variation

Variation is the simple fact that no two individuals are exactly alike. Look around a crowd and you see endless differences in face, height, and voice; beneath the surface, the DNA of any two people differs at millions of positions. Variation is the raw material that makes each of us distinct.

Genetic variation refers specifically to differences in DNA sequence among individuals in a population. These differences range from a single changed letter to large stretches of DNA that are duplicated, deleted, or rearranged. They arise originally from mutation and are then shuffled and spread through reproduction.

Variation is essential, not a defect. It is the supply on which natural selection acts: without differences among individuals, there would be nothing for selection to favour or disfavour, and populations could not adapt to changing conditions. Much human medical genetics is the study of which variants matter and how.

An important nuance: most genetic variation has no visible effect at all, sitting in regions of DNA that do little, or causing changes too subtle to notice. Only a small fraction of variants strongly influence traits, and fewer still cause disease. Difference at the DNA level is the norm, not the exception.