Answer:
1. Mutation- increase genetic variation
2. Selection- may increase, decrease or maintain genetic variation
3. Genetic drift- decrease genetic variation.
4. Gene flow- may increase or decrease genetic variation
Explanation:
1. A mutation refers to the changes in the DNA in an organism. The mutation of genes in DNA produces new adaptive features to evolve which increases the genetic variation of the organisms.
2. The natural selection favors the species which are best suited to live in that natural conditions. The natural selection, therefore, may increase or decrease the variation in a population.
3. Genetic drift is the phenomenon occurs due to random event which changes the gene pool of the population. When the genetic drift phenomenon is observed than the genetic variation of that population usually decreases.
4. Gene flow is the phenomenon in which the gene pool changes die to immigration and emigration. Therefore the phenomenon may increase or decrease genetic variation.