In the particular order from top to bottom , the answers are;
Genetic drift
Gene flow
Natural Selection
Gene flow occurs during migration where alleles from one population are introduced to another population hence increasing the population diversity.
Genetic drift occurs when the allelic frequency of a population changes due factors such as calamities and emigration or natural selection.
Natural selection is the force of ‘survival for the fittest’ that is applied differentially on the population by nature.