Heritable variation is the key to evolution -- it supplies the raw material on which natural selection acts. Darwin's famous Galapagos Island finches displayed great variation in the size and shape of their beaks, which were adapted to different diets and ecological niches.
Darwin understood that if this variation was inherited, natural selection could produce evolutionary change in a population by selecting "for" some variations and "against" others. But what was the source of variation in the first place, and how was it inherited? In what "secret writing" in the blood, as Darwin thought of it, lay the biological blueprints for a range of beak shapes, fur, feathers, sizes of limbs and organs -- a huge storehouse of biological possibility?
It would be decades before genes and DNA were discovered, but they provided the answer Darwin sought. Each organism carries a set of genes that are passed along to the next generation. In sexually reproducing life forms, half of the genes of each parent are combined, and the offspring each inherit a different, random assortment of genes (except in the case of identical twins).
Moreover, an individual's set of genes can be altered by mistakes, or mutations, in copying the DNA code. These mutations are not only an additional source of variation, they are the ultimate source of all genetic variation.
Variation in a population means that some individuals have a better chance than others of surviving and reproducing. If the environment changes, a diverse population has a better chance of some individuals making it to reproductive age and carrying on the species.
Degrees of heritable variation can be great or small, and given large enough populations and enough time, even minute variations can have powerful effects. For example, Peter Grant, who with his wife Rosemary has done the definitive study of Darwin's finches, says that half a millimeter's difference in the size of the beak of a finch can mean the difference between life or death under certain conditions.