In a mouse population, some mice have thicker fur than others, but there are more mice with thinner fur. The climate in which these mice live has slowly gotten colder.

As the climate gets colder, mice with thicker fur are more likely to stay warm and survive long enough to reproduce. Over generations, they can pass on traits associated with thicker fur, so thicker fur is selected over thinner fur. A change in the population’s inherited traits over generations is evolution, so the population will evolve.

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