In a cave, a bear gives birth to two cubs one long dark night. In the morning, the weak winter light reveals something strange: the cubs' fur is white, in stark contrast to the dark fur of their mother. They are freaks... or are they?
What is evolution? Easy, you might think: it's the way living organisms change over time, driven by natural selection. Well, that's not wrong, but it's not really how evolutionary biologists think of it.
Picture those bear cubs. Here we see a dramatic physical change, but it isn't evolution. Among black and brown bears, white bear cubs are not that uncommon. But white bears don't have more cubs than other bears, so the gene variants for white fur remain rare.
Among one group of brown bears living in the Arctic, though, white ...
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