Selective forces need to be maintained at the same level throughout an evolutionary run, so as to balance mutational forces and maintain a similar degree of genetic convergence throughout. Basing selection directly on absolute fitness values does not achieve this, and some system based on ranking of the population must be used. This implies that the fittest member of the population has the same expected number of offspring whether it is far better than the rest, or only slightly better. Truncation selection (reproducing only from a top slice of the population) is one way of achieving this, but generally is too severe in restricting exploration by the less fit mutants. Less severe methods are recommended such as linear ranking; for instance giving the top ranker twice the average expected number of offspring, and reducing this amount linearly as one goes down the ranks, towards zero.
One way to achieve an effect comparable to linear ranking in a steady state GA is through tournament selection. Rather than replacing the whole population by a similar number of offspring at each generation, only one new offspring at a time replaces a fatality in an otherwise unchanged population. Two parents for the offspring can each be chosen by picking the fittest of a randomly picked pair (the tournament), and the fatality chosen at random from the whole population; alternatively, the parents can be picked at random from the whole population, and the fatality selected as the loser of a tournament.
Elitism is often advocated in GAs when used for optimisation. This is the requirement that the current fittest member of the population is never deleted to make way for another that is less fit. In real world applications such as hardware design, however, evaluations are likely to be noisy. Since in this case one can never be certain which is the fittest, elitism cannot be relied upon.