They developed the concept of Darwinian medicine, described in the 1995 book “Why We Get Sick.’’ The authors offered explanations for questions like why appetite decreases during a fever or why children often loathe dark green vegetables.

By choosing important subjects, Dr. Williams remained relevant. His ideas were approachable because he wrote in clear, simple prose and largely without the use of mathematics