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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 variation is environmental, 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. This means sperm and eggs contain half the normal chromosomes number. 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 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 biologically male or female is a result of 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 you will become heavier. If you eat too little you will become lighter. A plant in the shade of a big tree will grow taller to reach more light.

Other examples of features that show environmental variation include:

• scars
• language and accent
• flower colour in hydrangeas as these plants produce blue flowers in acidic soil and pink flowers in alkaline soil

Genetic and environmental causes together

Phenylketonuria (PKU) is an example of how the genes and the environment may work together. The disease is caused by a buildup of phenylalanine in the bodies of people who have a variant of the PAH gene which codes for an enzyme which does not work properly. The disease is managed by excluding phenylalanine from the diet.

Another example is height. Tall parents will pass genes on to their children for height. Their children have the genetic potential to also be tall. However, if their diet is poor then they will not grow very well. Their environment also has an impact on their height.

Learn more about variation with Dr Alex Lathbridge.

Listen to the full series on ����ý Sounds.