Why, I ask Richard Dawkins, is he so driven to explain and persuade people of the truth of Darwinism? "Oh, well, because it's true and because it's so mind-blowing that we are in a position to understand why we are here. We are privileged to be in one sense lumps of matter like any other lump of matter in the universe, but we are lumps of matter that have the apparatus that would be needed to understand why we are here. And how could you let anybody die without assisting them to take up that opportunity?"

To Dawkins, natural selection is not simply true, it is the Truth. Its awesome explanatory power, its sinuous consistency, its staggering simplicity combine to make it the single most popularly persuasive scientific theory of all. And Dawkins is its primary British apostle, a man driven to such exaltation by Darwinism's beauty that it is not enough to contemplate it in the study or the lab. He must tell the world.

As a result, he has become a national institution. Attacks on the theory are met with vehement public rebuttals. His letters to erring newspapers are rancorous and anguished. Like many zealots, he finds it almost beyond belief that people should not see the light. And, again, like many zealots, he often finds it necessary to stray far beyond the confines of his belief.

This is odd. Zoologists may often be passionate about animals, but it is rare, if not unique, to find one so passionate about ideas. We may feel relaxed, almost cosy with the spectacle of David Attenborough or Desmond Morris cooing over insects or apes, but there is something distinctly strange about Dawkins dashing out of his study to seize us by the throat and tell us he knows exactly why we are here.

At first, it becomes even stranger in the context of Dawkins the man. The letters suggest a fiery evangelist, a tough-minded, angry visionary. In reality, he is a gentle, slightly fussy man with the fragile air of a devout and excessively unworldly curate. "He is", a friend of his once whispered to me, "a complete child."

Like a child, he seems to take everything far too personally. Once during a public debate I accused him, routinely I thought, of straying far beyond the bounds of scientific competence. He winced, blushed, shuffled his papers and generally looked so injured that I felt rather brutal. At another conference, a philosopher and I pointed out, perfectly amiably, that he had misunderstood the Catholic doctrine on war. He was speechless, not taking in our straightforwardly factual points because, I thought, he could not bring himself to regard a philosopher and a writer as reliable authorities.

The other side of this coin is that the unworldliness is very much part of his persuasiveness and charm. His lecturing style is lucid in content yet tentative in manner. He smiles easily and quickly wins supporters and fans. He is groomed and good-looking and has the instantly seductive air of a brilliant, distracted man who needs looking after. It also helps that though he is 54, he appears to be 44.

Dawkins lives in a flat near his Oxford college with his third wife, the actress Lalla Ward, who first made her name as one of Doctor Who's "young assistants". This, she confesses over college lunch, has dogged her somewhat: she still gets invited to Doctor Who conventions - effectively colloquies of nerds.

The flat is cluttered with a collection of fairground horses belonging to Lalla and a mass of rather ethnic looking objects. Lalla is also an artist and Dawkins was wearing a white silk tie decorated with her paintings of wart hogs. Many of the objects turn out to be painted or embroidered by Lalla and there is a cabinet decorated by his mother, each panel depicting an aspect of Dawkins's life. An air of domestically creative femininity pervades.

Dawkins was brought up in Nyasaland - now Malawi - and Kenya, where his father was in the colonial service. One immediately assumes that the fauna of Africa lit the zoological flame in young Dawkins. But no.

"I don't think I ever got into zoology as a naturalist. I wish I had. But my interests have always been rather more cerebral and philosophical. Now going back to Africa in middle life, I love the wild life, but I've never been a naturalist."

This if the first clue to the oddity of Dawkins. The natural world is to him a puzzle requiring a solution. Unlike most zoologists, he does not revel in its abundance, rather he is driven to seek its underlying pattern.

"Biology for me answers deep questions about existence - in a way that's my dominant motivation. Understanding how complicated things work to me means understanding their parts and the interactions of their parts. I suppose that is a reductionist drive.

"I certainly don't wish to devalue the work of people who make it their business to know the plurality of life, the great, sweeping diversity. That seems to me admirable and enviable. But what I mean by understanding means asking the question where does it all come from? Why is it here? Why is it the way it is?"

Soon after the War, Dawkins's father inherited a farm in Oxfordshire and the family came to England. Through prep school and boarding school Dawkins seems to have been a somewhat colourless boy. He drifted towards biology, but largely because his father had been a biologist.

