Haldane's rule or Haldane's law was formulated in 1922 by the British evolutionary biologist J.B.S. Haldane. It describes hybrid sterility in species and is extended to describe speciation in evolutionary theory, in two parts: the rule of hybrid sterility and the rule of hybrid inviability. According to the rule of hybrid sterility, when the resulting offspring of parents crossed from different lineages exhibits sterility but is otherwise healthy, that offspring will tend to be of the heterogametic sex, which exhibits two different sex chromosomes (as in XY). Heterogametes are also typically subject to greater morphological deformities than homogametic hybrids. Haldane's accompanying rule for hybrid inviability states that when two parental lineages become so evolutionarily divergent as to exhibit genetic differences, but do not become mechanically isolated, fusion of the gametes may be uncooperative, causing inviability and the preferential production of the homogametic sex, which exhibits two of the same sex chromosomes (as in XX). The heterogametic sex is rare in this case.
This pattern holds true in both organisms in which males are the heterogametic sex (as in Drosophila and mammals, including humans) as well as in organisms where the reverse is true, females being heterogametic (as in birds). [1]
According to the rule, the first stage of speciation occurs when all offspring produced by two differing lineages are hybrids, although viable. As genetic divergence continues, incompatibilities preventing the fusion of gametes, the development of the embryo, or the development of the offspring to adult maturity arise, causing hybrid inviability. The development of prezygotic barriers to reproduction ensues at this stage, in the form of anatomical, temporal, or behavioral incompatibility. By definition, the two lineages may be referred to as different species, as they may no longer interbreed naturally.
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Although Haldane's rule may prove to be one of "the strongest patterns in evolutionary biology," a sufficient explanation of the occurrence of hybrid sterility and hybrid inviability has only been theorized. It is unclear as to why hybrids of the heterogametic sex are much more greatly afflicted by deformities, or even why only homogametic offspring are almost always produced in hybridization. A variety of factors have been hypothesized to contribute to these phenomena.
Data from multiple phylogenetic groups support a "dominance and faster X–chromosome" theory [2].
The dominance hypothesis is the core of the composite theory and the X-linked recessive/dominance effects have been demonstrated in many cases to cause hybrid incompatibilities. There is also supporting evidence for the faster male hypothesis and meiotic drive hypothesis. For example, a significant reduction of male-driven gene flow is observed in Asian elephants, suggesting a faster evolution of male traits.[3]
Haldane's rule has a correspondence with the observation that some negative recessive genes are sex-linked and express themselves more often in men than women, such as color blindness or haemophilia.
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 species lacking chromosomal sex determination, such as haplodiploids.[4]
There are notable exceptions to Haldane's rules where the homogametic sex turns out to be unviable while the heterogametic sex is viable and fertile. This has been most commonly noted in Drosophila,[5] where it is proposed to function through maternal effect genes, and their interaction with species specific heterochromatin.[6]
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