 A uniform invasion barrier is a natural concept of evolutionary stability. It says that, when a population all follows the same strategy \(\sigma\), except for a small proportion of mutants, all of whom follow a single strategy \(\mu\), the incumbent strategy \(\sigma\) has a strictly higher expected fitness in the mixed population than the invading strategy  \(\mu\).[2]  Hence there would be selection against the invading strategy and, in a sufficiently large population with a sufficiently small number of mutants, \(\sigma\) would be evolutionarily stable.