The Classical Problem of Fitness

In the context of evolutionary biology, the concept of fitness is central. It is used to describe an organism’s propensity to survive, reproduce, and pass on its genes. It is also a central element in the explanation of how variation leads to differential reproduction rates. For this reason, it is vital that the concept of fitness be interpreted correctly. Unfortunately, there is considerable disagreement amongst geneticists about what exactly the term means.

One view of fitness is to use a “design problems” framework: a given genotype x is fitter than another y if y’s traits enable it to solve the design problems set by the environment more fully. This has the advantage of being simple and intuitive. It has the disadvantage, however, of being ambiguous and thus potentially confusing. Furthermore, the notion of “design problems” may simply hide the problem of distinguishing fitness from reproductive rates, rather than solving it.

A second way to understand fitness is to use a probabilistic propensity framework. This is a more sophisticated version of the design problems approach and has the advantage of being counterexample-free. It is, however, difficult to pin down a mathematical expression of the probabilistic propensity that constitutes fitness. This difficulty stems from features of natural selection that we must accommodate. The failure to do so risks limiting the concept of fitness to an indefinitely large number of practical measures that lack any explanatory power.

The third way to understand fitness is to treat it as a cause of differential reproduction rates. This is the most popular view of the term and, as such, it has received substantial philosophical support. It is the only approach that avoids the threat of tautology but, as we will see, it has its own difficulties.

Despite its widespread popularity, it is doubtful that any of these approaches fully captures what Darwin meant by “fitness” or the idea of being fitted. Indeed, in the Origin, the word “fitness” appears only three times (as part of Spencer’s phrase “the fittest shall survive”) and never independently. In any event, none of these views explains the relationship between variation and differential reproduction rates in the way that natural selection does.

In this article, we will argue that the solution to this debate lies in a return to Darwin’s own formulation of the idea. We will explore some of the epistemological problems that this involves and then propose a new, more precise definition of fitness. We will then show that this solution, while preserving the explanatory power of natural selection, also addresses several other issues that plague discussions of evolution. The Classical Problem of Fitness is available on the Internet and can be accessed here. The article is published under an Attribution-NonCommercial-NoDerivatives 4.0 International license. To republish or redistribute this work in any format, please contact the Author. For requests for permission to reuse this work, please review the FAQ. For more information about the Attribution-NonCommercial-NoDerivatives license, please visit our FAQ page.

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