Haldane himself described the rule as: Haldane's rule applies to the vast majority of heterogametic organisms. This includes the case where two species make secondary contact in an area of sympatry and form hybrids after allopatric speciation has occurred. The rule includes both male heterogametic and female heterogametic, and some dioecious plants such as campions. Hybrid dysfunction is a major form of post-zygotic reproductive isolation, which occurs in early stages of speciation. Evolution can produce a similar pattern of isolation in a vast array of different organisms. However, the actual mechanisms leading to Haldane's rule in different taxa remain largely undefined.
Hypotheses
Many different hypotheses have been advanced to address the evolutionary mechanisms to produce Haldane's rule. Currently, the most popular explanation for Haldane's rule is the composite hypothesis, which divides Haldane's rule into multiple subdivisions, including sterility, inviability, male heterogamety, and female heterogamety. The composite hypothesis states that Haldane's rule in different subdivisions has different causes. Individual genetic mechanisms may not be mutually exclusive, and these mechanisms may act together to cause Haldane's rule in any given subdivision. In contrast to these views that emphasise genetic mechanisms, another view hypothesizes that population dynamics during population divergence may cause Haldane's rule. The main genetic hypotheses are:
Dominance: Heterogametic hybrids are affected by all X-linked alleles causing incompatibilities due to divergent alleles being brought together. However, homogametic hybrids are only affected by dominant deleterious X-linked alleles. Heterogametic hybrids, which carry only a single copy of a given X-linked gene, will be affected by mutations regardless of dominance. Thus, an X-linked incompatibility between diverging populations is more likely to be expressed in the heterogametic sex than in the homogametic sex.
The "faster male": Male genes are thought to evolve faster due to sexual selection. As a result, male sterility becomes more evident in male heterogametic taxa. This hypothesis conflicts with Haldane's rule in male homogametic taxa, in which females are more affected by hybrid inferiority. It therefore only applies to male sterility in taxa with XY sex determination, according to the composite theory.
The "faster X": Genes on hemizygous chromosomes may evolve more quickly by enhancing selection on possible recessive alleles causing a larger effect in reproductive isolation.
Differential selection: Hybrid incompatibilities affecting the heterogametic sex and homogametic sex are fundamentally different isolating mechanisms, which makes heterogametic inferiority more visible or preserved in nature.
Data from multiple phylogenetic groups support a combination of dominance and faster X-chromosome theories. However, it has recently been argued that dominance theory can not explain Haldane's rule in marsupials since both sexes experience the same incompatibilities due to paternal X-inactivation in females. The dominance hypothesis is the core of the composite theory, and X-linked recessive/dominance effects have been demonstrated in many cases to cause hybrid incompatibilities. There is also supporting evidence for the faster male and meiotic drive hypotheses. For example, a significant reduction of male-driven gene flow is observed in Asian elephants, suggesting faster evolution of male traits. Although the rule was initially stated in context of diploid organisms with chromosomal sex determination, it has recently been argued that it can be extended to certain species lacking chromosomal sex determination, such as haplodiploids and hermaphrodites.
Exceptions
There are notable exceptions to Haldane's rule, where the homogametic sex turns out to be inviable while the heterogametic sex is viable and fertile. This is seen in Drosophila fruit flies.
