Final answer:
Genetic changes in a population of organisms occur through mutation and sexual reproduction, which leads to genetic diversity.
Step-by-step explanation:
Genetic changes in a population of organisms occur through two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles and genetic variation in a population. Sexual reproduction leads to genetic diversity by combining unique combinations of alleles from two parents to produce different genotypes and phenotypes in each offspring.