Fitness and Frequency: The Key to Fixation

The likelihood that a behavioral variant will reach fixation is primarily determined by whether it confers a fitness advantage that increases as its frequency rises. If the fitness of individuals carrying the variant is always higher than those without it, positive selection will drive the variant toward fixation.

Let’s interpret the typical fitness-frequency curves:

• Positive Frequency-Dependent Selection: The fitness of the variant increases as it becomes more common. If the fitness is always above that of the alternative, the variant is likely to reach fixation.

• Negative Frequency-Dependent Selection: The variant is favored when rare but loses its advantage as it becomes common, often maintaining polymorphism rather than fixation.

• Constant Fitness Advantage: If the variant’s fitness is always higher, regardless of frequency, it will be driven to fixation.

• Fitness Decreases with Frequency: The variant cannot reach fixation, as it becomes disadvantageous when common.

Interpreting the Graphs

Based on these principles, the behavioral variant is most likely to reach a frequency of 1 in cases where its fitness is always highest (relative to alternatives) at all frequencies. This is typically shown by:

• A monotonically increasing or constant fitness curve that is always above the baseline (cases b and d, for example, if those curves represent a consistent or increasing fitness advantage as frequency rises).

Conclusion

The behavioral variant is most likely to reach a frequency of 1 in cases b and d, where its fitness is always highest across all frequencies. This is because positive selection will continually favor the variant until it becomes fixed in the population.

Correct answer: (3) b and d