 To begin, recall how in the discussion above regarding the game of figure 1, we appealed informally to the idea of a mutant attempting to invade a population, all of which follow a single incumbent strategy. To make this idea precise, we need to introduce some notation: let \(\mu, \sigma\) be two strategies and let \(0 \lt \epsilon \lt 1\). Then \(\epsilon \mu + (1-\epsilon) \sigma\) denotes the strategy which plays \(\mu\) with probability \(\epsilon\) and plays \(\sigma\) with probability \(1-\epsilon\). However, we may also interpret this as representing the strategy used by a player chosen at random from a large population in which the majority \((1-\epsilon)\) follow the strategy \(\sigma\) and a minority (\(\epsilon\)) follow the strategy \(\mu\) which is attempting to invade.