Inclusive Fitness in Evolutionary Modelling

The notion of Darwinian fitness allows us to understand how natural selection influences the evolution of organismal traits. For example, we might explain a plant’s morphology, or a bird’s behaviour in terms of the fitness advantages (reproductive or survival benefits) such morphology or behaviour confers. The notion of inclusive fitness is used specifically to understand how selection will act on those traits that influence the fitness of several genetically-related individuals simultaneously. Inclusive fitness recognizes that an individual may have a “genetic share” in the fitness of its neighbours, and so it is a more general (more “inclusive”) measure of evolutionary success. The development of inclusive fitness is usually credited to the biologist W. D. Hamilton, who used the idea to explain the selective advantage of altruistic traits [12]. Before Hamilton’s seminal work was published, the advantage of altruistic traits was difficult to explain using fitness alone. The fact that an individual would be willing to decrease its fitness by some amount (traditionally denoted, c) to increase the fitness of a neighbour by some other amount (traditionally denoted, b) seemed counterintuitive to many evolutionary biologists. Hamilton, showed, that the selective advantage of these “problematic” traits is clear provided the recipient of the altruistic act is genetically related to the actor, and provided one adopts an inclusive fitness perspective. In this case, the fitness of the actor is 1− c, and the fitness of the recipient is 1 + b; if the actor and the recipient are related to one another by a factor r, then the actor’s inclusive fitness is simply, 1− c+ (1 + b)r. Relative to the situation in which the actor does nothing (i.e. the situation in which both actor and recipient fitness is 1), the inclusive-fitness change is −c+ br. (1)