Q.Define gene flow (gene migration). Explain its effect on the allele frequencies of both the population that individuals leave and the population that they join.

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 to average out. The founder effect is a striking special case of genetic drift, in which a small number of individuals becoming geographically isolated from a larger source population (such as colonising a remote island) establish a new population whose allele frequencies can differ substantially, purely by the chance composition of the founders, from those of the original source population.