Men are supposed to mellow in their mid-60s. Richard Dawkins appears to be going the other way. Never one to tolerate fools at the best of times, he's become noticeably less patient as the years roll by. "It does appear that I've become rather more grumpy," he says, without appearing that bothered one way or another. And despite a contented home life with his third wife, the actor Lalla Ward, there's a great deal to be grumpy about.

Back in 1976, as a 30-something research fellow recently returned to Oxford after the obligatory two-year stint in the US at the University of California at Berkeley, Dawkins secured his reputation with The Selfish Gene as a cutting-edge thinker and a man blessed with the common touch. Long before popularising science became a career route for academics, Dawkins managed to advance the scientific understanding of the evolutionary process, while making that knowledge accessible to the general reader.

There were two key parts to The Selfish Gene. The first was Dawkins's inversion of the process of natural selection. Instead of trotting out the established view that organisms use genes to self-replicate, Dawkins made the revolutionary suggestion that genes use organisms to propagate themselves, an idea that immediately answered many of the difficult questions of Darwinism, such as the apparent selflessness of some animal behaviour. The second important theme was the rehabilitation of memes, self-replicating cultural transmissions - "viruses of the mind" - that are passed on both vertically and horizontally within families. And it is the meme, or rather one particular meme, that is the prime cause of Dawkins's current grumpiness.

According to memetic theory, memes are subject to the same process of natural selection as genes. And yet one meme, the religious meme, steadfastly refuses to die. You can see where the religious meme sprung from: when the world was an inexplicable and scary place, a belief in the supernatural was both comforting and socially adhesive. But as our understanding of the world grew, you might have expected the religious meme to give way to rationalism. Yet the opposite has happened. Despite overwhelming scientific evidence for the Darwinian explanation of evolution, religious belief - and fundamentalist religion at that - remains as ingrained as ever.

Religion offends every bone in Dawkins's rational, atheist body. "You can see why people may want to believe in something," he acknowledges. "The idea of an afterlife where you can be reunited with loved ones can be immensely consoling - though not to me. But to maintain such a belief in the face of all the evidence to the contrary is truly bewildering." If individual faith is, for Dawkins, an expression of an ignorance, collective faith and organised religion embody something much more pernicious. That is what drove him to make two films for Channel 4, the first of which was shown last night, and to write his new book, The God Delusion, to be published in September.

Dawkins describes these projects as "consciousness-raising exercises" but the films come across as full-frontal assaults. Protestantism, Catholicism, Judaism and Islam all get both barrels. Powerful and well-argued, they are; subtle, they ain't. Richard Harries, Bishop of Oxford, gets a walk-on role as the liberal voice of religion, but mostly it's the fundamentalists of all faiths who fall under Dawkins's scrutiny. "They are profoundly wrong," he says, "but in some ways I have more sympathy with their views than I do with the so-called more liberal wings. At least the fundamentalists haven't tried to dilute their message. Their faith is exposed for what it is for all to see."

No such thing

What angers Dawkins most is the way religion gets such an easy ride. "We treat it with a politically correct reverence that we don't accord to any other institution," he says. "Even secularists talk about Jewish, Catholic and Muslim children. There's no such thing. Children aren't born with a particular religious gene. What they are is children of Jewish, Catholic and Muslim parents. If you started to talk about monetarist or Marxist children, everyone would consider you abusive. Yet for religion we make an exception. We are incapable of distinguishing between race and religion. There is some statistical correlation between the two, but they are very different entities and we shouldn't allow them to be confused."

Predictably, Dawkins has no time for faith schools. "Segregation has no place in the education system," he argues. "Take Northern Ireland. You could get rid of the climate of hostility within a generation by getting rid of segregated schooling. Separating Catholics and Protestants has fomented centuries of hostility." But Dawkins reserves his greatest scorn for creationists. "How any government could promote the Vardy academies in the north-east of England is absolutely beyond me. Tony Blair defends them on grounds of diversity, but it should be unthinkable in the 21st century to have a school whose head of science believes the world is less than 10,000 years old."

Evolution offers Dawkins all the explanations he needs - "if there are other worlds elsewhere in the universe, I would conjecture they are governed by the same laws of natural selection" - but he does acknowledge there are still large gaps in our knowledge. "Of course, we would love to know more about the exact moment of Big Bang," he says, "but interposing an outside intelligence does nothing to add to that knowledge, as we still know nothing about the creation of that intelligence."

