How can we determine how large a population size needs to be to avoid the risk due to chance?

How can we determine how large a population size needs to be to avoid the risk due to chance?

• A. Estimate the mutation rate
• B. Count the number of alleles
• C. Calculate the effective population size
• D. Measure the migration rate

Answer: (C)

Genetic drift—the random change in allele frequencies from generation to generation—depends on the effective population size. The effective population size is the number of individuals that effectively contribute to the next generation, and it often differs from the actual census size because of factors like unequal family sizes, sex ratio imbalances, and overlapping generations. To determine how large a population needs to be to minimize drift, you estimate the effective population size (Ne) using life-history data and standard formulas (for a diploid species with separate sexes, Ne ≈ (4NfNm)/(Nf + Nm), with Nf and Nm the number of breeding females and males). Once Ne is known, you can judge how large the census size should be to achieve the desired level of drift or genetic stability. The other options don’t directly tell you how large the population must be to reduce drift: mutation rate indicates how new variation arises, the number of alleles gives a snapshot of diversity but not the required size, and migration rate affects gene flow rather than the size needed to minimize chance effects.

Genetic drift—the random change in allele frequencies from generation to generation—depends on the effective population size. The effective population size is the number of individuals that effectively contribute to the next generation, and it often differs from the actual census size because of factors like unequal family sizes, sex ratio imbalances, and overlapping generations. To determine how large a population needs to be to minimize drift, you estimate the effective population size (Ne) using life-history data and standard formulas (for a diploid species with separate sexes, Ne ≈ (4NfNm)/(Nf + Nm), with Nf and Nm the number of breeding females and males). Once Ne is known, you can judge how large the census size should be to achieve the desired level of drift or genetic stability. The other options don’t directly tell you how large the population must be to reduce drift: mutation rate indicates how new variation arises, the number of alleles gives a snapshot of diversity but not the required size, and migration rate affects gene flow rather than the size needed to minimize chance effects.