Figure 2 .

(A) Schematic illustration of the fitness distribution within a population. We refer to the most-fit class as the “nose” of the distribution. (B) A typical realization of the stochastic dynamics at the nose. In the event of extinction of the nose, n₁ becomes the new nose and relative nose fitness decreases by s. Conversely, when a more-fit nose is established, the relative nose fitness increases by s.