Variation is the differences between individuals of the same species. It can be inherited (caused by genes) or caused by the environment. Understanding variation helps explain natural selection and evolution.
Continuous and discontinuous variation
Continuous variation shows a range of values with no distinct categories, e.g. human height or mass. It is usually controlled by several genes and strongly influenced by the environment. When plotted it gives a smooth bell-shaped curve.
Discontinuous variation shows distinct categories with no in-between values, e.g. ABO blood group or the ability to roll the tongue. It is controlled by one or a few genes and little affected by the environment. It is shown as a bar chart.
Key idea
Continuous variation → range of values → bell curve → many genes + environment. Discontinuous variation → separate categories → bar chart → one/few genes.
Causes of variation
Genetic variation arises from meiosis (which shuffles alleles), random fertilisation and mutation. A mutation is a random change in the base sequence of DNA and is the original source of all new alleles. Environmental factors such as diet, climate and scars also cause variation but are not inherited.
Worked example
Tongue rolling is either possible or not — two clear groups — so it shows discontinuous variation. Body mass forms a continuous range because it depends on both genes and diet (an environmental factor).
Remember
- Continuous = bell curve; discontinuous = distinct groups.
- Mutation is the source of new alleles.
- Environmental variation is not inherited.