Abstract

Fitness is a fundamental concept in evolutionary biology, as it is intimately connected with evolution by natural selection. The term ‘fitness’ entered biological lexicon when Herbert Spencer recommended to Darwin that he use the phrase ‘survival of the fittest’ to help explain how natural selection works. In the course of the 20th century the fitness concept underwent a series of subtle transformations, thanks to the successful mathematization of evolutionary theory, but it remains central to evolutionary theory. Despite this, the fitness concept is rather elusive, as a number of authors have noted, seemingly lacking a fully general, precise definition. The concept has been called “subtle”, “vague”, “impossible to define”, as having “many meanings”, “lacking a comprehensive definition”, and being the object of “conceptual confusion”. In the light of this situation, it is not surprising that since the 1970s philosophers of biology have sought to clarify the fitness concept. However the philosophical discussions, though useful in some ways, generally make insufficient contact with the technical literature on fitness in evolutionary theory itself.

This talk is part of a broader project that re-visits the fitness concept, tries to make sense of the controversies surrounding it, and to integrate the philosophical and biological discussions. The basic idea is to regard “fitness” as a theoretical term which is to be defined via its theoretical role, that is, the job that it is meant to do. This sounds simple; however, closer inspection shows that there are actually four slightly different “fitness roles”. Moreover, in a particular evolutionary model, a given quantity, definable from the model parameters, may realize one of these fitness roles but not others. Taken together, this explains why the fitness concept has caused so much confusion, why the term “fitness” is polysemic in evolutionary biology, and why theorists can disagree about the “right” fitness measure despite the underlying science not being in dispute.