Final answer:
A mutation is a change in an organism's DNA sequence that contributes to genetic diversity within a population and can have various impacts on phenotype and fitness.
Step-by-step explanation:
A change in the nucleotide sequence of an organism's DNA, ultimately creating genetic diversity, is known as a mutation. Genetic diversity is critical for the evolution of species and can lead to individuals within a population having different traits. Mutations are changes in DNA that can produce new alleles, contributing to the variation in a population. While mutations can result in a range of effects on an organism's phenotype and its fitness, not all changes are detrimental; some can be beneficial or neutral.