Richard Dawkins has published an article in Free Enquiry titled The Power of Darwin. Here are the opening paragraphs.
Charles Darwin had a big idea, arguably the most powerful idea ever. A powerful idea assumes little to explain much. It does a lot of explanatory “heavy lifting” while expending little in the way of assumption or postulation. It gives you plenty of bang 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.Natural selection does NOT explain all of life and its consequences. A great deal of what we see in modern species is a consequence of accident and happenstance where contingency rules over natural selection. Natural selection does NOT explain diversity.
Power of a theory = That which it explains/That which it needs to assume in order to do the explaining
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 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).
Power of Darwin’s theory = The diverse complexity of life/Non-random survival
Natural selection is an improbability pump—a process that generates statistical improbability. It systematically seizes the minority of random changes that have what it takes to survive and accumulates them, step by tiny step over unimaginable timescales, until evolution eventually scales mountains of improbability and diversity whose height and range seem to know no limit.
Darwin's big idea was to convince us that life has evolved rather than being created. That's a simple concept that explains a lot.
Natural selection explains adaptation. That's extremely important and extremely interesting but it's only a small part of evolution. Random genetic drift, which Darwin does not get credit for, explains much more because more of evolution is due to drift than to adaptation.
The contributions of Charles Darwin are enormous. That's why he gets credit for being the greatest scientist who ever lived. It does a disservice to his achievements to exaggerate them in celebration of the 150th anniversary of the publication of Origin of Species.