Why are worker ants sterile? Why do birds sometimes help their parents raise more chicks, instead of having chicks of their own? Why do bacteria explode with toxins to kill rival colonies? In 1964, the British biologist William Hamilton published a landmark paper to answer these kinds of questions. Sometimes, he argued, helping your relatives can spread your genes faster than having children of your own. For the past 46 years, biologists have used Dr. Hamilton’s theory to make sense of how animal societies evolve. They’ve even applied it to the evolution of our own species. But in the latest issue of the journal Nature, a team of prominent evolutionary biologists at Harvard try to demolish the theory. The scientists argue that studies on animals since Dr. Hamilton’s day have failed to support it. The scientists write that a close look at the underlying math reveals that Dr. Hamilton’s theory is superfluous. “It’s precisely like an ancient epicycle in the solar system,” said Martin Nowak, a co-author of the paper with Edward O. Wilson and Corina Tarnita. “The world is much simpler without it.”
Other biologists are sharply divided about the paper. Some praise it for challenging a concept that has outlived its usefulness. But others dismiss it as fundamentally wrong. “Things are just bouncing around right now like a box full of Ping-Pong balls,” said James Hunt, a biologist at North Carolina State University. Dr. Hamilton, who died in 2000, saw his theory as following logically from what biologists already knew about natural selection. Some individuals have more offspring than others, thanks to the particular versions of genes they carry. But Dr. Hamilton argued that in order to judge the reproductive success of an individual, scientists had to look at the genes it shared with its relatives. We inherit half of our genetic material from each parent, which means that siblings have, on average, 50 percent of the same versions of genes. We share a lower percentage with first cousins, second cousins and so on. If we give enough help to relatives so they can survive and have children, then they can pass on more copies of our own genes. Dr. Hamilton called this new way of tallying reproductive success inclusive fitness.