In a group of frogs, some are light green and some are dark green. Predators can see the light green frogs more easily.
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Predation: Light green frogs get eaten more often than dark green frogs.
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Survival: Dark green frogs survive longer because they are harder to see.
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Reproduction: The dark green frogs have more babies because they live longer.
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Allele Frequency: Over time, more frogs will be dark green. The genes (alleles) for dark green become more common in the population.
This is how natural selection changes the number of certain traits in a group of frogs.