Final answer:
Mutation and gene flow increase genetic variation, while natural selection and genetic drift often lead to decreases in genetic variation within a population.
Step-by-step explanation:
Evolutionary processes that increase allele variation include mutation, which introduces new alleles, and gene flow, which can bring different alleles from other populations. Conversely, evolutionary processes that decrease allele variation include natural selection, particularly through directional and purifying selection, which tends to favor certain alleles over others, and genetic drift, which can randomly reduce variation especially in small populations.
Mutations are the primary source of genetic variation and can have positive, negative, or neutral effects on evolutionary fitness. Gene flow occurs when individuals migrate between populations, mixing genetic material. Genetic drift can cause random fluctuations in allele frequencies, often having a stronger effect in smaller populations. Natural selection acts on heritable variations, increasing the frequency of beneficial alleles while decreasing the frequency of harmful ones.