The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [49]
In Praise of Unilateral Disarmament
By and large, however, the common interest of the genes prevails over the ambitions of the outlaws. As Egbert Leigh has put it, a ‘parliament of genes’ asserts its will.19 Yet the reader is surely restless. ‘This little tour of the cellular bureaucracy,’ he says, ‘fun though it was, has brought us no closer to the question asked at the beginning of the chapter – why there are two genders.’
Have patience. The road we have chosen – to seek conflicts between sets of genes – leads to the answer. For gender itself may prove to be a piece of cellular bureaucracy. A male is defined as the gender that produces sperm or pollen: small, mobile, multitudinous gametes. A female produces few, large, immobile gametes called eggs. But size is not the only difference between male and female gametes. A much more significant difference is that there are a few genes that come only from the mother. In 1981, two scientists at Harvard whose perspicacity we will re-encounter throughout the book, Leda Cosmides and John Tooby, pieced together the history of an even more ambitious genetic rebellion against this parliament of genes, one that forced the evolution of animals and plants into strange new directions and resulted in the invention of two genders.20
So far I have treated all genes as similar in their pattern of inheritance. But this is not quite accurate. When a sperm fertilizes an egg it donates just one thing to that egg, a bag full of genes called a nucleus. The rest of it stays outside the egg. But a few of the father’s genes are left behind, because they are not in the nucleus at all; they are in little structures called organelles. There are two main kinds of organelles, mitochondria, which use oxygen to extract energy out of food, and chloroplasts (in plants), which use sunlight to make food out of air and water. These organelles are almost certainly the descendants of bacteria that lived inside cells and were ‘domesticated’ because their biochemical skills were of use to the host cells. Being descendants of free-living bacteria, they came with their own genes, and they still have many of these genes. Human mitochondria, for example, have thirty-seven genes of their own. To ask ‘Why are there two genders?’ is to ask ‘Why are organelle genes inherited through the maternal line?’21
Why not just let the sperm’s organelles into the egg, too? Evolution seems to have gone to extraordinary lengths to keep the father’s organelles out. In plants a narrow constriction prevents the father’s organelles from passing into the pollen tube. In animals the sperm is given a sort of strip search as it enters the egg, to remove all the organelles. Why should this be?
The answer lies in the exception to this rule: an alga called Chlamydomonas that has two genders called plus and minus rather than male and female. In this species, the two parents’ chloroplasts engage in a war of attrition that destroys ninety-five per cent of them. The five per cent left standing are those of the plus parent, which by force of sheer numbers overwhelm the minus ones.22 This war impoverishes the whole cell. The nuclear genes take the same dim view of it as the prince takes in Romeo and Juliet of the war between two of his subjects:
Rebellious subjects, enemies to peace,
Profaners of this neighbour-stainèd steel –
Will they not hear? What ho! – you men, you beasts,
That quench the fire of your pernicious rage
With purple fountains issuing from your veins!
On pain of torture, from these bloody hands
Throw your mistempered weapons to the ground
And hear the sentence of your moved prince.
Three civil brawls, bred of an airy word
By thee, Old Capulet, and Montague,
Have thrice disturbed the quiet of our streets
… If ever you disturb our streets again,
Your lives shall pay the forfeit of the peace.
Romeo and Juliet, Act I, Scene 1