(of a trait) The increase in reproductive success that the trait confers on its possessor's genetic constitution, or genotype. For example, the function of nest-building is to provide a nest to keep the occupants warm, and to help protect them from predation. This implies that these consequences of nest-building confer higher reproductive success on the genotype of the individuals that practise it.
In most cases, traits that increase the reproductive success of the genotype that produces them also increase the success of their individual carriers, but this is not always true. For example, genes that cause their carriers to assist other individuals sharing the same genotype (i.e. relatives) may be favoured by natural selection and may spread through the population, even if they lower the reproductive success of their carriers. This phenomenon is called kin selection.
Functions are always complex and usually multiple. For example, the function of incubation in herring gulls (Larus argentatus) is dependent upon a number of consequences of incubation. Thus, incubation keeps the eggs warm, but it prevents the sitter from foraging for food. It protects the eggs from predation, but it exposes the adult to attack by larger predators. Thus incubation has both costs and benefits. The function of incubation, therefore, has to be seen in terms of all the consequences of incubation that affect reproductive success, in comparison with the consequences of not incubating, or of incubating in a different manner.
Subjects: Zoology and Animal Sciences.