Individual amoebae of the slime mold Dictyostelium discoideum aggregate to form a spore-producing fruiting body. Cheaters make spores, but do not contribute to the stalk. Since many more spores with cheating genotypes are dispersed, they have higher fitness. Why does directional selection not lead to fixation of the cheater genotype?
a. This is actually a case of frequency-dependent selection. With too many cheaters in a population, the fruiting body's stalk is not built well, and all individuals have lower fitness.
b. Noncheating genotypes are maintained for the harmony and balance of nature.
c. Individual amoebae compete against groups of multicellular predators; the noncheating genotype is favored during this competition.
d. Individual amoebae form "slugs" only with other amoebae that have the same genotypes.
e. Reproductive success is independent of whether an organism exists as a single cell or as part of a multicellular "slug."

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