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← Theoretical A

Q28 — Distribution of Fitness Effects and Effective Population Size

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Genetic Drift, Gene Flow & Mutation

Mutations have different effects ranging from strongly deleterious to highly adaptive. The frequency with which a particular mutation appears de novo is described by the fundamental property of each species — distribution of fitness effects (DFE). There are two direct methods of finding DFE: induce mutations and check the fitness of each mutation, or prepare mutation accumulation lines and compare their fitness distribution with the initial line. DFE can also be inferred indirectly from sequencing data that compares rates of non-synonymous (N) and synonymous (S) substitutions using the dN/dS ratio between different species.

Schematic of a mutation-accumulation-line experiment: a preserved ancestor line is compared against descendant lines allowed to accumulate mutations over many generations (Gen 1, Gen 2, … Gen N). Figure 1. Scheme of mutation accumulation line experiment.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. Individual effectively neutral mutations are fixed only due to genetic drift.
B. Species with large Ne have higher adaptability.
C. The mutation accumulation line method always underestimates the frequency of deleterious mutations.
D. dN/dS is proportional to the ratio of deleterious mutations.
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Question reproduced from IBO 2024, Theoretical Exam Part A, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)