(Image shamelessly scraped from BuildUpThatWall.com)

From “Why Darwin Matters“, by 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.

[...]

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.