A recessive deleterious allele (h = 0) with selection coefficient s = 0.5 is present at very low frequency (q = 0.001) in a large population. Why does natural selection eliminate it so slowly despite the strong selection pressure?
AThe selection coefficient weakens as the allele becomes rare, reducing selection pressure at low frequency
BAlmost all copies of the allele are in heterozygotes, where h = 0 means the allele confers no fitness disadvantage and is invisible to selection
CGenetic drift counteracts selection when allele frequency is very low, effectively neutralizing it in large populations
DThe allele mutates to a neutral form once it becomes sufficiently rare
When a recessive allele is rare, the vast majority of copies exist in heterozygotes (Aa), not homozygotes (aa). With h = 0 (completely recessive), heterozygotes have the same fitness as AA — the allele has no fitness effect when paired with a dominant copy. Selection only 'sees' the allele in the rare aa homozygotes. At q = 0.001, the frequency of aa homozygotes is only q² = 0.000001, so the allele is almost completely hidden from selection. This sheltering of recessive alleles in heterozygotes is why deleterious recessive mutations persist for thousands of generations. Option A is a misconception: s is a constant in this model — frequency-dependent selection is a separate phenomenon.
Question 2 Multiple Choice
An allele with selection coefficient s = 0.0001 exists in two populations: one with effective population size N = 1,000 and one with N = 10,000,000. In which population is drift most likely to cause this allele to increase in frequency despite being slightly deleterious?
AThe large population (N = 10,000,000) — more individuals means more random variation in offspring number
BThe small population (N = 1,000) — the threshold for selection to overcome drift is 1/(2N) = 0.0005, which exceeds s = 0.0001
CBoth populations equally — the selection coefficient alone determines allele fate, not population size
DNeither — any s > 0 means selection reliably eliminates the allele regardless of population size
The key threshold is s vs. 1/(2N). When s < 1/(2N), genetic drift is stronger than selection and the allele behaves as if neutral — it can increase or decrease by chance alone. For N = 1,000: 1/(2×1,000) = 0.0005, which exceeds s = 0.0001, so drift dominates and the allele can drift toward fixation despite being harmful. For N = 10,000,000: 1/(2×10,000,000) ≈ 0.00000005, far less than s, so selection dominates and reliably eliminates the allele. This is the foundation of nearly neutral theory: alleles that are effectively neutral in small populations are effectively selected against in large ones.
Question 3 True / False
A selection coefficient of s = 1 means that most individuals carrying the affected genotype will die before reaching reproductive age.
TTrue
FFalse
Answer: False
s = 1 means the genotype has zero relative fitness — it contributes no offspring to the next generation compared to the most-fit genotype. This can happen through failure to reproduce, sterility, or any mechanism yielding zero reproductive output, not necessarily death before adulthood. Additionally, if the allele is recessive and h < 1, only the homozygous genotype (aa) has fitness 1 − s = 0; heterozygotes (Aa) may still reproduce and carry the allele forward. The selection coefficient is defined in terms of relative reproductive contribution, not survival.
Question 4 True / False
Selection is most effective at changing allele frequencies when the allele under selection is at intermediate frequency — it slows when the allele is either very rare or very common.
TTrue
FFalse
Answer: True
The per-generation change in allele frequency (Δq) under selection depends on both s and the current frequency q. When q is very small, there are few copies to select against and most recessive alleles are sheltered in heterozygotes — progress is slow. When q is very large (allele nearly fixed), only rare copies of the alternative allele are under selection — again slow change. The rate peaks at intermediate frequencies where both allele types are common and phenotypic differences are most visible. This frequency-dependence of selection effectiveness shapes the entire trajectory of allele frequency change.
Question 5 Short Answer
Why does the effectiveness of natural selection in eliminating a deleterious recessive allele decline as that allele becomes rarer in the population?
Think about your answer, then reveal below.
Model answer: As the frequency q of a recessive allele decreases, the proportion of allele copies found in homozygotes (aa, frequency q²) drops much faster than the frequency itself — at q = 0.1, about 9% of allele copies are in visible homozygotes; at q = 0.01, only about 1%. Most copies shelter in heterozygotes (Aa, frequency 2pq), which are phenotypically identical to dominant homozygotes (AA) when h = 0. Since selection acts only on expressed phenotypes, the sheltered copies in heterozygotes escape selection entirely. The result is diminishing returns: selection efficiently purges visible homozygous alleles early but becomes increasingly ineffective as remaining copies concentrate in the invisible heterozygote pool.
This asymptotic elimination explains why harmful recessive alleles like those causing cystic fibrosis persist at low but nonzero frequencies — selection pressure becomes negligible long before the allele disappears, and recurrent mutation continually replenishes it.