Causes of variation
Genetic variation exists because of changes to chromosomes or genes (DNA).
Mutations are random changes in the number of chromosomes (e.g. Down syndrome) or the structure of a gene (e.g. cystic fibrosis).
Independent assortment during meiosis increases variation because it results in genetically different gametes.
The random nature of sexual reproduction also gives rise to variation.
An individual’s environment can also cause variation.
For example, human height is genetically controlled but actual height reached will depend on the nutrition available.