Figure 1. Genealogical structure in the background selection limit when .

(A) In “fitness space,” the genealogy is perfectly star-like, with the most recent common ancestor (MRCA) rooted in the mutation-free class [78]. Deleterious mutations (red circles) occur every time an ancestor changes fitness classes. (B) In the standard (time-based) representation, deleterious mutations occur in a short delay phase of duration , when ancestral lineages migrate through the fitness distribution. After this point, all ancestral lineages are mutation free, and coalescence proceeds according to the neutral expectation with an effective population size . Since , silent mutations (blue circles) will primarily occur in the coalescence phase.