What determines biological fitness? The problem of the reference environment

Organisms’ environments are thought to play a fundamental role in natural selection. However, existing conceptions of environment are so vague as to make it indeterminate which biological types are fittest, depriving the concept of natural selection of any clear sense. The “problem of the reference environment” is the problem of specifying principles which pick out those environmental conditions which determine biological fitness. I sketch two solutions to the problem, each appropriate in a different context. The first solution specifies the environment relevant to evolution over a specified period of time. The second specifies an environment relevant to fixation and extinction of types in a population; it is this kind of environment which is relevant to long-term evolution.