A species of butterfly lives in a warm climate. The butterflies feed on the nectar of tropical flowers. Birds eat the butterflies, and they prefer to eat the butterflies with brighter wings.

Birds are more likely to eat butterflies with brighter wings, so darker-winged butterflies are less likely to be eaten. If a mutation makes some butterflies’ wings darker, they may survive longer and produce more offspring, passing on the trait. A decrease in birds could reduce selection pressure, but it would not directly favor a particular wing color as clearly as darker wings would.

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