Questions: Frequency-Dependent Selection and Polymorphism

Question 1

In a prey population, blue morphs are at 80% frequency and green morphs at 20%. Predators have formed a strong search image for blue prey. What will happen to morph frequencies in the next generation?

• A. Blue morphs will increase further because they are more numerous and better adapted to the environment
• B. Green morphs will increase because their rarity makes them harder for predators to detect, giving them a survival advantage
• C. Both morphs will decline sharply as predators eliminate prey from the population
• D. The population will fix on blue morphs because the majority phenotype always prevails under natural selection

Answer: (B)

This is the core mechanism of negative frequency-dependent selection: rare phenotypes have a fitness advantage precisely because they are rare. Predators with a search image for blue prey hunt blue morphs efficiently while overlooking green morphs. As green morphs survive and reproduce more, they become more common — at which point the advantage shifts, and predators begin to form a search image for green. This oscillation maintains both morphs indefinitely. Options A and D reflect the misconception that common phenotypes are always favored — true under directional selection but false under negative frequency dependence.

Question 2

Warning coloration in toxic species (Müllerian mimicry), where multiple toxic species converge on the same color pattern, is an example of:

• A. Negative frequency-dependent selection, because rare warning patterns are harder for predators to learn and avoid
• B. Positive frequency-dependent selection, because the most common warning pattern provides the best predator education and protection
• C. Balancing selection through heterozygote advantage, because individuals heterozygous for color alleles survive best
• D. Directional selection driving one color pattern to extinction while another fixes

Answer: (B)

In Müllerian mimicry, predators learn to avoid the most common warning pattern most efficiently — the more individuals share a pattern, the stronger the learned avoidance response. This means common patterns are favored: positive frequency-dependent selection. Crucially, positive FDS does the opposite of maintaining polymorphism — it pushes the population toward fixation on one common form. This is why positive FDS cannot be invoked to explain long-term polymorphism; that explanation requires negative FDS.

Question 3

Negative frequency-dependent selection can maintain two or more phenotypes in a population indefinitely without any individual organism consciously preferring or choosing rarity.

• T. True
• F. False

Answer: True

The mechanism is entirely ecological, not intentional. Predator search images emerge from statistical regularities in predator learning — predators encounter common prey more often and form stronger recognition templates for them. No organism chooses to be rare, and no organism 'prefers' novelty. The rare-advantage emerges from the interaction between predator cognition and prey frequency, not from any preference within the prey population.

Question 4

Positive frequency-dependent selection is the primary mechanism responsible for maintaining genetic polymorphism in natural populations.

• T. True
• F. False

Answer: False

Positive frequency-dependent selection does the opposite — it favors common phenotypes and tends to reduce or eliminate polymorphism by driving the population toward fixation on a single form. The mechanism that maintains polymorphism is NEGATIVE frequency-dependent selection, where rare phenotypes are favored. When population geneticists invoke frequency-dependent selection to explain sustained genetic variation, they almost always mean the negative form.

Question 5

Explain why negative frequency-dependent selection prevents directional selection from fixing a single allele in the population.

Model answer: Under directional selection, the fittest allele increases in frequency monotonically until it reaches fixation. Negative frequency-dependent selection breaks this monotonic increase by changing the fitness landscape as frequencies change: as a phenotype becomes more common, its fitness advantage declines (and may reverse), while the rarer phenotype gains a fitness advantage. This creates a stable equilibrium frequency for each phenotype — the frequency at which neither has a net advantage. Any perturbation away from equilibrium is self-correcting: if the common phenotype becomes too common, selection favors the rare one; if the rare phenotype becomes too common, selection shifts back. The result is indefinite coexistence of multiple phenotypes.

The contrast with directional selection is the key insight: under directional selection, fitness is fixed and the fittest wins; under negative FDS, fitness is dynamic and depends on frequency, so no single phenotype can achieve permanent dominance.