Fig. 1.

Schematic for the transition rates between adjacent classes under the sequential-fixation model for the case of $L=4$ sites. This schematic readily generalizes to any value of $L$. $u_{01}$ and $u_{10}$ are the mutation rates from $-$ to $+$ allelic states, and vice versa, and $f_{ij}$ is the probability of fixation of a newly arisen mutation to allele $j$ from a background of $i$, defined as SI Appendix, Eq. S5. The number of $-$ alleles in a class is denoted by $m$, and except for the two extreme classes ($m=0$ and $m=4$), there are multiple equivalent genotypic states within each class of genotypic values.