Unfortunately for Dawkins, it is into precisely these gaps that faith and superstition insinuate themselves, a problem made worse for secularists when scientists declare a religious affiliation. "I think the figures are somewhat overstated in this country," he says tersely, "as it's generally the same three scientists making their voices heard. Most scientists use the term God in the way that Einstein did, as an expression of reverence for the deep mysteries of the universe, a sentiment I share.

"In the US, the picture is rather different. Coming out as an atheist can cost an academic his or her job in some parts of America, and many choose to keep quiet about their atheism. In a recent survey, 40% of US scientists said they believed in God; however, when the sample was narrowed to those in the National Academy [the US equivalent of the Royal Society] the figure was down to 10%."

He didn't start out as an unbeliever. Dawkins was born into a middle-class family that went to church each Christmas. At school, Anglicanism, if not rammed down the throat, was at least a given. "I had my first doubts when I was nine," he recalls, "when I realised there were lots of different religions and they couldn't all be right. However I put my misgivings on hold when I went to Oundle and got confirmed. I only stopped believing when I was about 15."

Opponents have claimed that Dawkins offers a bleak view of humanity, something he categorically denies. "The chances of each of us coming into existence are infinitesimally small," he argues, "and even though we shall all die some day, we should count ourselves fantastically lucky to get our decades in the sun." But even he expresses regret at our long-term prospects. "Within 50 million years, it's highly unlikely humans will still be around and it is sad to think of the loss of all that knowledge and music."

Greatest skill

Dawkins's greatest skill has been to synthesise other people's material and come up with different ways of thinking about problems that revolutionise future research. But to write him off as an ideas man, pure and simple, is to lose sight of the man. He may not do any white-coat lab work these days but he can number-crunch with the best of them. In person, he's friendly rather than approachable, and there's a hint of distance that suggests someone more at home in front of a computer than with other people.

"I did used to be addicted to computer programming," he admits. "In the early days, there was no off-the-shelf software and I wrote everything, from my own word-processing programmes to more complex programmes simulating cricket sounds that were necessary for my research. However, I now view programming as a vice, so I don't allow myself to do it."

This split between the nerd and the populist has been evident all through his career. The nerd may have been more in evidence early on - not least when he was doing his doctorate and ignored the advice of his Nobel prize-winning supervisor, Nikolaas Tinbergen, and opted for a stats fest, "a classic piece of Popperian science", instead of a fluffier study of animal behaviour - but it's still around. Though Dawkins has held the Charles Simonyi Chair in the Public Understanding of Science at Oxford since 1995 and gets more attention than most other scientists, you sense there's still a part of him that's not altogether comfortable in the public gaze.

It seems self-evident that his recent work has become more polemical, though he becomes strangely reticent when you suggest he's now a political figure as much as a scientist. "I don't know about that," he says. "I wouldn't want to make those claims." But then he adds that he wishes more scientists would stand up to be counted in the public arena.

There are similar competing pulls elsewhere. After declaring himself a recently converted anti-monarchist and delivering a withering attack on Prince Charles - "he's clearly soft on religion, just as he is on every dopey, half-baked failure to think" - he pulls back, saying he has nothing against Prince Charles as a person and giving the thumbs up to the Queen.

Even so, no one's ever going to die wondering what Dawkins really thinks. He may agonise over the thinking process and worry about how his ideas are interpreted, but the real voice always emerges in the end. Perhaps it is the populariser's dilemma: you get remembered for the soundbite rather than the complexity.

Put on the spot, Dawkins reveals he believes his lasting contribution to science is his 1984 book, The Extended Phenotype. Most lay people have long since forgotten or never heard of the book in which he argued that genes extend beyond their physical organisms - think beavers' dams and birds' nests - to ensure their survival.

But phenotypes have to remain on hold for the time being as it's religion that Dawkins has in his sights for the forseeable future. And what if, by some mischance, he were to find there is a God when he dies? He looks at me as if I were mad. "The question is so preposterous that I can hardly grace it with a hypothetical answer," he says finally. "But, to quote Bertrand Russell, I suspect I would say, 'There's not enough evidence, God'."

Curriculum vitae

Name: Richard Dawkins

Age: 64

Job: Charles Simonyi Chair in the Public Understanding of Science at Oxford

Likes: walking the dog

Dislikes: back-to-front baseball caps, gratuitous noise

Books: The Selfish Gene, The Extended Phenotype, The Blind Watchmaker, Climbing Mount Improbable, The Ancestor's Tale

Married: to Lalla Ward (of Dr Who fame), one daughter from previous marriage