Mendel's demon

on the shelves of any evolutionary biologist or student of the subject and you are likely to see a textbook by Mark Ridley. But Ridley, formerly at Emory University in Atlanta, and now in Oxford, has had another book in his mind for quite some time. He believes there are some ideas sparked by the Austrian monk, Gregor Mendel, that are of broader interest and has set out to explore some of them in a new book already published in the UK and shortly to appear in the US. Ridley's key question is how did complex, multicellular life come about. Why isn't life all simple? The Earth teems with single-celled life which occupies a stunning array of niches from deep-sea vents to mountain glaciers. Estimates suggest that single-celled life forms such as bacteria together contain about the same amount of carbon tied up in living organisms as all of the plants on the Earth today. With small genomes, they have been able to develop error repair mechanisms to combat mutations and maintain the information in their genomes. And with largely clonal reproduction, genetic information can be transferred straightforwardly to subsequent generations. Complex life, in contrast, involves organisms of increasing size and larger genomes which stretch the abilities of error repair mechanisms to spot and repair mutations. In addition, sexual reproduction leads to only half of any individual's genes being passed on to any particular offspring. What had not been known until recently is the sheer number of mutations that occur. A human being makes 200 or so copying mistakes every time he or she breeds, says Ridley. This is a big number, making it something of a mystery how we can cope. " If that number of mutations are introduced each generation, a similar number must somehow be purged. Otherwise our genes would be steadily randomised over time by mutation, and a random set of genes can no more code for a living creature than a random set of letters can code for a Shakespeare play, " he says. This revelation has suggested one possibly important function of sex in complex organisms: the removal of bad genes. This theory has been developed largely by Alexey Kondrashov. John Maynard Smith has described an analogy: imagine you take broken-down cars of the same make, one immobilized by a faulty ignition system and the other with broken brakes. By swapping parts it is …