Clear the Mud: No one knows: "No one knows it's 'time to change'. Change happens. And sometimes a useful change happens at the right time. And so the change sets into the population. But, rest assured, no one is aware of it. What they - we - are aware of is that we are living our lives the best we can."
This is one of the clearest explainations for natural selection I've seen. I need to send it to my Auntie!