Which evolutionary mechanism is most likely to cause a new allele to become fixed in a population?

Genetic drift is a mechanism of evolutionary change that can lead to new alleles becoming fixed in a population purely by chance. It is particularly significant in small populations, where random events can have a larger impact on allele frequencies. When a new allele arises, for instance through mutation, genetic drift can affect how likely it is for that allele to become common or even fixed, which means reaching a frequency of 100% in the population.

In small populations, the effects of genetic drift are magnified; random fluctuations can lead to certain alleles being passed down more frequently than others regardless of their potential adaptive value. This contrasts with natural selection, where the survival and reproductive success of individuals are influenced by their alleles, potentially leading to the increase or decrease of certain traits based on their benefits to fitness. While natural selection can indeed fix alleles in a population, it operates on the premise that beneficial alleles confer an advantage, rather than being a random process.

Migration can introduce new alleles to a population, which may increase genetic diversity but does not guarantee that a new allele will become fixed. Similarly, while mutation is the source of new alleles, it is the subsequent processes—like genetic drift—that determine if and how quickly these alleles