The Unity of Fitness

It’s been argued that fitness cannot always be defined as expected number of offspring; different, more complex functions are required for different contexts. Brandon (1990) argues that fitness therefore merely satisfies a common schema. Other authors (Ariew and Lewontin, 2004; Krimbas, 2004) argue that no unified mathematical characterization of fitness is possible. I focus on comparative fitness, explaining that though comparative fitness must be relativized to an evolutionary effect which fitness differences help cause, thus relativized it can be given a unitary mathematical definition in terms of probabilities of producing offspring of various types and various other effects. Fitness will sometimes be defined in terms of probabilities of effects occurring over the long term, but I argue that these probabilities nevertheless concern effects occurring over the short term.