Genetic diversity refers to variation within and variation among populations of species.
It is measured in terms of difference between genes or between DNA or amino acid sequences. All species require a diversity of genes spread among their populations if they are to retain their ability to adapt to changing environments. Genetic diversity also is necessary for food production, forestry, and new pharmaceuticals. It is the selection and manipulation of genetic variability in crop plants that provides the raw material for agriculture. Because genetic variation is the material from which new species evolve, it is also key to biodiversity over the long haul.