Variation and natural selection (CCEA)Causes of variation
Organisms of the same species vary in many ways. There are two types of variation – continuous and discontinuous. Natural selection explains how evolution occurs.
Genetic variation exists because of changes to chromosome Genetic structures that usually occur in functional pairs in the nucleus of cells (except gametes and bacteria). or geneA short length of DNA that codes for a particular characteristic. (DNA).
mutationA random and spontaneous change in the structure of a gene, chromosome or number of chromosomes. are random changes in the number of chromosome Genetic structures that usually occur in functional pairs in the nucleus of cells (except gametes and bacteria). (e.g. Down syndrome) or the structure of a gene (e.g. cystic fibrosis).
Independent assortment during meiosis A type of cell division that produces haploid cells (gametes). Mitochondria – the site of cell respiration. increases variation because it results in genetically different gameteSex cell (sperm in males and ova/eggs in females)..
The random nature of sexual reproduction also gives rise to variation.
Environmental 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.