Final answer:
Natural selection is the evolutionary force that favors a fitter allele and leads to its fixation in a population due to its survival and reproductive advantages.
Step-by-step explanation:
Natural selection favors a fitter allele and leads to its fixation in the population. The key principle behind natural selection is that certain alleles confer benefits that make organisms more likely to survive and reproduce. When an allele increases an organism's fitness, it becomes more common over successive generations, eventually becoming fixed in the population. This process can dramatically shift allele frequencies, particularly if the favored allele offers a significant reproductive advantage or enhances survival. Examples can include better camouflage, increased resistance to diseases, or more efficient metabolism.
Contrastingly, genetic drift involves random changes in allele frequencies, and although it can lead to allele fixation, it does not inherently favor alleles based on fitness. Mutation introduces new alleles and hence new genetic variation into a population, while gene flow involves the transfer of alleles between populations. Both mutation and gene flow can introduce beneficial, neutral, or even deleterious alleles, but it is natural selection that typically steers their prevalence based on fitness contributions.