The concept of fitness is introduced, and a simple derivation of the Fundamental Theorem of Natural Selection (which states that the average fitness of a population increases if its variance is nonzero) is given. After a short discussion of the adaptative walk model, a short review is given of the quasispecies approach to molecular evolution and to the error threshold. The relevance of flat fitness landscapes to molecular evolution is stressed. Finally a few examples which involve wider concepts of fitness, and in particular two-level selection, are shortly reviewed. 1 Fitness and the fundamental theorem of natural selection The term “fitness ” derives from the phrase “survival of the fittest ” that the philosopher Herbert Spencer suggested to use instead of “natural selection”. In the struggle made by the evolutionary theorists to avoid th
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