Question

Explain the difference between gene flow and genetic drift.

Answer 1

Brief definitions
- Gene flow (migration): the movement of alleles between populations when individuals, gametes, or seeds move and reproduce in a new population. It changes allele frequencies by introducing alleles from other populations.
- Genetic drift: random fluctuations in allele frequencies from one generation to the next due to chance sampling of gametes. Drift is strongest in small populations.

Key differences (quick comparison)
- Cause: gene flow is caused by migration; drift is caused by random sampling.
- Directionality: gene flow tends to move allele frequencies toward those of the source population (directional toward homogenization); drift is random with no directed tendency.
- Dependence on population size: gene flow effects depend on migration rate (m) relative to population size; drift is much stronger in small effective population size (Ne) and weaker in large populations.
- Effect on variation:
- Gene flow increases genetic variation within populations (by introducing new alleles) and decreases genetic differentiation among populations.
- Genetic drift reduces genetic variation within populations (by chance loss or fixation of alleles) and increases differentiation among populations.
- Evolutionary consequences: gene flow can impede local adaptation and speciation by homogenizing populations; drift can cause divergence and sometimes contribute to speciation, especially after bottlenecks or founder events.

Simple quantitative expressions
- Gene flow (island-model): change in allele frequency in one population ≈ Δp = m (p_migrant − p_resident), where m is the fraction of migrants each generation.
- Genetic drift (diploid): variance of allele frequency change ≈ p(1 − p) / (2Ne) per generation; probability of fixation of a neutral allele equals its current frequency; expected time to fixation of a neutral allele is on the order of Ne (more precisely ~4Ne generations for a new neutral mutation to either fix or be lost).

Examples
- Gene flow: pollen or seed dispersal between plant populations, human migration introducing alleles between populations, introgression from Neanderthals into Homo sapiens.
- Genetic drift: founder effects in island colonization, population bottlenecks (e.g., northern elephant seals), random allele loss in very small populations.

Takeaway
Gene flow is migration-driven exchange of alleles that tends to homogenize populations; genetic drift is random sampling that tends to reduce within-population variation and increase divergence, especially in small populations.