Rank-based selection is another method that was inspired by the observed drawbacks of fitness proportionate selection. It preserves a constant selection pressure by sorting the population on the basis of fitness, and then allocating selection probabilities to individuals according to their rank, rather than according to their actual fitness values. Let us assume that the ranks are numbered so that an individual’s rank notes how many worse solutions are in the population, so the best has rank μ-1 and the worst has rank 0. The mapping from rank number to selection probability can be done in many ways, for example, linearly or exponentially decreasing. As with FPS above, and any selection scheme, we insist that the sum over the population of the selection probabilities must be unity – that we must select one of the parents.