Final answer:
Genetic drift is one way in which allele frequencies can change randomly without any advantage to the population. Natural selection and genetic drift usually occur simultaneously in populations. Founder effects can also lead to changes in allele frequencies.
Step-by-step explanation:
Sometimes, allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies. We call this phenomenon genetic drift. Natural selection and genetic drift usually occur simultaneously in populations and are not isolated events. It is hard to determine which process dominates because it is often nearly impossible to determine the cause of change in allele frequencies at each occurrence. We call an event that initiates an allele frequency change in an isolated part of the population, which is not typical of the original population, the founder effect. Natural selection, random drift, and founder effects can lead to significant changes in a population's genome.