Could someone explain to me why gene flow (migration) tends to increase genetic variation within populations and decrease variation between populations?
And for genetic drift, genetic variation tends to increase between population and decrease within populations?

Question

Could someone explain to me why gene flow (migration) tends to increase genetic variation within populations and decrease variation between populations? 
And for genetic drift, genetic variation tends to increase between population and decrease within populations?

koikkara · Answer

@Enerz The essential mechanism of gene flow is movement of individuals (or their gametes) between populations. For example, gene flow can occur in plant species when pollen is carried by bees or blown by the wind from one population of flowering plants to another. Another example: Imagine two different groups of rabbits, one group has blue eyes where as the other one has brown eyes. These two groups of rabbits could share these alleles in a few ways, but the two ways I will focus on is migration between populations, and nonrandom mating. Ref: http://academygenbioii.pbworks.com/w/page/38938116/Chapter%2024-5%3A%20Migration%20and%20Nonrandom%20Mating