Inherited and environmental variation

What is variation?

Variation means the differences between individuals of the same species. Some of these differences are easy to see (height, eye colour). Others are hidden (blood group).

Variation comes from two sources:

• Inherited variation: the characteristic is controlled by genes passed from parents to children, for example eye colour and blood group.

• Environmental variation: the characteristic is caused by the surroundings or lifestyle, for example a scar, a suntan or the language you speak.

Many characteristics are controlled by both. Your genes set the possible range of your height, but nutrition and health decide where you end up in that range.

How variation arises

Where do inherited differences come from? In sexual reproduction there are two main sources:

1. Mixing of genes from two parents. Each parent gives half of their genes to the child. Each egg cell and each sperm carries a different combination, so every child gets a new mix.

2. Random fertilisation. Any sperm could fertilise the egg, so the combination that meets is a matter of chance.

A third source is mutation: a random change in DNA, which creates a completely new version of a gene.

Continuous and discontinuous variation

Variation can be shown in two different patterns.

Discontinuous variation: there are a few clear groups with nothing in between. Examples: blood group (A, B, AB or O), tongue rolling (you can or you cannot), being able to taste a certain chemical. These are usually controlled by one gene and are mostly not affected by the environment.

Continuous variation: there is a whole range of values, with every value in between. Examples: height, mass, foot size. These are controlled by many genes and are often also influenced by the environment. A histogram (frequency diagram) of the data usually forms a bell shape, with most people near the middle.

Genes and environment together

How can we tell whether a difference comes from genes or from the environment? Scientists use clues.

Identical twins have exactly the same genes. If they are raised apart and still differ, the differences must be down to the environment. Suppose twins grow up in different parts of Ireland: one eats well and plays sport, the other spends hours outdoors in the sun. They may differ in height and skin tone, but their blood group and eye colour stay the same.

Asexual reproduction gives a useful comparison. Offspring made by asexual reproduction come from one parent and are genetically identical to it, so there is very little inherited variation between them. Any differences that appear must come from the environment. Sexual reproduction mixes genes from two parents, so it produces much more inherited variation.

Plants show it clearly. Hydrangeas of the same variety have blue flowers in acidic soil and pink flowers in alkaline soil. The genes are the same, so the soil causes the colour change.

Environmental factors that affect height include nutrition, illness and sleep. Over generations, better nutrition can raise the average height of a population even though the genes have not changed.

Why variation matters

Variation is not just a curiosity. It helps a species to survive when conditions change.

Imagine mice in a field in Co. Clare. Some are brown and some are white. Hawks hunt them, and white mice are easier to see against dark soil.

1. There is variation: brown and white mice.

2. Hawks catch more white mice, so more brown mice survive.

3. Surviving brown mice breed and pass on the inherited brown colour to their young.

4. Over many generations, brown mice become more common.

This is natural selection: individuals with helpful inherited characteristics survive and reproduce more often. Without variation, no individual would be better suited and a change such as a new disease could wipe out the whole species.

If snow covered the soil for many years, white mice would be hidden and brown mice would stand out. Then the selection would favour white mice, and the change could be reversed over generations, as long as white mice are still present in the population.

Back to the start: even identical twins differ a little because the environment shapes them differently, while brothers and sisters differ because of the new mix of genes each time.

Summary