 Usually, all cells in an individual in a given species (plant or animal) show a constant number of chromosomes, which constitute what is known as the karyotype defining this species (see also List of number of chromosomes of various organisms), although some species present a very high karyotypic variability.In humans, mutations that would change an amino acid within the protein coding region of the genome occur at an average of only 0.35 per generation (less than one mutated protein per generation).Sometimes, in a species with a stable karyotype, random variations that modify the normal number of chromosomes may be observed.