Ch 23: The Evolution of Populations Overview Microevolution is a change in allele frequencies in a population over generations Concept 23.1: Genetic variation makes evolution possible Genetic Variation Genetic variation among individuals is caused by differences in genes or other DNA segments Phenotype is the product of inherited genotype and environmental influences Natural selection can only act on variation with a genetic component Formation of New Alleles New genes and alleles can arise by mutation or gene duplication A mutation is a heritable change in nucleotide sequence of DNA Only mutations in cells that produce gametes can be passed to offspring A point mutation is a change in one base in a gene The effects of point mutations can vary: