Figure 1.

Mutation rate and evolutionary trajectories on fitness landscapes. When mutations are rare (A), the population is nearly homogeneous and mutations are fixed sequentially via sweeps. Evolution can therefore be represented by a single path on the fitness landscape. Several distinct clones coexist at any given time in the population when mutation rate is high. (B) If each clone is allotted its own trajectory, the evolution of the population can be represented by a bundle of trajectories which split and terminate on the way to the summit. At any given time the population can be represented by a probability density function in sequence space.