Haldane's rule
Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J.B.S. Haldane, that states that if in a species hybrid only one sex is inviable or sterile, that sex is more likely to be the heterogametic sex. The heterogametic sex is the one with two different sex chromosomes; in therian mammals, for example, this is the male. Overview. 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 (XY or XO-type sex determination, such as found in mammals and "Drosophila" fruit flies) and female heterogametic (ZW or Z0-type sex determination, as found in birds and butterflies), and some dioecious plants such as campions. Hybrid dysfunction (sterility and inviability) 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 emphasize genetic mechanisms, another view hypothesizes  that population dynamics during population divergence may cause Haldane's rule. The main genetic hypotheses are: 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.