The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [36]
Sexual species can call on a sort of library of locks that is unavailable to asexual species. This library is known by two long words which mean roughly the same thing: heterozygosity and polymorphism. They are the things that animals lose when their lineage becomes inbred. What they mean is that in the population at large (polymorphism) and in each individual as well (heterozygosity) there are different versions of the same gene at any one time. The ‘polymorphic’ blue and brown eyes of westerners are a good example: many brown-eyed people carry the recessive gene for blue eyes as well – they are heterozygous. Such polymorphisms are almost as puzzling as sex to true Darwinists because they imply that one gene is as good as the other. Surely, if brown eyes were marginally better than blue (or, more to the point, if normal genes were better than sickle-cell anaemia genes) then one would gradually have driven the other extinct. So why on earth are we stuffed full of so many different versions of genes? Why is there so much heterozygosity? In the case of sickle-cell anaemia it is because the sickle gene helps to defeat malaria, so the heterozygotes (those with one normal gene and one sickle gene) are better off than those with normal genes where malaria is common, whereas the homozygotes (those with two normal genes or two sickle genes) suffer from malaria and anaemia respectively.37
This example is so well worn from over-use in biology textbooks that it is hard to realize that it is not just another anecdote, but an example of a common theme. For it transpires that many of the most notoriously polymorphic genes, such as the blood groups, the histocompatibility antigens and the like, are the very genes that affect resistance to disease – the genes for locks. Moreover, some of these polymorphisms are astonishingly ancient. They have persisted for aeons. For example, there are genes that have several versions in mankind, and the equivalent genes in cows also have several versions, but what is bizarre is that the cows have the very same versions of the genes as mankind. This means that you, reader, might have a gene that is more like the gene of a certain cow than it is like the equivalent gene in your spouse. This is considerably more astonishing than it would be to discover that the word for, say, meat was viande in France, Fleisch in Germany, viande again in one uncontacted stone-age village in New Guinea and Fleisch in a neighbouring village. Some very powerful force is at work ensuring that most versions of each gene survive, and that no version changes very much.38
That force is almost certainly disease. As soon as a lock gene becomes rare, the parasite key gene that fits it becomes rare, so that lock gains an advantage. In a case where rarity is at a premium, the advantage is always swinging from one gene to another and no gene is ever allowed to become extinct. To be sure, there are other mechanisms that can favour polymorphism: anything that gives rare genes a selective advantage over common genes. Predators often do this, by overlooking rare forms and picking out common forms. Give a bird in a cage some concealed pieces of food, most of which are painted red but a few painted green, and it will quickly get the idea that red things are edible, and will initially overlook green things. J. B. S. Haldane was the first to realize that parasitism, even more than predation, could help to maintain polymorphism, especially if the parasite’s increased success in attacking a new variety of host goes with reduced success against an old variety – which would be the case with keys and locks.39
The key and lock metaphor deserves closer scrutiny. In flax,