Haldane’s rule, paraphrased: The heterogametic sex is more likely to be absent, rare, or sterile in the offspring of two species; the homogametic is less likely to be affected by these things. This rule holds in most animals possessing sex chromosomes. Obviously this wouldn’t hold true for sex determined by temperature during incubation and such.
- Explanations of Haldane’s Rule
- Dominance Theory: Hinges on alleles that act recessively in hybrids. Hybrid inviability may affect the heterogametic sex just like X-linked disorders. If your one copy of a chromosome / gene is bad, then there’s nothing to override the negative effects.
- Faster Male Theory: Incompatibilities afflicting heterogametic hybrids are more common than those afflicting homogametic hybrids. Thus, hybrid male sterils are more common than hybrid female sterils. Spermatogenesis inherently sensitive? Faster evolution due to sexual selection on males? Males sterile more often even when females are heterogametic. Does not explain hybrid inviability, only sterility.
- Faster-X Theory: if X-linked genes have a disproportionately large effect on hybrid fitness.
- Meiotic Drive: Selfish genetic elements; alleles distort mendelian ratios to their own advantage, often by inactivating sperm that carry a homologous chromosome (X sperm might inactivate Y sperm). Imposes a fertility cost on the bearers and other genes by distorting the sex ratio.
Dominance and Faster Male cause Haldane’s Rule. The other 2 forces may also play a role, but evidence is ambiguous.
Hybrid male sterility involves more genes than hybrid female sterility or hybrid inviability.
It is difficult to determine which / how many genes were involved in the initial evolution of hybrid incompatibility, simply because the species have continued to involve over time.
“Genes causing inviability typically affect both sexes, while genes causing sterility typically affect one sex only. Second, maternal effects on viability are common, while those on fertility are rare. Third, male sterility typically involves postmeiotic problems. — Intrinsic postzygotic isolation typically involves “ordinary” genes that have normal functions within species.” (Not novel genetic factors)
