In biology, the function of a feature of an organism is frequently defined as that role it plays which has been responsible for its genetic success and evolution. Thus although the brain weighs down the shoulders, this is not its function, for this is not why entities with brains are successful. A central question will be the unit whose adaptation is in question: these may be persons, or their genes, or clusters of genes, or gene pools; it may be said that a person is a gene's way of making another gene, just as a scholar is a library's way of making another library. There are also difficulties in distinguishing fortuitous roles that an adaptation may come to serve from its function proper.
Philosophy dictionary. Academic. 2011.