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Individuals in a population are usually similar to each other, but not identical. Some of the variation within a species is genetic, some is as a result of the environment they live in, and some is a combination of both.

Genetic causes of variation

Children generally look a little like their mother and their father, but are not identical to either. They inherit their features from each parent's DNA.

Every sperm and egg cell contains half of the genetic information needed for an individual. Each sex cell is known as haploid, which has half the normal number of chromosomes. When the chromosomes fuse during fertilisation, a new cell is formed, which is known as a zygote. It has all the genetic information needed for an individual, which is known as diploid and has the full number of chromosomes.

Examples of genetic variation in humans include blood group, skin colour and natural eye colour.

Whether you have lobed or lobeless ears is due to genetic causes.

Sex is also an inherited variation - whether you are male or female is a result of genes you inherited from your parent.

Environmental causes of variation

Some examples of variation are not caused by the inheritance of genetics. Whether or not you have a scar or tattoo was not determined when the sperm fertilised the egg to begin your life. This variation is often caused by the environment we live in and so is called environmental variation.

Genetic and environmental causes of variation

Many kinds of variation are influenced by both environmental and genetic factors, because although our genes decide what characteristics we inherit, our environment affects how these inherited characteristics develop. For example:

• a person might inherit a tendency to be tall, but a poor diet during childhood will cause poor growth
• plants may have the potential for strong growth, but if they do not receive sufficient mineral resources from the soil, they may hardly grow at all

Identical twins are a good example of the interaction between inheritance and environment, because such twins are genetically the same. Any differences you may see between them – for example in personality, tastes and particular aptitudes – are due to differences in their experience or environment.

Types of data from surveys on variation

Variation in a population is often investigated by completing a survey. The data that is collected from this can be one of two types:

Examples of continuous variation are: height, weight, heart rate, hand span and leaf length.

Examples of discontinuous variation are: blood group, eye colour and tongue rolling.

The causes and effects of mutations

Mutations are changes that can occur in genes. These changes are random and can be caused by background radiation and chemicals that we come into contact with, eg the chemicals in cigarette smoke. The change causes an alteration to the base sequence in the genetic code.

Sometimes these changes can be so severe that the cell dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Occasionally, the changes may even be beneficial to us and produce new and useful characteristics.

Passing on mutations

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.

It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.