Consider a species of group-living bird in which individuals produce alarm calls to alert group members of the presence of a predator. The alarm call confers fitness benefits to the caller as it helps group members (composed of genetic relatives) escape the predator. However, alarm calling also makes the caller more conspicuous to the predator. Individuals of this species in a population have four phenotypes for the loudness of alarm calls they produce in the order P < Q < R < S. The graph below gives the cost and benefit functions for alarm calling behavior for the four phenotypes.

Which one of the following phenotype frequencies represents the correct outcome of natural selection?

Correct option is C

The correct answer is: (3) Phenotype frequency: Q > (P = R) > S

Explanation:
In this case, the fitness cost and benefit functions indicate how the alarm call behavior (based on the loudness of the call) impacts the fitness of the individuals:

• Phenotype S produces the loudest alarm calls, but the cost of being more conspicuous to the predator outweighs the benefit. In the graph, the benefit increases up to a point, but the cost increases more rapidly after this point. Therefore, S is less beneficial than Q.

• Phenotype Q represents a good balance between cost and benefit, where the fitness benefit outweighs the fitness cost. This leads to Q being the most favored phenotype in the population.

• Phenotypes P and R are less fit compared to Q because, while they may have lower costs or higher benefits, the overall fitness value (benefit – cost) is lower. The values for P and R are similar, so they are grouped together.

Thus, the most common phenotypes will be Q > (P = R) > S under natural selection, as Q has the best benefit-to-cost ratio, and S is the least favorable phenotype.

Information Booster:

1. Natural selection favors the phenotype that maximizes fitness, which is a balance of benefit (alarm call effectiveness) and cost (increased predator conspicuousness).

2. Phenotype Q has the most favorable balance of benefit and cost, making it the most common under natural selection.

3. Phenotypes P and R are similarly fit, but they are less favored than Q due to a lower benefit-to-cost ratio.

4. Phenotype S is the least favored due to its high cost relative to the benefit, making it the least common phenotype.

Additional Information:

• Option 1 (P = Q = R < S): This option incorrectly suggests that P, Q, and R are equally fit, which does not match the graph showing a significant advantage for Q.

• Option 2 (S > R > Q > P): This suggests that S is the most favored phenotype, which contradicts the graph, as Q is the most favored.

• Option 4 (Q > P = R = S): This does not align with the graph, as S should not be equal to P and R in fitness.