Final answer:
Natural selection, by promoting the survival and reproduction of individuals with advantageous traits, can lead to evolution by changing the frequency of certain alleles in a population over time. Mutation is also a source of new alleles, but its effect on evolution is small unless it interacts with selection.
Step-by-step explanation:
Natural selection is a mechanism that can lead to evolution by changing the frequency of certain alleles in a population over time. This occurs by promoting the survival and reproduction of individuals with advantageous traits. For example, if a certain allele confers a phenotype that allows an individual to have more offspring that survive and reproduce, that allele will be in greater frequency in the next generation. On the other hand, detrimental alleles may be swiftly eliminated from the population. Mutation is also a source of new alleles, but its effect on evolution is small unless it interacts with selection. Harmful mutations are removed from the population by selection, while beneficial mutations spread through the population through selection.