In directional selection, natural selection favors an extreme phenotype — for example, the fastest individuals. If selection for the extreme phenotype continues through time, the population will become faster and faster so long as sufficient heritable variation in speed is present on which natural selection can act.
Think about a cheetah and its prey. All other things being equal, faster is always better, so natural selection acts to increase the speed of both the predator and the prey from one generation to the next. (Some physical limits exist on how fast animals can run, of course, but this process has generated some very fast creatures on the plains of the Serengeti!)
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