The Classical Problem of Fitness

Two thousand years ago, being in tip-top physical shape was a matter of life or death. In fact, the ability to run fast and punch hard was essential for the military strength of many ancient civilizations. The emergence of the Renaissance brought with it a rebirth of cultural learning from ancient Greece, and the ideals of ancient Greek athleticism became a major inspiration for fitness gurus of the time. This rebirth of fitness was driven by the desire to learn, as well as by the need to be strong and healthy.

The classic problem of fitness focuses on the question, “What is fitness?” and is a central issue in evolutionary theory. The answer to this question determines whether fitness has any explanatory power and, if so, what its role in evolutionary explanation should be. It is important to note that, despite the wide popularity of the concept, there are a number of different views concerning this issue. Some think of fitness as a pattern or series of patterns that are caused by non selective processes, thus depriving it of any causal or explanatory power. Others, such as Walsh, Lewens, Ariew, and Matthen, think of fitness as a particular statistical feature that is influenced by a number of other statistical features of a population in a given environment, and yet others, such as Millstein and Stephens, think of fitness as a cause operating at the population level to bring about the differential reproductive rates that are the result of natural selection.

All of these views face serious problems. In some cases, it is difficult to pin down the specific mathematical expression that is supposed to constitute the probabilistic propensity to reproduce that is supposedly defined by this schematic “definition.” In other cases, it turns out that in particular selective scenarios, it is not possible to have a sufficiently precise definition of fitness such that it can be used to predict the outcome of an experiment. Yet in other cases, the definition of fitness that is formulated by Pence and Ramsey may be too general and may not fully capture the important features of natural selection that Darwin and his successors uncovered.

There is a general consensus among philosophers of biology that the solution to this problem is to define fitness as a garden-variety dispositional concept like the concepts of magnetism or fragility, that is, a property that is distinguished from the actual behaviour that it causes no more than a magnetic object can be distinguished from the iron filings that it attracts or a fragile object from its breaking point. This approach has the advantage of being relatively harmless, but it may also miss some of the important work that ecological fitness can do that no definition of fitness in terms of differential reproductive rates-actual, expected, or dispositional-can do. This is because it may be necessary to distinguish between different kinds of design problems that a particular trait can solve.

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