Answer:
The evolutionary force of MIGRATION tends to increase genetic variation within a population but decrease genetic variation between populations, while the evolutionary force of GENETIC DRIFT tends to decrease genetic variation within a population but increase genetic variation among populations.
Step-by-step explanation:
Migration is the movement of individuals from one population to another and results in a transfer of alleles from one population to another. When the immigrated individuals interbreed, they add new alleles to the gene pool of the recipient population. Therefore, migration increases the genetic variations within the population. When sufficient alleles flow from one population to another by migration, they become more similar genetically.
Genetic drift refers to random changes in the allele frequencies of a small population by any chance event. Genetic drift may eliminate an allele of a gene irrespective of the adaptive nature of another allele. Therefore, genetic drift reduces the genetic variations within a population but tends to increase the genetic differences among different populations.