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There is a fresh evolutionary breeze blowing through this book. Psychiatrist Randolph Nesse and evolutionary biologist George Williams join forces to challenge the traditional focus of medicine on what they call the "how?" of disease (bacteria, viruses and so forth). We should be asking, they claim, not "how?" but "why?". Why should there be diseases at all? From an evolutionary perspective, this is the central mystery of medicine. Natural selection has wrought miracle on miracle of subtle biological engineering. Why, then, should so many and such diverse faults remain? Are we really the best nature can do? Are we, the survivors of three and a half billion years of cut-throat competitive selection, the furthest that fitness can go?

It is remarkable that this question has been so long coming. Darwin watchers have had a positive feast in recent years: books on evolution must be well up with consciousness and the cosmos in the literary top ten. Medicine, too - and especially genetic medicine - is a headline grabber. While as for "healing", well, new-age journalists please copy.

The "why?" of disease, moreover, once asked from an evolutionary perspective, is also a remarkably fertile question. Nesse and Williams offer a series of evolutionary answers. In the first place, they say, disease itself, viewed in an evolutionary perspective, is a hopelessly anthropocentric notion. The syphilis spirochete, the leprous mycobacterium, the malarial plasmodium, the viruses of HIV and Ebola . . . these are not diseases, evolutionarily speaking, but competitors. From the germ's point of view it is the host's defences that are the diseases.

Other diseases reflect the internal economy of evolution. Natural selection operates in a win-lose environment. For every gain there is a price to pay: walking upright frees our arms and hands but makes us vulnerable to back problems. Similarly, the gene that offers protection from malaria is also the gene that causes the severe blood disease sickle cell anaemia. Still other diseases are defences gone wrong: the immune response, essential to survival, may turn against the body's own tissues. Some diseases are simple failures of the evolutionary engineer: natural selection, it seems, is not fast enough to keep up with the pace of change of culture and technology, leaving us exposed to a range of modern ills from heart disease to sick building syndrome.

Nesse and Williams make a good case for the importance of the evolutionary perspective in medicine. It explains so much. The biologist's answer to the physicist's TOE (Theory of Everything), it touches, as Nesse and Williams describe, ageing, allergy, sex, pregnancy, cancer, Aids, indeed every concern of modern medicine. It is also a well-balanced perspective. The win-lose economy of evolution underlines the fact that not all symptoms are necessarily bad: the cough that is merely suppressed is the defence that is lowered.

The evolutionary perspective, furthermore, provides a salutary reminder of the limitations of medical science. Medicine has taken on, or has had thrust upon it, the role of a superhero, capable, in principle, of the final elimination of disease. It has had some brilliant successes - smallpox and polio, for example. But so long as we live in an evolutionary environment, mutation and counter-mutation, coupled with the pressures of natural selection, will ensure that we and our microscopic competitors remain locked in what Nesse and Williams graphically describe as an "arms race without end".

But will the evolutionary perspective, apparently so fruitful, catch on in medicine? Nesse and Williams are evangelical. Those whose work they describe are "pioneers and visionaries". But will medicine respond? Will there be the wholesale shift for which Nesse and Williams call from the conventional "how?" of disease to the evolutionary "why?".

Medicine has evolved recently by leaps and bounds, not in the light of good philosophy, still less through enthusiasms, (though both have been necessary), but pragmatically. A theory survives in medicine if it works. If it leads to new and better treatments, if indeed it leads to healing, it will, ultimately, flourish. If not it will be selected against and disappear into the fossil record of medical history.

By this pragmatic criterion Darwinian medicine has still to prove its evolutionary fitness. Nesse and Williams are careful to define the limits of their book. Its purpose is to stimulate interest, not just among doctors, but among all those concerned with health care. All the same, if it is to be a scientifically valid endeavour, as they claim, we might legitimately expect rather more prospect and less retrospect.

The book is certainly full of fascinating new insights, but these are mainly at the margins: morning sickness is an echo of the need to protect the foetus from toxins in the roots and berries of our ancestral hunter-gatherer diet: this is also why toddlers will not eat their greens. Fine. But where are the hypotheses, the experimental designs, the predictions, that will persuade the hard-nosed assessors at the Medical Research Council to fund evolutionary research on cancer or HIV?

Evolutionary theory, although traditionally better at explaining the outcome of the last race than predicting the winner of the next, could have a crucial role to play as we move into a future of gene therapy, transgenic animals, human-machine "symbiots", and other high-tech departures from biological evolution. But something better than a promissory note will be needed if it is to become more than merely a sideshow.

The (self-imposed) limitations of Nesse and Williams's book are most evident in the chapter on mental disorders. That there should be such a chapter at all in a book on medicine is a mark of their refreshingly innovative stance. Psychiatry remains, in popular as well as in medical culture, the Cinderella health-care discipline. Nesse and Williams give the subject the prominence it deserves. Yet even they fail to capitalise on its full potential for their evolutionary perspective.

As in the rest of medicine, evolutionary biology has had little direct impact on the development of modern psychiatry. But psychiatrists have a venerable tradition of association with evolution. Darwin corresponded with Henry Maudsley (one of the founders of modern scientific psychiatry) about the emotions; and the reforming Victorian psychiatrist, W. A. F. Browne, is said to have directly influenced the timing of publication of On The Origin Of Species. Throughout this century, moreover, psychiatrists have speculated about the evolutionary origins of the severest forms of mental disorder, such as schizophrenia. Directly anticipating Nesse and Williams's central question of the "why?" of disease, they have sought to explain why schizophrenia, which is in part genetically determined, should be maintained at a remarkably steady prevalence of about 2 per cent in most countries around the world, despite the (evolutionarily fatal) disadvantage of an average age of onset that coincides with sexual maturity.

Nesse and Williams touch on this. The gene survives, clearly, because of some counterbalancing evolutionary advantage - the gene for schizophrenia could code for social skills, say, or creativity or intelligence. These and many other possibilities have been aired over the years. What is dramatically new, though, and yet not mentioned by Nesse and Williams, is the way in which this evolutionary perspective, and their central question of the "why?" of disease, are now, for the first time, being linked with developments in high-tech medicine to shape specific and testable hypotheses. There is a remarkable cross-disciplinary cauldron here, involving artificial intelligence, the philosophy of language, animal psychology and dynamic brain imaging. But as the Oxford research scientist Tim Crow has recently argued, the critical paradigm is evolutionary.

Here, then, is an area of medical research where an evolutionary perspective, combined with the powerful tools of modern high-tech science, and the mathematical and conceptual resources of a reinvigorated philosophy and psychology of mind, is already making a distinctive prospective contribution to medicine. Here indeed is an area in which, as in so many others, psychiatry, far from lagging behind the rest of medicine, could be in the forefront. Yet for Nesse (the psychiatrist) and Williams, the value of evolutionary biology for psychiatry is merely that it could help to convert the subject into a properly scientific medical discipline, to "bring the study of mental disorders back to the fold of medicine" .

Evolution and Healing is a very good book that amply achieves its main purpose of stimulating wider interest in Darwinian medicine. It is witty, enthusiastic, packed with lively detail, clearly written, highly accessible to the non-specialist, often provocative to the specialist. Whether the evolutionary breeze Nesse and Williams have started in medicine blows up into a genuine wind of change, or whether it remains mere (if fascinating) evolutionary huff and puff, will depend on the survival potential of evolutionary theories in the pragmatic environment of medical science.

Bill Fulford is professor of philosophy and mental health, University of Warwick.