Variation and Evolution » Variation

Why no two organisms are exactly the same

Look at the people in your class. Nobody looks exactly the same, even brothers and sisters. The same is true of cats, daisies and oak trees. Every population is full of differences.

These differences are what biologists call variation. Understanding where variation comes from is the first step to understanding how species change over time.

Key terms:

• Variation: the differences in the characteristics of individuals in a population.
• Genetic causes: differences that come from the genes an organism has inherited.
• Environmental causes: differences that come from the conditions in which an organism has developed.

How the genome and the environment build the phenotype

An organism's genome is the full set of instructions in its DNA. Those instructions are the starting point. They are not the whole story.

As an organism grows, its genome interacts with its environment. The environment includes things like food, light, temperature, water and the soil it grows in. The result of this interaction is the phenotype, which is the characteristics you can actually observe.

So you can think of it like this:

genome (the instructions) + environment (the conditions) = phenotype (the organism you see)

Two organisms with the same genome can end up with different phenotypes if they grow up in different conditions.

The three causes of variation

Variation may be due to differences in genes, differences in conditions, or a combination of both. Here is how to sort any example.

Examples of genes and environment together

Height. Genes you inherit from your parents set how tall you could grow. But a person who does not get enough food while growing may not reach that height. Height is caused by genes and the environment.

Plants. Take two cuttings from the same plant. They are clones, so they have identical genes. Grow one on a bright windowsill and one in a dark cupboard. The first is green and sturdy. The second is pale and spindly. The difference between them is environmental.

Skin. The amount of pigment in your skin is controlled by genes, but sunlight makes the skin darker. So skin colour is caused by both.

Genetic variation within a population

In most species there is usually extensive genetic variation within a population. This means that individuals have many different combinations of genes. In sexual reproduction, each offspring gets a mixture of genes, so most individuals are genetically different from each other.

This genetic variation matters because it gives a population many different phenotypes. When conditions change, some individuals may be better suited than others.

All variants come from mutations

Where do the different versions of genes come from in the first place? Every variant arose from a mutation, which is a change to the genome. Mutations occur continuously, all the time, in all living things.

But not every mutation matters. In fact, most do very little. You need to recall these three groups:

The molecular detail of how a mutation changes a protein belongs to the lesson on Protein Synthesis and Mutations. Here, you only need to know how often each type of effect happens.

A rare new phenotype can change a species

Very rarely, a mutation leads to a new phenotype. Sometimes the environment changes, for example the climate becomes cooler or a new disease appears.

If a new phenotype happens to be suited to the environmental change, it can lead to a relatively rapid change in the species.

How this process works step by step is the subject of the next lesson, Evolution by Natural Selection. For now, remember the chain:

• Mutations occur continuously.
• Very rarely, one gives a new phenotype.
• If it suits an environmental change, the species can change quite quickly.

Exam-style question

Two cuttings (clones) were taken from the same parent plant, so they have identical genes. Cutting A was grown in good light. Cutting B was grown in a dark cupboard. After two weeks, A was green and B was pale and tall and thin.

(a) Give the cause of the difference between A and B. [1 mark]

(b) Explain why the cuttings could end up with different phenotypes even though their genomes are the same. [2 marks]

(c) State what proportion of mutations have no effect on the phenotype. Choose from: most, some, very few. [1 mark]