Frequency dependent selectionFrequency-dependent selection occurs when the fitness of a genotype depends on its frequency in the population. It is possible for the fitness of a genotype to increase (positively frequency-dependent) or decrease (negatively frequency-dependent) as the genotype frequency in the population changes.
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Example: In the picture above are African cichlids a type of fish with asymmetrical jaws. Some jaws are more the the left side while others are more the right. Jaw orientation determines what side of their prey they eat from. When there are more right jawed fish in a population, there is more competition for for the left side of their prey leaving more food for the left jawed fish. Furthermore, the prey adapts to protecting its left side against the right jawed fish. In this case, the left jawed fish has the evolutionary advantage. Over time, the number of right jawed fish will decrease while the left jawed fish will increase. Now, the right jawed fish have the advantage. This is an example of negative frequency dependent selection because the less common phenotype is more favored at any given time.