Why doesn't directional selection lead to the fixation of cheater genotypes in Dictyostelium discoideum?

Directional selection typically favors a particular phenotype, but in the case of Dictyostelium discoideum, the dynamics of cooperative behavior and social interactions exhibit a different scenario. Frequency-dependent selection plays a crucial role here as it describes situations in which the fitness of a phenotype depends on its frequency relative to other phenotypes in the population.

In Dictyostelium, when cheater genotypes begin to increase in prevalence, their success can lead to changes in the environment that ultimately counteract their advantage. For example, a higher proportion of cheaters can negatively impact the overall success of the group, leading to lower survival rates for these cheaters when their population becomes too large. Conversely, cooperative genotypes may increase in frequency as their advantages become more significant in response to the rising frequency of cheaters. This creates an oscillating dynamic where the success of genotypes depends not only on their inherent characteristics but also on their relative frequencies in the population.

This interplay means that cheaters cannot simply dominate the population; instead, cooperation can maintain stability and support the population's overall fitness, preventing any single genotype from becoming fixed, thus carving a niche that promotes a balance between cooperation and cheating in the evolutionary process.