Figure 9.

Mutation order determines the potential for a derived‐ancestral incompatibility. Red and blue stars mark the position of the first and second mutations producing incompatible alleles. The ancestral genotype for the two loci is denoted “ab”, with the mutations producing derived alleles “A” and “B”. (A) Derived–ancestral incompatibility from the derived allele in the S ₁ lineage with the ancestral allele in the S ₃ lineage. (B) No incompatibility forms because the derived–ancestral genotype persists in the S ₂ lineage.