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Individuals in a population are usually similar to each other, but not identical. Some of this variation within a species is genetic, some is environmental, and some is a combination of both.

Genetic causes of variation

Children usually look like their mother and father, but they will not be identical to either one of them. This is because they get half of their inherited features from each parent.

Each sperm cell and egg cell contains half of the genetic information needed for an individual. Each one is haploid, ie it has half the normal number of chromosomes. When these join at fertilisation, a new cell is formed. This zygote has all the genetic information needed for an individual. It is diploid, ie it has the normal number of chromosomes.

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

Gender is also an inherited variation – whether you are male or female is a result of the genes you inherited from your parents.

Environmental causes of variation

Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much food you will gain weight, and if you eat too little you will lose weight. Another example is that of a plant that grows in the shade of a big tree - it will grow taller to reach more light.

Other examples of features that show environmental variation include:

• a scar or accent
• flower colour in hydrangeas (these plants produce blue flowers in acidic soil and pink flowers in alkaline soil)

Genetic and environmental causes combined

Some features vary because of a combination of genetic and environmental causes. For example, identical twins inherit exactly the same features from their parents. But if twin A eats more than twin B (and all other conditions stay the same), then twin A is likely to end up heavier.