Variability refers to the differences in heritable traits exhibited by the individuals of a species. One of the major contributions of Darwin to the study of evolution was his book of 1868, 'The Variation of Animals and Plants under Doxnestication'. Danyin pointed out that populations of living organisms are not composed of identical individuals.
It is indeed true that even monozygotic twins may express differences between them. Onecan cite a number of examples to indicate the differences in different traits in a population. Certain traits are continuously varying such as the height or colour, and certain other traits exhibit two or a few distinct, different categories of individuals with no intermediates. For example in human population, there are P'C tasters and non-tasters; individuals with blood groups A or B or AB or 0, individuals with black or brown or blue eyes, individuals suffering from sickle cell anaemia and individuals who are normal, so on and so forth. Sweet peas may have either red flowers or white ones; Drosophila may have normal or vestigial wings, red or white eyes etc. Essentially the major source of variability in organisms is the genetic variability caused by mutations of all kinds and genetic recombination. Also the subtle effects of environment add another dimension to the variability of traits in natural populations. An oft cited example is the differences exhibited in a number of traits of monozygotic quadruplets of a nine-banded armadillo.