Final answer:
The mechanism responsible for creating new alleles in a population's gene pool is mutation, which provides the essential genetic variation needed for evolution.
Step-by-step explanation:
The mutation is the process responsible for creating new alleles within a population's gene pool. Mutation introduces new genetic variations by altering the DNA sequence. In sexually reproducing species, only mutations that occur in gametes can be inherited by offspring, meaning these mutations can contribute to evolutionary changes. Although the probability of a mutation occurring in any given gene is quite low, mutations are the foundational source of genetic variation, enabling other evolutionary forces such as natural selection, gene flow, and genetic drift to act.
While other mechanisms like genetic drift, gene flow, and natural selection affect allele frequencies, they do not create new alleles themselves. It is important to understand that without mutation supplying new genetic material, evolution would not have the raw material needed to produce the diversity of life observed on Earth today.