Final answer:
Natural populations maintain significant variation primarily due to the balance between mutation, which introduces new genetic variants, and selection, which influences the frequency of these variants. Genetic drift and gene flow also contribute to the maintenance of genetic variation. The correct option is B.
Step-by-step explanation:
Natural populations exhibit significant variation mostly due to the balance between mutation and selection. Variations arise through processes such as mutation and gene flow, which introduce new genetic material into the population.
These variations are maintained in the population by natural selection, which acts upon the existing genetic differences among individuals. If certain traits increase the chance of survival and reproduction, they become more common in the population over generations. Conversely, traits that are detrimental tend to decrease in frequency.
Genetic drift also plays a role, particularly in small populations, where random events can lead to changes in allele frequencies. Events like natural disasters (bottleneck effect) or the establishment of a new population by a few individuals (founder effect) can significantly impact genetic variation and are examples of genetic drift.
In summary, the continuous process of mutation provides new genetic variation, while selection mechanisms such as natural selection, genetic drift, and gene flow determine the frequency of these variations in a population.
Without such variation, natural selection could not occur, as there would be no genetic differences for it to act upon. Therefore, the correct answer is B) Balance between mutation and selection.