Fitness

# Fitness

 Fitness is an assumed property of a system that determines the probability that that system will be selected, i.e. that it will survive, reproduce or be produced.
Technically, the fitness of a system can be defined as the average number of instances of that system that can be expected at the next time step or "generation", divided by the present number of instances. Fitness larger than one means that the number of systems of that type can be expected to increase. Fitness smaller than one means that that type of system can eventually be expected to disappear, in other words that that type of system will be eliminated by selection. (see a definition of fitness with transition probabilities) High fitness can be achieved if a system is very stable, so that it is unlikely to disappear, and/or if it is likely that many copies of that system will be produced, by replication or by independent generation of similar configurations (for example, though snow flakes are unstable and cannot reproduce, they are still likely to be recurrently produced under the right circumstances). The fitter a configuration, the more likely it is to be encountered on future occasions (Heylighen, 1994).

Although this technical interpretation may seem rather far removed from the intuitive notion, the English word "fit" is eminently suited for expressing the underlying dynamic. Its spectrum of meanings ranges between two poles: 1) "fit" as "strong", "robust", "in good condition"; 2) "fit" as "adapted to", "suited for", "fitting". The first sense, which may be called "absolute fitness", points to the capability to survive internal selection, i.e. intrinsic stability and capacity for (re)production. The second sense, which may be called "relative fitness", refers to the capability to survive external selection, i.e. to cope with specific environmental perturbations or make use of external resources.

It must be noted that "internal" and "external" merely refer to complementary views of the same phenomenon. What is internal for a whole system, may be external for its subsystems or components. For example, the concentration of oxygen in the air is an external selective factor for animals, since in order to survive they need a respiratory system fit to extract oxygen. Similarly, the concentration of carbon dioxide is an external selective factor for plants. However, when we consider the global ecosystem consisting of plants and animals together, we see that the concentrations of both oxygen and carbon dioxide are internally determined, since oxygen is produced out of carbon dioxide by plants, and carbon dioxide out of oxygen by animals. Survival of the global system requires an internal "fit" of the two halves of the carbon dioxide - oxygen cycle: if more oxygen or carbon dioxide would be consumed than produced, the whole system would break down.

Similarly, when we look at a crystal as whole system, we see it as a stable structure that is unlikely to disintegrate, i.e. it is absolutely fit and survives internal selection. However, when we look at the molecules as the parts that make up the crystal, we see that they must have the right connections or bonds, i.e. fit relations, to form a stable whole. The exact configuration of each molecule is externally selected by the other molecules to which it must fit. In this way, every absolute or intrinsic fitness characterizing a whole can be analysed as the result of a network of interlocking relational fitnesses connecting the parts .

In summary, a system will be selected if: 1) its parts "fit together", i.e. form an intrinsically stable whole, 2) the whole "fits" its environment, i.e. it can resist external perturbations and profit from external resources to (re)produce.