Big enough to undermine the idea of creation but simple enough to be stated in a sentence, the theory of natural selection is a masterpiece, writes Richard Dawkins

Charles Darwin had a big idea, arguably the most powerful idea ever. And like all the best ideas it is beguilingly simple. In fact, it is so staggeringly elementary, so blindingly obvious that although others before him tinkered nearby, nobody thought to look for it in the right place.

Darwin had plenty of other good ideas - for example his ingenious and largely correct theory of how coral reefs form - but it is his big idea of natural selection, published in On the Origin of Species, that gave biology its guiding principle, a governing law that helps the rest make sense. Understanding its cold, beautiful logic is a must.

Natural selection's explanatory power is not just about life on this planet: it is the only theory so far suggested that could, even in principle, explain life on any planet. If life exists elsewhere in the universe - and my tentative bet is that it does - some version of evolution by natural selection will almost certainly turn out to underlie its existence. Darwin's theory works equally well no matter how strange and alien and weird that extraterrestrial life may be - and my tentative bet is that it will be weird beyond imagining.

Explanation ratio

But what makes natural selection so special? A powerful idea assumes little to explain much. It does lots of explanatory "heavy lifting", while expending little in the way of assumptions or postulations. It gives you plenty of bangs for your explanatory buck. Its Explanation Ratio - what it explains, divided by what it needs to assume in order to do the explaining - is large.

If any reader knows of an idea that has a larger explanation ratio than Darwin's, let's hear it. Darwin's big idea explains all of life and its consequences, and that means everything that possesses more than minimal complexity. That's the numerator of the explanation ratio, and it is huge.

Yet the denominator in the explanatory equation is spectacularly small and simple: natural selection, the non-random survival of genes in gene pools (to put it in neo-Darwinian terms rather than Darwin's own).

You can pare Darwin's big idea down to a single sentence (again, this is a modern way of putting it, not quite Darwin's): "Given sufficient time, the non-random survival of hereditary entities (which occasionally miscopy) will generate complexity, diversity, beauty, and an illusion of design so persuasive that it is almost impossible to distinguish from deliberate intelligent design." I have put "which occasionally miscopy" in brackets because mistakes are inevitable in any copying process. We don't need to add mutation to our assumptions. Mutational "bucks" are provided free. "Given sufficient time" is not a problem either - except for human minds struggling to take on board the terrifying magnitude of geological time.

A certain kind of mind

It is mainly its power to simulate the illusion of design that makes Darwin's big idea seem threatening to a certain kind of mind. The same power constitutes the most formidable barrier to understanding it. People are naturally incredulous that anything so simple could explain so much. To a naive observer of the wondrous complexity of life, it just must have been intelligently designed.

But intelligent design (ID) is the polar opposite of a powerful theory: its explanation ratio is pathetic. The numerator is the same as Darwin's: everything we know about life and its prodigious complexity. But the denominator, far from Darwin's pristine and minimalist simplicity, is at least as big as the numerator itself: an unexplained intelligence big enough to be capable of designing all the complexity we are trying to explain in the first place!

Here may lie the answer to a nagging puzzle in the history of ideas. After Newton's brilliant synthesis of physics, why did it take nearly 200 years for Darwin to arrive on the scene? Newton's achievement seems so much harder! Maybe the answer is that Darwin's eventual solution to the riddle of life is so apparently facile.

Claims to priority were made on behalf of others, and by Patrick Matthew in the appendix to his work On Naval Timber, as was punctiliously acknowledged by Darwin in later editions of the Origin. However, although Matthew understood the principle of natural selection, it is not clear that he understood its power. Unlike Darwin and Alfred Russel Wallace, who hit on natural selection independently, prompting Darwin to publish his theory, Matthew seems to have seen selection as a purely negative, weeding-out force, not the driving force of all life. Indeed, he thought natural selection so obvious as to need no positive discovery at all.

Garbled versions

Although Darwin's theory can be applied to much beyond the evolution of organic life, I want to counsel against a different sense of Universal Darwinism. This is the uncritical dragging of some garbled version of natural selection into every available field of human discourse, whether it is appropriate or not.

Maybe the "fittest" firms survive in the marketplace of commerce, or the fittest theories survive in the scientific marketplace, but we should at very least be cautious before we get carried away. And of course there was Social Darwinism, culminating in the obscenity of Hitlerism.

Less obnoxious but still intellectually unhelpful is the loose and uncritical way in which amateur biologists apply selection at inappropriate levels in the hierarchy of life. "Survival of the fittest species, extinction of poorly adapted species" sounds superficially like natural selection, but the apparent resemblance is positively misleading. As Darwin himself was at pains to point out, natural selection is all about differential survival within species, not between them.

I'll end on a subtler legacy of Darwin's big idea. Darwin raises our consciousness to the sinewy power of science to explain the large and complex in terms of the small and simple. In biology we were fooled for centuries into thinking that extravagant complexity in nature needs an extravagantly complex explanation. Darwin triumphantly dispelled that delusion.

There remain deep questions, in physics and cosmology, that await their Darwin. Why are the laws of physics the way they are? Why are there laws at all? Why is there a universe at all? Once again, the lure of "design" is tempting. But we have the cautionary tale of Darwin before us. We've been through all that before. Darwin emboldens us - difficult as it is - to seek genuine explanations: explanations that explain more than they postulate.

Richard Dawkin FRS is the Charles Simonyi Professor of the Public Understanding of Science at Oxford. His latest book is The God Delusion