Originally posted by lee_merrill
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The landscape is changing as well; what was once a hilltop may now be a valley and vice versa. Passenger pigeons had parasites: fleas, tapeworms and the like. That was once a nice large hilltop when there were immense population of the pigeons. That hilltop disappeared.
Evolution works in populations, so it has multiple starting points. Hence it can climb multiple hilltops. Some humans have climbed the "digest milk as an adult" hill while others have not, for example.
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