The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [48]
Call such a gene Cain. Now it so happens that Cain is Abel’s virtually identical twin, so he cannot kill his brother without killing himself. This is because the weapon he uses against Abel is merely a destructive enzyme released into the cell – a chemical weapon, as it were. His only hope is to attach to himself a device that protects him – a gas mask (though it in fact consists of a gene that repels the destructive enzyme). The ‘mask of Cain’ protects him from the gas he uses against Abel. Cain becomes an ancestor and Abel does not. Thus a gene for chromosomal fratricide will spread as surely as a murderer will inherit the earth. Segregation distorters and other fratricidal genes go under the general name of ‘meiotic drive’ because they drive the process of meiosis, the division of the partnerships, into a biased outcome.15
Meiotic-drive genes are known from flies and mice and a few other creatures, but they are rare. Why? For the same reason that murder is rare. The interest of the other genes has been reasserted through laws. Genes, like people, have other things to do than kill each other. Those genes that shared Abel’s chromosome and died with him would have survived had they invented some technique to foil Cain. Or, to put it another way, genes that foil meiotic drivers will spread as surely as meiotic drivers will spread. A Red Queen race is the result.
David Haig and Alan Grafen believe that such a response is indeed common and that it consists of a sort of genetic scrambling, the swapping of chunks of chromosomes. If a chunk of chromosome lying next to Abel suddenly swapped places with the chunk lying next to Cain, then the mask of Cain would be unceremoniously removed from Cain’s chromosome and plonked on to Abel’s. The result: Cain would commit suicide and Abel would live happily ever after.16
This swapping is called ‘crossing over’. It happens between virtually all pairs of chromosomes in most species of animal and plant. It achieves nothing except a more thorough mixing of the genes – which is what most people thought its purpose was before Haig and Grafen suggested otherwise. But Haig and Grafen are implying that crossing over need not serve any such function; it is merely a piece of intracellular law enforcement. In a perfect world policemen would not exist because people would never commit murder. Policemen were not invented because they adorn society but because they prevent the disruption of society. So, according to the Haig–Grafen theory, crossing over polices the division of chromosomes to keep it fair.
This is not, by its nature, the sort of theory that lends itself to easy confirmation. As Haig remarks, in a dry Australian manner, crossing over is like an elephant repellant. You know it’s working because you don’t see any elephants.17
Cain genes survive in mice and flies by hugging their masks close to them so that they are not likely to be parted by crossing over. But there is one pair of chromosomes that is especially plagued by Cain genes – the ‘sex chromosomes’, which do not engage in crossing over. In people and many other animals gender is determined by genetic lottery. If you receive a pair of X chromosomes from your parents you become a female; if you receive an X and a Y you become a male (unless you are a bird, spider or butterfly, in which case it is the other way round). Because Y chromosomes contain the genes for determining maleness, they are not compatible with Xs and they do not cross over with them. Consequently, a Cain gene on an X chromosome can safely kill the Y chromosome and not risk suicide. It biases the sex ratio of the next generation in favour of females, but that is a cost borne by the whole population equally, whereas the benefit