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Natural selection changes the frequencies of particular alleles and phenotypes within a population through directional, stabilizing, and disruptive selection.

In a typical population, most individuals have intermediate traits, while a few individuals have extreme traits. In directional selection, environmental or sexual pressures favor one extreme phenotype, causing the population mean to shift toward that trait.

Imagine a population of lizards living on an island with no competitors or predators.

In this population, females prefer bright, colorful, and fit males. This preference is called sexual selection.

As females mate more often with bright, colorful males, the frequency of this phenotype increases. Over time, the population shifts toward the bright phenotype. This is directional selection.

Now, consider a different situation. A predator arrives on the island, and bright, colorful males become easy prey.

At the same time, dull males survive but attract few females.

Because both extremes face disadvantages, males with intermediate color survive and reproduce more successfully. This is called stabilizing selection.

The last type of selection is disruptive selection, in which both extreme phenotypes are favored, and the intermediate phenotype is selected against.

In this lizard population, females prefer bright males, so these males mate more often.

At the same time, dull males that look similar to females avoid aggression from dominant males and gain mating opportunities without direct competition. Intermediate males do not have either advantage.

Because both bright and dull males reproduce successfully, their frequencies increase, while the frequency of intermediate males decrease.