The world is divided into things that look designed (like birds and airliners) and things that don't (rocks and mountains). Things that look designed are divided into those that really are designed (submarines and tin openers) and those that aren't (sharks and hedgehogs). The diagnostic of things that look (or are) designed is that their parts are assembled in ways that are statistically improbable in a functional direction. They do something well: for instance, fly.
Darwinian natural selection can produce an uncanny illusion of design. An engineer would be hard put to decide whether a bird or a plane was the more aerodynamically elegant.
So powerful is the illusion of design, it took humanity until the mid-19th century to realise that it is an illusion. In 1859, Charles Darwin announced one of the greatest ideas ever to occur to a human mind: cumulative evolution by natural selection. Living complexity is indeed orders of magnitude too improbable to have come about by chance. But only if we assume that all the luck has to come in one fell swoop. When cascades of small chance steps accumulate, you can reach prodigious heights of adaptive complexity. That cumulative build-up is evolution. Its guiding force is natural selection.
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