Genealogical structure in the background selection limit when .

<p>(A) In “fitness space,” the genealogy is perfectly star-like, with the most recent common ancestor (MRCA) rooted in the mutation-free class <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004222#pgen.1004222-Kao1" target="_blank">[78]</a>. 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 <i>delay phase</i> 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.