A reduction in the quantity of variety is the process of selection: some of the possibilities or alternatives are eliminated, others are retained. The result is a constraint: a limitation of the number of possibilities.
Selection processes are endemic in
systems theories. The most significant is the natural selection
of Darwinian evolution, but in this context we can recognize natural selection
outside of the context of biological evolution as any selection process
which eliminates distinctions. Also included in the concept of selection
are all forms of stability and equilibrium.
Variation on its own, without further constraints, produces entropy or disorder, by diffusion of existing constraints or dependencies. The equivalent for DNA is called "genetic drift".
However, variation is generally held in check by selection. Selection is the elimination or reduction of part of the variety of configurations produced by variation. Selection decreases disorder or entropy, by reducing the number of possibilities (Heylighen, 1992). A system that undergoes selection is constrained: it is restricted in the number of variations it can maintain. The existence of selection follows from the fact that in general not all variants are equivalently stable or capable of (re)production: those that are more easy to maintain or generate will become more numerous relative to the others (Heylighen, 1992). If all possible configurations are equally likely to be produced or conserved, there is no selection, and the only possible outcome of the process is maximization of statistical entropy, as in the cloud of gas molecules that diffuses to homogeneously fill its container. Selection can be internal, as when an unstable system (e.g. a radio-active atom) spontaneously annihilates, or external, as when a system is eliminated because it is not adapted to its environment (Heylighen, 1991a). Fitness is a measure of the likeliness that a configuration will be selected.