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

It is an example of frequency-dependent selection. Due to several cheaters in the population, the fruiting body's stalk is not made properly. So, all individuals have lesser fitness.

Explanation:

Natural selection:  

The population contains both superior as well as an inferior organism where natural resources are limiting so it will cause competition between organisms. As a result of competition, it will select superiors, and inferiors are deleted and they are given reproductive advantages. Due to this reproductive advantage new population emerges. It is more suitable for the environment.  

Natural selection divides into three parts that are directional, disruptive, and stabilizing selection.

The given question is the example of frequency-dependent selection. Due to several cheaters in the population, the fruiting body's stalk is not made properly. So, all individuals have lesser fitness. Directional selection will not lead to fixation of cheater genotype. This is because cheating can be controlled through high relatedness in social groups, resulting from kin discrimination.  

It is controlled through positive pleiotropy, where the cooperation gene has an additional vital function. Cheating will be controlled if stalk vs. spore is a result of environmental, not genetic factors. An example, spore fate can be a result of the position in the mitotic cell cycle.

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