*A* | Measure of postmating asymmetry, measured as a difference between the more successful minus the less successful direction of hybridization (11) |

*C* | Threshold value of the hybrid breakdown score that leads to complete postmating isolation; the value is scaled as a multiple of the average effect of the most deleterious DMI (44) |

CV | Coefficient of variation of effects of both B and U DMIs (13) and (24) |

*g*_{B} | 1 − *g*_{U}, fraction of substitutions that can contribute to B DMIs (23) |

*g*_{U} | Fraction of substitutions relevant to U DMIs that occur at “female-acting” loci (17) |

*h*_{0} | Average contribution of B DMIs to the hybrid breakdown score, assuming that the U DMIs have been scaled to have an average effect of 1 (21) |

| No. of bidirectional (B) DMIs accumulated after divergence time *t* (1) |

| No. of unidirectional (U) DMIs experienced in a cross with a taxon 1 maternal parent |

*p* | Probability that an allelic difference at two B loci produces a B DMI (22) |

*p*_{U} | Probability that allelic differences at one “male-acting” locus and one “female-acting” locus produce a U DMI (5) |

| Contribution to the hybrid breakdown scores (both and ) from B DMIs (1) and (3) |

S_{ij} | Hybrid breakdown score produced with a taxon *i* maternal parent (1) |

| Contribution to the hybrid breakdown score from U DMIs |

*T*_{C} | Geometric mean of and (9) |

| Divergence time at which *E*(*S*_{ij}) = *C*, (9), *i.e*., the time at which the expected breakdown score corresponds to complete postmating isolation |

*v*(*S*) | Function describing how fitness declines as the hybrid breakdown score, *S*, increases (10) |

α | Exponent in the function describing how fitness declines as DMIs accumulate (10) |

β | *p*(1 − *g*_{U})/(*p*_{U}*g*_{U}), ratio of the expected no. of B DMIs to the expected no. of U DMIs (26) |

δ_{1} | υ_{1} − , difference in the relative taxon-specific rates of evolution of female-acting *vs*. male-acting loci that contribute to U DMIs (4b) |

η | *h*_{0}β, ratio of the expected contribution to the hybrid breakdown scores caused by B DMIs (which have average effect *h*_{0}) to the expected contribution from U DMIs (which are assumed to have average effect 1) (28) |

τ | *t/T*_{C}, scaled divergence time between the taxa hybridized (19) and (25) |