  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.
