Mutation refers to a permanent alteration in the nucleotide sequence of the genome of an organism. These changes can occur at various levels of biological organization, including point mutations (affecting a single base pair), insertions, deletions, or larger structural rearrangements of chromosomes. Mutations are the ultimate source of genetic variation, driving the evolution of species by introducing novel alleles into the gene pool. While some mutations are neutral or beneficial, allowing adaptation and survival, others can introduce deleterious changes, often leading to disease or reduced fitness. The processes underlying mutation are categorized by their cause, ranging from spontaneous chemical errors during replication to exposure to environmental mutagens.