Associate Professor, Department of Philosophy, Lewis & Clark College

The Tautology Problem and Fitness as a Propensity

Added on by jay odenbaugh.
Suppose we define ’fitness’ as,
Fitness of an individual or type of individual as the actual number of offspring produced.
Likewise, suppose the average number of offspring of type x is greater than the average number of offspring of type y.

Why did x on average leave more offspring than y? Because x is fitter than y.

But, given the above definition,

Why did x on average leave more offspring than y? Because x on average left more offspring than y.

This is no explanation -- it seems to be a "tautology." Mills and Beatty write,

"The whole idea of setting up empirical investigations to deter- mine whether fitness differences are correlated with actual descendent contribution differences seems absurd, given the above definitions of fitness."
How does the Propensity Interpretation of Fitness resolve this problem?