Which factors commonly cause Ne to be smaller than census size?

• A. Unequal sex ratio, variation in family size, fluctuating population size, overlapping generations.
• B. Equal sex ratio, constant population size, monogamy.
• C. Large, stable populations with random mating.
• D. High mutation rate.

Answer: (A)

Ne is the number of individuals that effectively contribute genes to the next generation, and it tends to be smaller than the census size when reproduction is uneven or occurs across time in a way that concentrates genetic contributions.

Unequal sex ratio reduces the pool of breeders because if one sex is underrepresented, there are fewer individuals passing on genes than the total counted in the population. For example, many more females than males (or vice versa) means the actual breeding pair pool is smaller than the total population.

Variation in family size also lowers Ne. When some individuals have many offspring while others have few or none, genetic drift has a stronger effect because a small subset of the population drives most of the gene flow to the next generation.

Fluctuations in population size over time reduce Ne through the harmonic mean effect. If the population undergoes bottlenecks or large swings, the years with small census size dominate the long-term genetic contribution, pulling Ne below the census size.

Overlapping generations contribute to a smaller Ne because multiple age cohorts breed simultaneously, spreading genetic contributions across a longer time and effectively reducing the number of breeders that pass genes in a given generation.

In contrast, a balanced sex ratio with stable population size and random mating tends to push Ne closer to the census size, and a high mutation rate doesn’t directly set Ne, though it influences genetic variation.