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Natural selection can depend on how common or rare a phenotype is in a population. This is called frequency-dependent selection. Here, a phenotype’s fitness, or the ability to survive and reproduce, depends on its frequency in the population.

In positive frequency-dependent selection, as a phenotype becomes more common, its fitness within the population also increases.

For example, many Heliconius butterfly species have toxic morphs with bright color patterns. When one pattern is common, birds learn to recognize and avoid it. But when a new color pattern is rare, birds may not recognize it and may attack those butterflies until they learn that it is also toxic. Once they do, the survival and frequency of that morph increase.

In negative frequency-dependent selection, a phenotype’s fitness decreases as it becomes more common.

For instance, viceroy butterflies mimic the toxic monarch butterflies in color and pattern. When viceroys are fewer in number, birds avoid them, mistaking them for monarchs. But if viceroys become too common, birds learn that many of these butterflies are safe to eat, which decreases the viceroys’ fitness in the population.