How can allele frequency time series help distinguish selection from drift?

How can allele frequency time series help distinguish selection from drift?

• A. Consistent directional change with expected variance smaller than drift; drift yields fluctuating trajectories with no consistent direction
• B. Random fluctuations with no direction
• C. No change over time
• D. Mutation rate differences drive changes

Answer: (A)

Tracking allele frequencies over time lets us see whether there is a steady push in one direction or just random wandering. Natural selection adds a consistent, directional force, so an allele with a fitness advantage tends to rise (or fall) in frequency in a fairly steady way, with less random spread around that trend. Genetic drift, on the other hand, is a random sampling effect that makes frequencies wander up and down without a predictable direction, often producing larger fluctuations, especially in small populations. So if the time series shows a clear, monotonic change toward higher (or lower) frequency with relatively little scatter, that points to selection. If the trajectory is erratic with no persistent direction, drift is the better explanation. Across multiple independent populations, a similar directional trend further supports selection, while random, uncoordinated changes argue for drift.

Tracking allele frequencies over time lets us see whether there is a steady push in one direction or just random wandering. Natural selection adds a consistent, directional force, so an allele with a fitness advantage tends to rise (or fall) in frequency in a fairly steady way, with less random spread around that trend. Genetic drift, on the other hand, is a random sampling effect that makes frequencies wander up and down without a predictable direction, often producing larger fluctuations, especially in small populations. So if the time series shows a clear, monotonic change toward higher (or lower) frequency with relatively little scatter, that points to selection. If the trajectory is erratic with no persistent direction, drift is the better explanation. Across multiple independent populations, a similar directional trend further supports selection, while random, uncoordinated changes argue for drift.