Variation = differences in the phenotype
It is often possible to count or to measure these differences and show them in bracket form. Every individual has a phenotype and the appearance for an individual for any of the characteristics is because of their genotype which is be variant to various degrees accordingly to the environment. (Individual Phenotype = Genotype + Environment)
Variation in the population is due to the variation of the genotype and the variation of the environment they occupy and develop in.
Examples of this:
In the first example it shows variation that depends entirely on the genotype with no role of the environment. For example the blood groups.
In the second example it shows where the variation depends on the environment. With one example such as height, one might have been inherited the genotype to be short, however, a good diet may affect his height.
In the third example, it shows a variation in population which depends entirely on the environment. Genes have no roles to play here, for example, languages.