Natural Selection

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Principles of Natural Selection

Natural selection is the process that leads to evolution in a population. In order to understand how this process functions, it is important to learn the key principles of natural selection.

Random mutation

Mutations are changes in the gene sequence of DNA.
Random mutations can arise spontaneously.
New mutations can result in new alleles of a gene.

Variation in mutations

Most mutations are harmful because they alter the normal functioning of a gene.
But by chance, some mutations might confer a beneficial trait.
An individual with a beneficial mutation will be more likely to survive and reproduce than an individual without the mutation.

Inheritance

When an individual reproduces, their offspring will inherit half of their genes.
When an individual with an advantageous mutation reproduces, the allele is inherited by their offspring.

Change in allele frequency

Individuals in the next generation who have the advantageous mutation are also more likely to reproduce and pass on the allele.
Over many generations, the new allele will increase in frequency in the population.
The change in allele frequency over time is called evolution.

Stabilising Selection

Stabilising selection is the process where natural selection favours an average phenotype.

Selection pressure

Natural selection acts on an individual by imposing a selection pressure.
The selection pressure is an external factor that influences the reproductive success of an individual.
- E.g. Disease acts as a selection pressure that increases the reproductive success for individuals who are immune to the disease.

Stabilising selection

In stabilising selection, natural selection favours an average phenotype.
The selection pressures in stabilising selection select against the extreme phenotypes.
Stabilising selection takes place in environments that do NOT change.

E.g. Human birth weights

Human birth weight is subject to stabilising selection.
If a baby has a below average weight, it is unlikely to survive.
If a baby has an above average weight, it is more likely to encounter difficulties during birth.
Stabilising selection selects for a phenotype within the average weight range.

Directional Selection

Directional selection is the process where natural selection favours an extreme phenotype.

Directional selection

In directional selection, natural selection favours one extreme phenotype.
The selection pressures in directional selection select against all other phenotypes.
Directional selection takes place after an environment has experienced a change.

E.g. Antibiotic resistance

Directional selection is seen in bacterial populations.
If bacteria are exposed to antibiotics, most of the population will not be resistant to its effects and will not survive.
If a mutation evolved that confers resistance, selection will strongly favour bacteria with the resistant phenotype.
The extreme, resistant phenotype is favoured.