Image Caption
There is differential reproduction. Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
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Natural selection (1 of 4) Variation
There is variation in traits. For example, some beetles are green and some are brown.
Natural selection (3 of 4) Heredity
There is heredity. The surviving brown beetles have brown baby beetles because this trait has a genetic basis.