Q.Distinguish gene flow from genetic drift in terms of the mechanism by which each brings about a change in a population's allele frequencies, and the typical population size in which each effect is most pronounced.

Gene flow (gene migration) occurs when individuals move from one population into a geographically separate population of the same species and interbreed there, carrying their own alleles into the new population's gene pool; this changes the allele frequencies of both populations — the donor population loses the departing alleles, the recipient population gains them — and, sustained over many generations, tends to make the two populations' allele frequencies more similar over time. Genetic drift is a fundamentally different, random, chance-driven change in a population's allele frequencies from one generation to the next, arising because only a limited sample of one generation's alleles happens by chance to be passed on; unlike selection, drift has nothing to do with an allele's adaptive value, and its effect is far more pronounced in small populations than in large ones, where chance fluctuations tend t …