Frequencydependent selection maintains genetic variation within populations

Natural selection often preserves variation as a polymorphism: the coexistence within a population, at frequencies greater than mutations can produce, of two or more alleles at a locus. A polymorphism may be maintained when the fitness of a genotype (or phenotype) varies with its frequency relative to that of other genotypes (or phenotypes) in a population. This phenomenon is known as frequency-dependent selection.

A small fish that lives in Lake Tanganyika, in East Africa, provides an example of frequency-dependent selection. The mouth of this scale-eating fish, Perissodus microlepis, opens ei-

Perissodus Microlepis Mouth

"Right-mouthed" Perissodus attack prey from the left rear side.

"Left-mouthed" Perissodus attack prey from the right rear side.

"Right-mouthed" Perissodus attack prey from the left rear side.

"Left-mouthed" Perissodus attack prey from the right rear side.

23.20 A Stable Polymorphism Frequency-dependent selection maintains equal proportions of left-mouthed and right-mouthed individuals of the scale-eating fish Perissodus microlepis.

ther to the right or to the left as a result of an asymmetrical jaw joint; the direction of opening is genetically determined (Figure 23.20). P microlepis approaches its prey (another fish) from behind and dashes in to bite off several scales from its flank. "Right-mouthed" individuals always attack from the victim's left; "left-mouthed" individuals always attack from the victim's right. The distorted mouth enlarges the area of teeth in contact with the prey's flank, but only if the scale-eater attacks from the appropriate side.

Prey fish are alert to approaching scale-eaters, so attacks are more likely to be successful if the prey must watch both flanks. Vigilance by the prey favors equal numbers of right-mouthed and left-mouthed scale-eaters, because if one form were more common than the other, prey fish would pay more attention to potential attacks from the corresponding flank. Over an 11-year period in which the scale-eaters in Lake Tanganyika were studied, the polymorphism was found to be stable: The two forms of P microlepis remained at about equal frequencies.

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