Why is coalescent theory under selection more complex than under neutrality?

• A. Under selection, allele frequencies remain constant over time.
• B. Coalescent times are unaffected by selection.
• C. Neutral coalescent has exact solutions for all selection regimes.
• D. Selection skews genealogies, affecting coalescent times and allele frequency spectra; neutral coalescent provides better tractable approximations.

Answer: (D)

Selection changes how genealogies are formed, making coalescent theory under selection more intricate than the neutral case. When a allele has a fitness advantage or disadvantage, its frequency changes over time and lineages tied to that allele coalesce in ways that depend on those dynamic frequencies and on the strength and mode of selection. This bias alters the shape of the genealogy, often speeding up coalescence for lineages linked to beneficial alleles during a sweep or changing the overall tree balance, so the waiting times to common ancestry are no longer the simple, constant-rate patterns assumed under neutrality. In addition, the allele frequency spectrum becomes distorted away from neutral expectations, reflecting the selective pressures that shape which variants rise or fall in frequency. The neutral coalescent, in contrast, relies on random drift with no fitness differences and yields tractable, well-understood results for coalescent times and spectra. Because selection introduces dependence on allele trajectories and linkage, exact solutions become much more complex or intractable, and researchers often rely on approximations that capture general skewing of genealogies rather than exact neutral-like behavior.