Haldane's rule relating to hybrids of species and extended to speciation in evolutionary theory is easily stated: // In biology, hybrid has two meanings. ... In biology, a species is one of the basic units of biodiversity. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... This article is about evolution in biology. ...

When in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex.

It was originally formulated in 1922 by the British evolutionary biologist J. B. S. Haldane. Since it appears to be usually but not universally true, it is described as a rule rather than a law. (Haldane's Law is "Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we CAN suppose.") Heterozygote cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. ... Evolutionary biology is a subfield of biology concerned with the origin and descent of species, as well as their change, multiplication, and diversity over time. ... John Burdon Sanderson Haldane (November 5, 1892 – December 1, 1964), who normally used J.B.S. as a first name, was a British geneticist and evolutionary biologist. ... J. B. S. Haldanes statement that he suspected that the Universe is not only queerer than we suppose, but queerer than we can suppose is often described as Haldanes Law. ...

In many organisms, such as mammals or Drosophila flies, males are the heterogametic sex, in that they have XY sex chromosomes, whereas females are homogametic, with XX chromosomes. However, in some other animals (i.e. birds, butterflies) and plants, the reverse is usually true. Haldane's rule have been shown in a number of different hybrids crosses where either the male or the female is the heterogametic sex. Subclasses Allotheria* Order Multituberculata (extinct) Order Volaticotheria (extinct) Order Palaeoryctoides (extinct) Order Triconodonta (extinct) Prototheria Order Monotremata Theria Infraclass Marsupialia Infraclass Eutheria The mammals are the class of vertebrate animals characterized by the production of milk in females for the nourishment of young, from mammary glands present on most species... Type Species Musca funebris Fabricius, 1787 Drosophila is a genus of small flies whose members are often called small fruit flies, or more appropriately vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies. ... The shield and spear of the Roman God Mars are often used to represent the male sex In heterogamous species, male is the sex of an organism, or of a part of an organism, which typically produces smaller, mobile gametes (spermatozoa) that are able to fertilise female gametes (ova). ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... The hand mirror and comb of the Roman Goddess Venus is often used to represent the female sex. ...

The fact that hybrid sterility and inviability can evolve due to Haldane's rule in such a vast array of different organisms is quite striking. However, the actual explanation of this phenomena is rather complicated. Many different theories have been advanced to explain the genetic basis of Haldane's rule.

1) The dominance theory: Heterogametic hybrids are affected by all, recessive and dominant, X-linked genes involved in incompatibilities, while homogametic hybrids are only affected by the dominant ones.

2) Faster male theory: Males genes evolve faster due to sexual selection.

3) Meiotic drive: In hybrid populations, selfish genetic elements inactivate sperm cells (i.e: A X-linked drive factor inactivates a Y-bearing sperm and vice versa).

4) Faster X theory: X-linked have a larger effect in reproductive isolation.

The dominance theory is the leading theory explaining Haldane's rule. However, it is not mutually exclusive and many causes might potentially act together and cause hybrid sterility and inviability in the heterogametic sex.

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. It has been suggested that this article or section be merged into Dominance relationship. ... Sex-linked genes are those carried on the mammalian X chromosome but not the Y chromosome. ... Color blindness in humans is the inability to perceive differences between some or all colors that other people can distinguish. ... This article or section does not adequately cite its references or sources. ...

References

• Coyne, J.A. (1985): The genetic basis of Haldane's rule. Nature 314(6013): :736-738. Retrieved 2007-JAN-26. NCBI Pubmed Abstract.

• Forsdyke, Donald (2005): Haldane's rule. Version of 2005-DEC-6. Retrieved 2006-OCT-11.

• Haldane, J. B. S. (1922): Sex ratio and unisexual sterility in hybrid animals. J. Genet. 12: 101-109.