Steven Pinker -- How the Mind Works - Hampshire High Italian ...

462 J HOW THE MIND WORKSwasting half their pregnancies on sons who lack the machinery to makegrandchildren and are nothing but sperm donors? Why do people andother organisms swap out half their genes for the genes of another memberof the species, generating variety in their offspring for variety's sake?It's not to evolve faster, because organisms are selected for fitness in thepresent. It's not to adapt to environmental change, because a randomchange in an already adapted organism is more likely to be for the worsethan for the better, there being vastly more ways to be badly adaptedthan to be well adapted. The best theory, proposed by John Tooby,William Hamilton, and others, and now supported by several kinds ofevidence, is that sex is a defense against parasites and pathogens (disease-causingmicroorganisms).From a germ's point of view, you are a big yummy mound of cheesecake,there for the eating. Your body takes a different view, and hasevolved a battery of defenses, from your skin to your immune system, tokeep them out or do them in. An evolutionary arms race goes onbetween hosts and pathogens, though a better analogy might be anescalating contest between lockpickers and locksmiths. Germs aresmall, and they evolve diabolical tricks for infiltrating and hijacking themachinery of the cells, for skimming off its raw materials, and for passingthemselves off as the body's own tissues to escape the surveillanceof the immune system. The body responds with better security systems,but the germs have a built-in advantage: there are more of them andthey can breed millions of times faster, which makes them evolve faster.They can evolve substantially within the lifetime of a host. Whatevermolecular locks the body has evolved, the pathogens can evolve keys toopen them.Now, if an organism is asexual, once the pathogens crack the safe ofits body they also have cracked the safes of its children and siblings. Sexualreproduction is a way of changing the locks once a generation. Byswapping half the genes out for a different half, an organism gives its offspringa head start in the race against the local germs. Its molecular lockshave a different combination of pins, so the germs have to start evolvingnew keys from scratch. A malevolent pathogen is the one thing in theworld that rewards change for change's sake.Sex poses a second puzzle. Why do we come in two sexes? Why do wemake one big egg and lots of little sperm, instead of two equal blobs thatcoalesce like mercury? It is because the cell that is to become the babycannot be just a bag of genes; it needs the metabolic machinery of the