"I was not very interested in anything that I can say. Music, I suppose. I played the clarinet. I'm one of those people who have the rudimentary natural gift to be able to pick up any instrument and get a tune out of it. Bland? Yes I suppose it's true. That's the way I was. I'm not saying I think I am now."

He appears to have been looking for a centre around which to organise his intelligence. He was, so to speak, potentially passionate, but had been offered nothing to be passionate about. Certainly the lukewarm Anglicanism of his upbringing did little more than leave him with a source of nostalgia for hymns and evensong.

Darwinism had crossed his path at school, but, like many others, he felt the idea seemed too simple, too small to do the job it was supposed to do. He took zoology at Oxford almost by accident - he had wanted to do biochemistry but was told that he would be no good at it, so took the offer to read zoology at Balliol.

By his second year, the subject had taken hold and Dawkins had found his destiny. Driven by the discipline of the weekly essay, he immersed himself in the impersonal patterns of the animal world.

"I loved the intellectual challenge of reading original research papers in the library and becoming, in effect, an authority on some little subject. Every week I would become obsessed with the topic of the week."

Dawkins went on to do research with Nikko Tinbergen, the Nobel prize winner. He adopted a characteristically Dawkinsesque approach - modelling mathematically what might be going on in an animal's head and then testing it against the behaviour of the real animal. Though Dawkins is no mathematician, he loves computers, using them to model the natural world. Again, this was an unusually abstract approach for a zoologist and the method was sufficiently original to get him noticed by the Americans and, in 1967, he became assistant professor of zoology at Berkeley in California.

This was the days of student revolt. Dawkins was involved - "tear gas and peace marches and things" - in some ways that he regrets. He has always been inclined to the left and he still feels the anti-Vietnam action was justified, but much of the rest, he acknowledges, involved manipulation by "rather clever political operators".

He came back to Oxford in 1970 into virtu-ally the job he still holds. But Edward Heath's three day week meant power to the labs was cut off, so Dawkins began writing what was to become The Selfish Gene.

"I got a sabbatical in 1975 and in a kind of passion of writing I finished the book. I wanted to write a popular book, I wanted it to be a bestseller. At the time I thought I was simply popularising what professionals in my field knew about. With hindsight that turned out to be not quite true. There is more thought-provoking innovation in The Selfish Gene than I realised."

The first point about the book is that it is rather startlingly well- written. Not only does Dawkins construct lucid sentences, he also has a journalistic flare for drawing the reader into a subject by starting from the most astonishing premises. His writing has some of the thrill of science fiction.

But the second point is that, whatever its popular appeal, the book was intended as an assault on the prevailing wisdom about evolution. Writers such as Konrand Lorenz and Robert Ardrey had presented natural selection as a rather cosy process in which friendship, co-operation and altruism all seemed to flow from the Darwinian hypothesis. Dawkins regards the sentimentality of it all with clear contempt.

"It was all a rather nice, rather benign view of life. It was anathema to me. It was rife in television - almost every natural history programme on TV was about the balance of nature and all creatures pulling for the good of the whole eco-system - dung beetles scurrying around being the dustmen of society, ants beavering away for the good of ecology. What I knew to be the case was that every individual was ruthlessly struggling for its own welfare and in order to explain that I went back to the roots of natural selection. I was really just trying to put people straight. Lorenz actually said that aggression was good for the species because it prevents over-population. This is all plain wrong, diametrically wrong."

What irked Dawkins was that the sentimental balance-of-nature interpretation assumed selection worked at the level of the species or group. But, as far as he was concerned, the driving force was the fundamental unit of replication and mutation - the gene. We and every other living thing are just mechanisms for the perpetuation of the gene. Any other interpretation is wishful thinking. In effect, sentimental biologists were smuggling in the old religious belief in a benignly-designed world through the scientific back door.

But there was also a moral point. The soft left Dawkins wished to make it clear that looking to nature for our morality was a waste of time. There we would find only the crudest self-interest. Unusually for a zoologist, he is, in this sense, anti-nature. He feels morality has to be a social, artificial construction. Yet, ironically, The Selfish Gene was taken by some as a Thatcherite work, glorifying greed and contemptuous of the altruistic demands of society.

