Final answer:
A mutation occurs when there is a change in the DNA sequence of a gene, which is crucial for introducing new genetic variations and alleles.
Step-by-step explanation:
A mutation occurs when there is a change in the DNA sequence of a gene. This event is crucial for the introduction of new genetic variations and serves as the ultimate source of new alleles in a population. Mutations can have various effects on an organism's survival and reproduction, and whether they are considered beneficial or harmful depends on the environmental context and their impact on an organism's fitness.
Mutations can lead to changes in allele frequencies when natural selection acts upon the phenotypic variations they produce. For instance, beneficial mutations that enhance an organism's ability to survive and reproduce will likely increase in frequency through selection, while harmful mutations tend to be eliminated. In small populations, the role of genetic drift becomes significant, as chance events can lead to dramatic shifts in allele frequencies independent of the mutation's adaptive value.