What Causes Variation Between Organisms?
Learn how genetic inheritance, mutations and environmental conditions cause variation between organisms for AQA GCSE Biology, with clear examples.
Variation between organisms is caused by genetic differences, environmental conditions, or a combination of both. Variation means the differences in characteristics between individuals in a population.
Genetic causes
Genetic variation is caused by differences in the genes organisms inherit. Different forms of the same gene are called alleles. For example, inherited alleles can affect blood group and natural eye colour.
New genetic variants arise through mutations, which are changes in DNA. Mutations occur continuously. Most have no effect on the phenotype, some influence it, and very few determine it.
Sexual reproduction also increases genetic variation. Genetic information from two parents is mixed when gametes join during fertilisation, so the offspring are genetically different from each other and from their parents.
Environmental causes
Environmental variation is caused by the conditions in which an organism develops. Examples include:
- diet affecting body mass
- sunlight affecting whether skin becomes tanned
- exercise affecting muscle development
- water, light and mineral availability affecting plant growth
These characteristics are not caused only by inherited alleles.
Genes and the environment together
Many characteristics are influenced by both genes and environmental conditions. For example, a person may inherit genes associated with being tall, but poor nutrition during development could prevent them from reaching their potential height.
Worked example: Two genetically identical plants are grown under different conditions. One receives enough light, water and mineral ions, while the other does not. Their different growth is caused by the environment because their genetic information is the same.
Common exam mistake
Do not say that all variation is caused by mutations. Mutations produce new genetic variants, but variation can also result from sexual reproduction, environmental conditions, or both genes and the environment.
This explanation assumes Higher tier, although the variation content covered here is assessed at both Foundation and Higher tier in AQA GCSE Biology and is not biology-only content.
