Figure 2.

Representative two-dimensional genotype distribution. Individuals with equal numbers of beneficial mutations in each trait are combined into classes (squares). Abundances of the bulk (grayscale squares) behave deterministically, while abundance of the stochastic front (light and deep blue squares) behave stochastically. The fittest genotypes in the population (deep blue squares) are referred to as the high-fitness front. The red dot marks the class with approximately average numbers of beneficial mutations in each trait. Equally fit classes lie along fitness isoclines, which are the lines parallel to the red line shown. In asexuals, beneficial mutations must occur on the fittest genetic backgrounds in order to contribute to the adaptive process. These mutations are represented by arrows from the bulk into the stochastic front. Classes in the stochastic front become part of the bulk once their abundances have become sufficiently large, advancing the stochastic front to include new classes. As classes below the population mean fitness isocline decline and those above increase exponentially, a two-dimensional traveling wave is produced. Simulation parameters: N = 10⁹, s = 0.02, and U = 10⁻⁵.