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worry about killing their hosts, because they have already moved on. But when most potential hosts are already infected or resistant, and the parasite has difficulty moving from host to host, it must take care not to kill its own livelihood. In the same way an industrial boss who pleads with his workers, ‘Please don’t strike, or the company will go bust’, is likely to be more persuasive if unemployment is high than if the workers already have other job offers. Yet, even where virulence declines, the host is still being hurt by the parasite and is still under pressure to improve its defences, while the parasite is continually trying to get around those defences and sequester more resources to itself at the host’s expense.29

Artificial Viruses

Startling proof of the fact that parasites and hosts are locked in evolutionary arms races has come from a surprising source: the innards of computers. In the late 1980s, evolutionary biologists began to notice a new discipline growing up among their more computer-adept colleagues called ‘artificial life’. Artificial life is a hubristic name for computer programs that are designed to evolve through the same process of replication, competition and selection as real life. They are, in a sense, the ultimate proof that life is just a matter of information; and that complexity can result from directionless competition, design from randomness.

If life is information, and life is riddled with parasites, then information, too, should be vulnerable to parasites. When the history of computers comes to be written, it is possible that the first program that will earn the appellation ‘artificially alive’ was a deceptively simple little two-hundred-line program written in 1983 by a graduate student at the California Institute of Technology, Fred Cohen. The program was a ‘virus’ that would insinuate copies of itself into other programs in just the same way that a real virus insinuates copies of itself into other hosts. Computer viruses have since become a worldwide problem. It begins to look as if parasites are inevitable in any system of life.30

But Cohen’s virus and its pesky successors were created by people. It was not until Thomas Ray, a biologist at the University of Delaware, conceived an interest in artificial life, that computer parasites first appeared spontaneously. Ray designed a system called Tierra that consisted of competing programs, which were constantly being filled by mutation with small errors. Successful programs would thrive at each other’s expense.

The effect was astonishing. Within Tierra, programs began to evolve into shorter versions of themselves. Programs seventy-nine instructions long began to replace the original eighty-instruction programs. But then suddenly there appeared versions of the program that were just forty-five instructions long: they borrowed half of the code they needed from longer programs. They were true parasites. Soon, a few of the longer programs had evolved what Ray called immunity to parasites. One program became impregnable to the attentions of one parasite by concealing part of itself. But the parasites were not beaten. A mutant parasite appeared in the soup that could find the concealed lines.31

And so the arms race escalated. Some of the times that he ran the computer, Ray was confronted with spontaneously appearing hyper-parasites, social hyper-parasites and cheating hyper-hyper-parasites. All within an evolving system of (initially) ridiculous simplicity. He had discovered that the notion of a host–parasite arms race is one of the most basic and unavoidable consequences of evolution.32

Arms-race analogies are flawed, though. In a real arms race, an old weapon rarely regains its advantage. The day of the longbow will not come again. In the contest between a parasite and its host, it is the old weapons, against which the antagonist has forgotten how to defend, that may well be the most effective. So the Red Queen may not stay in the same place so much as end up where she started from, like Sisyphus, condemned to spend eternity rolling