76 Mutations

Key Idea: Mutations are changes to an organism's DNA. Beneficial mutations may spread within a population but harmful mutations are not usually retained. A mutation is a permanent change to the DNA sequence of an organism. Mutations allow for new genetic material to arise and be tested within the current environmental conditions. Most mutations are harmful because they reduce fitness. They remain at low levels within a population or they are eliminated altogether. Heritable mutations that are beneficial (provide an advantage) may be passed on and become established within the population. Some mutations are silent, meaning they have no phenotypic effect. These may be carried in the genome and not subject to selection pressure as long as the environment stays the same. However, a change in the environment can alter selection pressure and may result in the mutation being beneficial or harmful. The effect of a mutation will always be determined by the selection pressures on the population at the time.

Some mutations are retained, others are eliminated

Original amino acid sequence

Mutated amino acid sequence

This mutation results in a different amino acid being added to the peptide chain. It changes the protein made.

A silent mutation is a DNA sequence change that has no phenotypic effect. This may be because the change occurs outside a protein-coding region (in introns), because there is no change in the amino acid (due to code degeneracy), or because an amino acid has been replaced by one with the same properties. Heritable silent mutations may be carried without effect and may only be subject to selection pressure when environmental conditions change.

Individual with the mutated protein

A mutation occurs. Mutations may arise through errors in DNA replication or from environmental factors (e.g. UV radiation). If the mutation is harmful (reduces fitness) in the current environment it is selected against and is usually eliminated from the population.

If the mutation is beneficial (increases fitness) and it is heritable (occurs in the gametes) it is selected for and retained in the population. It may become more common over several generations.

1. What is a mutation?

2. Why are some mutations retained within a population and others eliminated?

3. (a) What is a silent mutation?

(b) What is the potential advantage of a silent mutation being retained within a population?