The Propensity Interpretation of Fitness

The Propensity Interpretation of Fitness

Among the most prominent and influential explanatory conceptions in evolutionary biology is the concept of fitness as a disposition to survive and reproduce. The theory of natural selection posits that differences in fitness are cause and effect, with organisms of higher or lower fitness leaving more or fewer offspring than others.

Many philosophers and biologists have endorsed this concept of fitness. But the difficulty in establishing reproductive success for some clonal organisms, collectives of organisms and multi-species assemblages has forced some philosophers to entertain alternative concepts of fitness.

These concepts have included a proposal that fitness could act as an energy control mechanism for the evolution of a broader range of biological systems. The most influential proponent of this approach was Leigh Van Valen, who suggested that a ‘universal currency’ of fitness might be a measure of ecological persistence through time (Bouchard 2004, 2008, 2011).

This view is opposed by some philosophers and biologists who argue that reproductive success cannot be the universal currency of fitness because it does not apply to all clonal organisms, collectives or multi-species assemblages. In addition, some of these philosophers and biologists argue that the probabilityistic propensity definition of fitness is a natural and adequate definition for a purely population-level theory of natural selection.

The Propensity Definition of Fitness: a Counterexample to the Design Problem Conception

A significant issue facing the propensity definition of fitness has been its difficulty in identifying specific mathematical expressions that are appropriate and sufficient for providing a comprehensive general “definition” of this property. It is a problem that reflects features of natural selection that we have to accommodate, and which daunt any attempt to turn a probabilistic propensity schema for fitness into a complete general definition.

The first major obstacle to a probabilistic propensity definition is that such a definition will have to accommodate an indefinitely large number of operational measurements of the property of comparative fitness. These can include a broad range of statistical terms beyond variance.

Another major barrier to a propensity definition of fitness has been the problem that such a definition would have to exclude a variety of non-zero factors that are important to biological adaptation and selection. These include such things as quantum percolation, which is often thought to be a factor in mutations [Brandon 1990], or non-reproductive factors such as the temporal and spatial variation in the number of offspring left by organisms that are more fit than those that are less fit.

Nonetheless, some of these problems can be circumvented by interpreting the theory of natural selection as providing an explanatory mechanism for the evolutionary process in which differential reproduction rates are produced by stochastic processes that vary from species to species.

But this requires a radically different conceptualization of natural selection. This is the so-called ‘design problem’ interpretation of natural selection.

In this view, a ‘fitter’ species has the capacity to solve the design problems that it faces more completely than a ‘less fit’ species does. Hence, this means that a ‘fitter’ species will have a greater reproductive potential than a ‘less fit’ species and a ‘fitter’ species is more likely to evolve by natural selection than a ‘less fit’ species is.

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