Mutation
A mutation is a change in the DNA sequence of an organism that can be harmful, helpful, or neutral and is the source of new genetic variation.
Definition
A mutation is a permanent change in the DNA code, arising from copying errors during replication or from mutagens such as ultraviolet light, radiation, and certain chemicals, and it can be a substitution, insertion, or deletion of one or more bases, or a larger change affecting a whole section of a chromosome; only mutations in gametes are passed to offspring, while mutations in body cells affect just that individual. Mutation is random with respect to need, so a bacterium does not mutate to resist an antibiotic because it needs to, a resistant mutant simply already existed and survived when the antibiotic arrived. At the molecular level, mutations are classified by scale (point mutations, indels, copy-number variants, chromosomal rearrangements) and by their effect on a coding sequence (silent, missense, nonsense, frameshift); the human per-base mutation rate is about $10^{-8}$ per generation, giving each newborn roughly $50$ to $100$ new mutations, most neutral, a minority deleterious, and a rare few beneficial, supplying the raw variation that selection and drift then sort while somatic mutations accumulating in tissues drive cancer.
Example
Sickle-cell disease results from a single base change in the hemoglobin gene, from GAG to GTG, swapping one amino acid and bending red blood cells into a sickle shape, even though human cells copying about $3$ billion base pairs each division make only a handful of uncorrected errors. Lactase persistence in adults arose from a single base change in a regulatory region near the LCT gene and spread rapidly in dairying populations within the last $10{,}000$ years, while a frameshift from deleting one base shifts the reading of every downstream codon, usually producing a truncated protein.
Key Insight
Most mutations do nothing noticeable, and over long stretches of time the rare useful ones accumulate into new features. The mutation rate is itself under selection, since too high a rate lets the genome decay and too low a rate stalls adaptation, which is why proofreading polymerases and mismatch repair keep it low but never quite zero, ensuring evolution never stops.