Fitness for survival and reproduction

The fitness of a system can be defined as the rate of increase of that system at the next time step, or "generation". A fitness larger than one means that the system will increase in numbers. A fitness smaller than one means that the system can eventually disappear; so the system will be eliminated by natural selection. A high fitness can be achieved if a system is very stable so that it is unlikely to disappear, and/or if many copies of that system are reproduced. The fitter a configuration, the more likely it is to be encountered in the future. Applying this to a couple of man and woman, fitness to survival is when they produce two offspring; high fitness is when they produce more than two. See Darwin's "origin of species" on this subject.

Fitness implies two senses. The first sense, which may be called "internal or absolute fitness", points to intrinsic stability and capacity for reproduction. The second sense, which may be called "external or relative fitness", refers to the capability to cope with specific environmental perturbations (e.g. diseases in the case of man, environmental changes, e.g. climate) or make use of external resources to survive (e.g. reindeer, fish, whales, or seals, or wood etc...).

It must be noted that "internal" and "external" are complementary views of the same phenomenon. What is internal for a system, may be external for its subsystems. For example, fitness to fix oxygen from the air is external for animals, since in order to survive they need oxygen for their respiratory system. Similarly, fitness to fix carbon dioxide is external for plants since they also need to absorb carbon dioxide for building their body. However, when we consider the whole system 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 components of the carbon dioxide - oxygen cycle: if more oxygen or carbon dioxide is consumed than produced, the whole system would break down.

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 reproduce.

Mis à jour le 01/04/2016 pratclif.com