Final answer:
Mutation and gene flow are responsible for introducing new genetic variation within a species. Mutation leads to changes in the DNA sequence, while gene flow involves the transfer of genes between populations.
Step-by-step explanation:
The question at hand is which evolutionary force is responsible for producing new genetic variation within a species. The answer to this question is that both mutation and gene flow introduce new genetic variations into a population. Mutation refers to any change in the DNA sequence of an organism. These changes can occur spontaneously and can lead to new traits within a species. Mutations can be caused by various environmental factors such as UV radiation or can occur during DNA replication.
Meanwhile, gene flow occurs when there is migration of individuals between different populations, leading to an exchange of genes. This transfer of genes from one population to another can introduce new alleles (or variations of a gene) into a population. Although the process of genetic drift can change allele frequencies within a population, it does not introduce new genetic variation; instead, it can increase or decrease the prevalence of existing alleles by chance events. Natural selection does not create new genetic variation either; rather, it acts on existing variation, favoring traits that enhance survival or reproduction.
In conclusion, the correct option for the question '______ produces new genetic variation within a species' is b) Mutation and gene flow, as they are the main sources of introducing new genetic variation into a species.