Replace competition between two populations by choice between two alternatives
This is brilliant.
Fitness is the central concept in modern Darwinian genetics. It is measured simply by the rate at which an organism reproduces itself. If two organisms compete for occupancy of the same ecological niche, relative fitness determines which will survive. Even small differences in fitness can lead to enormous differences in reproductive success over only a few generations.
Replace competition between two populations by choice between two alternatives, and replace fitness by any measure associated with the outcomes of choice. Then if only one is confronted repeatedly with choices of some kind (for example, whether to take a second helping), slight difference in the probability of selecting one alternative or another can soon produce substantial differences in outcome (body weight). All of us are familiar with this example.
The win or loss of a chess game may be decided in the same incremental way. Especially when good players are nearly equal in skill, the game is seldom lost by a single bad move. Rather, the winning player secures a cumulative advantage by exploiting successive small weaknesses that are observed in the opponent's choices of moves.
— From the remarkable autobiography of the Nobel Prize winning social scientist and father of artificial intelligence, Herbert Simon.