William D. Hamilton (1936–2000) was an evolutionary biologist who is best known for his equation explaining kin selection, an evolutionary strategy which favours the reproductive success of an individual’s relations at the cost of the individual’s own survival.
One of the key problems with Darwin’s theory of evolution by natural selection was the lack of explanation for altruistic behavior. Natural selection was said to work by favouring the behaviour of an individual that increased biological fitness, such as increased production of offspring. However some animals acted in ways that benefited other individuals at a cost to themselves. An example of this would be when a bee stings a potential intruder, meaning that it will die, to protect its hive. The common explanation among biologists at the time was that the individual who displayed altruistic behaviour was acting in the interests of the group or for the good of the species.
Influenced by the work of R A Fisher and B S Haldane, in particular Fisher’s book The Genetical Theory of Natural Selection, Hamilton gradually formulated his own idea in which altruistic behaviour could be explained by the genetic relatedness of an individual to the recipient of the altruistic act. The more closely related two individuals are the higher the likelihood of an altruistic act between them. In other words, a parent might sacrifice themselves for their child as the survival of the child would mean that the net benefit in terms of the related genes that survived made it worthwhile. Hamilton named this behaviour ‘inclusive fitness’.
Published in the two-part paper The Genetical Evolution of Social Behavior in 1964 and expressed as a mathematical model, Hamilton’s rule, as it became known, was at first largely overlooked. However, as the paper became more widely known, it led to substantial innovations in the field of Neo-Darwinian biology and remains one of the most cited academic papers in the field to this day.
These 15 typeset pages, minus the introduction, and including handwritten corrections, were according to Hamilton’s annotation ‘about the version finally published’ of this influential paper.