Darwin spent five years sailing around the world on the Beagle, and everywhere he looked, he saw the same puzzle. Why did the Galápagos have so many types of finches, each with a different beak? Why did fossil armadillos in Argentina look so much like living ones? Why did island species differ from their mainland cousins?
The answer hit him while reading Malthus’s work on population growth. Malthus showed that populations grow faster than their food supply, leading to competition.
Here’s what he realized: All living things produce more offspring than can possibly survive. A single oak tree releases thousands of acorns, yet only a few become trees. If all the offspring of just one pair of elephants survived and bred, we’d have millions of elephants in a few centuries.
But nature keeps populations in check. Some individuals survive and reproduce while others don’t. The survivors usually have some advantage – stronger, faster, better camouflaged. The key point is that these useful variations get passed down to their offspring.
This process, repeated over millions of years, explains everything Darwin saw. The Galápagos finches? Ancestors arrived from the mainland, and birds with slightly different beaks survived better on different foods. Generation after generation, the beaks became more specialized. The same process explains how giraffes got their long necks and how insects match the color of leaves.
Darwin called this process “natural selection” because nature selects the best-adapted individuals to survive and reproduce, just as farmers select the best plants and animals to breed. The difference is that nature works slowly, without a plan—keeping what works and discarding what doesn’t.
This insight changed how we see the living world. Life isn’t static – it’s constantly changing. All living things are related through a vast family tree of descent with modification. We humans are part of this tree, intimately related to every other species on Earth.
That’s the simple truth Darwin discovered, though it took him twenty years to gather the courage to publish it. The most profound ideas are also the simplest. From this counterintuitive insight flows all the stunning diversity of life. We’re all the better for it.