Figure 1.

PoMo state-space using $N=3$. The four alleles represent the four nucleotide bases. Black and gray arrows indicate mutations, and genetic drift plus selection, respectively. Monomorphic or boundary states $\left\{N{a}_i\right\}$ are represented in the tetrahedron’s vertices, while the polymorphic states $\left\{n{a}_i,\left(N-n\right)a_j\right\}$ are represented in its edges. Monomorphic states interact with polymorphic states via mutation, but a polymorphic state can reach a monomorphic state only via drift or selection. Between polymorphic states, only drift and selection events occur.