"You can easily see why that happened if you take the view that that which is natural is what we ought to do. I was taking the view that what is natural has no connection with what we ought to do. You don't take your political and moral philosophy from nature because, if you do, you end up with Hitlerism or maybe Thatcherism.

The book transformed Dawkins's career. He abandoned his strictly academic work in favour of defending and explaining his Darwinism to the wider world. In 1982, he produced The Extended Phenotype, a largely technical work that advanced his arguments, and then, in 1986, came The Blind Watchmaker, another popular book intended to combat the idea that anything as complex and intricate as life must, somehow, have been consciously designed. His latest, River Out of Eden, simply extends that thesis, this time emphasising the central image of DNA, the genetic messenger, as a river of information flowing down through the ages.

As with his letters to newspapers, the books are concerned, as much as anything else, with crushing the opposition

"Well," he says, "you have to reckon on the provocation. All right, certain aspects of our understanding of evolution may be wrong. But, on the whole, people who argue against it are enormously ignorant. One is forced into the position of thinking, 'What can I do about this?' Suppose you were a classical scholar and you were confronted with a book that suggested that Rome never existed and Latin was a 19th century invention. If you come out with all fists flying - as I do - then you are branded as arrogant. It's difficult to know how to handle it. Tactically, I've misjudged it sometimes."

Much of his problem stems from the fact that his public persona is actually in conflict with his intellectual position. Though he says nature is morally neutral and natural selection is not a moral model, he acts as if they are. His often petulant outbursts are those of a man whose deepest beliefs have been challenged, not of one engaged in objective debate. Equally, his assaults on religion would be entirely meaningless and unnecessary unless he felt a morality was at stake.

"Yes, but I don't get as angry as I used to. I get pushed into being angry because so many other people are so pro-religion that that's what they ask me about. And I am angry about certain things - about the Pope and contraception and opposition to IVF. I'm angry about religious leaders interfering in other people's lives and telling them what they should and shouldn't do. I'm angry about the kind of philosophy in which people say they are right because of inner conviction rather than because of evidence. That seems to me to be subversive of intellectual values which I hold dear."

But, of course, this has nothing to do with zoology, and Dawkins is only involved in such debates because he has become a public figure.

There can be no doubt that his anger and his fierce evolutionary fundamentalism has gone too far. He has overstated his case to the point where the very overstatement makes him vulnerable. He insists, for example, on the overwhelming explanatory power of natural selection, but divides this absolutely from the moral and creative force of civilisation. Yet in this he has the same problem as the religious believer. The believer can never explain how a benign God allowed evil into the world. Equally, Dawkins cannot explain how the selfish gene allowed goodness into the world.

His response is that the brain simply has such a massive superfluity of power that it escapes from the grim, narrow demands of the gene.

"Yes I suppose this moral monster, the gene, has produced a kind of benign Frankenstein - although it's not entirely benign. But there are many people who are nice and generous and utterly unlike what you'd expect from selfish genes. They employ contraceptives, they give money to Oxfam, they give blood... We have this apparatus in our brains which was given us by natural selection and we are subverting it."

But this means that everything of ultimate importance lies outside zoology. The best that we are and the conflicts that we face have no meaningful connection with evolution, they are cultural creations. But Dawkins still jumps feet first into arguments with bishops when, by his own definition, a mere evolutionist can have nothing to say on such matters. In effect, he is still fighting the battles of the 19th century, a time when it was simply a straight war between monkey ancestors and Adam and Eve.

Yet Dawkins is easily forgiven. His insistence on a morality independent of our knowledge of nature sets him apart from the most bone-headedly scientific popularisers. His enthusiasm for his subject, both in his books and in conversation, is fresh, boyish and fun. He knows how to make ideas thrilling and how to communicate his own wonder. If we are to have serious chatter about evolution, then it is as well we have a Dawkins to put the case. It is hardly a serious crime that this occasionally spills over into petulance, defensive arrogance and an awkward intellectual inconsistency. And even his odd, slightly precious unworldliness has its uses - it makes you listen for fear of upsetting him.

The talk over, Dawkins beams and spreads his hands conclusively. We wind our way between the fairground horses. And then, from nowhere, Lalla re- appears to make the tea and slice his birthday cake which is covered with wart hogs, this time in chocolate.

'River Out of Eden' is published on 8 May by Weidenfeld and Nicolson at £9.99