The former Pope Benedict XVI has referred to The Selfish Gene as science fiction in a long letter to an Italian atheist, published in La Repubblica. This is not really fair to the book, nor to science fiction, but does capture an important point about popular science. What sells is not the stuff about science, but the stuff about human beings.

The first thing to be said about The Selfish Gene is that it is a very fine piece of pop science writing indeed. It is not as dense and thought-provoking as Richard Dawkins's second book, The Extended Phenotype – but without it, who would had bought or read the latter? – and it is not as accomplished as The Blind Watchmaker or Climbing Mount Improbable but those early books are much better than anything he has produced in his subsequent career. Their freshness and direct force is extraordinary.

The Selfish Gene must have inspired thousands of people to take up biology. Beyond that, it had a huge influence on the culture of nerds. There is nothing original in the biology and some can now be seen to be wrong, but that's the fate of any 30-year-old undergraduate text (it grew out of his lectures to students). What makes it so powerful is the vision of algorithmic biology: the idea that there are mathematical laws governing the development of living things which can be seen working themselves out over time and could, if you wanted them to, be modelled in computers.

Of course, the idea of mathematical regularities emerging through the operations of natural selection over the course of evolution isn't new or original to Dawkins. It is the fundamental matter of theoretical biology. The whole of the British school of population genetics, from Ronald Fisher, down through JBS Haldane to John Maynard Smith, worked with this stuff. But no one before Dawkins had brought this vision into the wider culture, and no one except him could have done. The nearest approach is perhaps Olaf Stapledon's Last and First Men which is infused with a Darwinian vision but which – as a work of science fiction – entirely lacks Dawkins's gift for the anchoring anecdote or biological fact. I think it's reasonable to claim that no one except perhaps William Gibson furnished more of the imaginations of Silicon Valley than Dawkins did.

But alongside the intellectual force and drive, wrapped round it and giving it shape, as histones give shape to DNA, came Dawkins's shadow side – the fact that he is his own greatest fan and believer. You may think the competition for this position is too great for there to be any single winner but I think it's safe to say that not even the most devoted of his groupies have their partner read out loud from his books at bedtime, as he does. But even if he does have readers more delighted in his cleverness than he is himself, they don't have quite the same corrupting effect on his understanding.

In particular, the ascription of agency to genes led him and his followers into endless confusion. The point is not merely whether genes can be selfish or generous, but whether they can be said to have any activity at all in the world. This is a point which he freely concedes and then forgets – his manner of dealing with most criticism. If a gene is defined, as he defines it, as a piece of chromosomal material subject to the pressures of selection, it is the pressures of selection which are the active and changing parts of the picture, and the DNA sequence is entirely passive.

It is still less true to imagine that genes "build" us into "great lumbering robots". The process by which a stretch of DNA sequence becomes a protein is complicated, and determined by cellular mechanisms which are in turn reacting to pressures from their environment. The process by which proteins become bodies is even more complicated.

The Selfish Gene is a brilliant phrase. It's also accurate, so long as you realise that "selfish" doesn't mean selfish, "gene" doesn't mean gene, and the definite article is a bit of an abstraction. But taken as the literal truth, it's about as much use as "In the beginning was the word". Given Dawkins's hostility to everyone else's metaphysics, this is an unfortunate weakness. "Science fiction" may not be the right term for the book but it does capture the sense in which its hold on the imagination depends on the parts that aren't science but dazzling metaphor.