The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [33]
Parasites and their hosts are locked in a close evolutionary embrace. The more successful the parasite’s attack (the more hosts it infects or the more resources it gets from each), the more the host’s chances of survival will depend on whether it can invent a defence. The better the host defends, the more natural selection will promote the parasites that can overcome the defence. So the advantage will always be swinging from one to the other: the more dire the emergency for one, the better it will fight. This is truly the world of the Red Queen, where you never win, you only gain a temporary respite.
Battles of Wit
It is also the inconstant world of sex. Parasites provide exactly the incentive to change genes every generation that sex seems to demand. The success of the genes that defended you so well in the last generation may be the best of reasons to abandon these same gene combinations in the next. By the time the next generation comes around, the parasites will have surely evolved an answer to the defence that worked best in the last generation. It is a bit like sport. In chess or in football, the tactic that proves most effective is soon the one that people learn easily to block. Every innovation in attack is soon countered by another in defence.
But of course the usual analogy is an arms race. America builds an atom bomb, so Russia does, too. America builds missiles; so must Russia. Tank after tank, helicopter after helicopter, bomber after bomber, submarine after submarine, the two countries run against each other, yet stay in the same place. Weapons that would have been invincible twenty years before are now vulnerable and obsolete. The bigger the lead of one superpower, the harder the other tries to catch up. Neither dares step off the treadmill while it can afford to stay in the race. Only when the economy of Russia collapses, does the arms race cease (or pause).27
These arms-race analogies should not be taken too seriously, but they do lead to some interesting insights. Richard Dawkins and John Krebs raised one argument derived from arms races to the level of a ‘principle’: the ‘life–dinner principle’. A rabbit running from a fox is running for its life, so it has the greater evolutionary incentive to be fast. The fox is merely after its dinner. True enough, but what about a gazelle running from a cheetah? Whereas foxes eat things other than rabbits, cheetahs eat only gazelles, so a slow cheetah never catches anything and dies. A slow gazelle might never be unlucky enough to meet a cheetah. So the down-side is greater for the cheetah. As Dawkins and Krebs put it, the specialist will usually win the race.28
Parasites are supreme specialists, but arms-race analogies are less reliable for them. The flea that lives in the cheetah’s ear has what economists call an ‘identity of interest’ with the cheetah: if the cheetah dies, it dies. Gary Larson once drew a cartoon of a flea walking through the hairs on a dog’s back carrying a placard that read: ‘The end of the dog is nigh’: the death of the dog is bad news for the flea, even if the flea hastened it. The question of whether parasites benefit from harming their hosts has vexed parasitologists for many years. When a parasite first encounters a new host (myxomatosis in European rabbits, AIDS in human beings, plague in fourteenth-century Europeans) it usually starts off as extremely virulent, but gradually becomes less so. But some diseases remain fatal while others quickly become almost harmless. The explanation is simple: the more contagious the disease, and the fewer resistant hosts there are around, the easier it will be to find a new host. So contagious diseases in unresistant populations need not