Types of Natural Selection

Selection always works the same way, but the shape of the result depends on which individuals are being removed. Take out one extreme and the population slides. Take out both extremes and it tightens. Take out the middle and it splits.

Directional selection

In directional selection the population changes towards one extreme of a range of variation, because that extreme becomes better adapted. This tends to happen when environmental conditions change.

The frost example

Suppose average temperatures fall, so plants now face frosts — spells where the air temperature drops below 0°C.

• If no allele exists in the population that gives any frost resistance, the species will become extinct in that habitat if the cold lasts more than one generation.
• If an allele does exist that gives some frost protection, those plants survive the frost and reproduce successfully.
• Such an allele might code for a protein that lowers the freezing point of the cell contents by a few degrees and prevents damaging ice crystals from forming.
• The plants that carry it have a selective advantage, so the allele spreads and the population shifts towards frost tolerance.

Because a strong new pressure can act on every individual at once, a species can change its genome abruptly by directional selection. It is the fastest of the three types.

Stabilising selection

Stabilising selection selects in favour of the average individual. It occurs when environmental conditions are stable and do not change, so the phenotype that already works keeps on working. This is the most common form of natural selection, which makes sense — most of the time, most environments are not changing dramatically.

• Selection favours individuals whose combination of alleles gives the most advantageous characteristic for the conditions.
• Coat colour in mice is one example. The colour settles on whichever shade gives the best camouflage against the local surroundings, such as brown fur on a forest floor.
• Human birth mass is the classic example. Birth mass shows a normal distribution clustered around a mean, and both very low and very high birth masses cause problems for an infant’s survival.
• Stabilising selection discards extreme phenotypes and favours the majority of the population that is already well adapted.
• It therefore decreases diversity within a population, and works mostly on polygenic traits — ones controlled by several genes, which is why they give a smooth bell-shaped curve.

Disruptive selection

Disruptive selection selects against the average individual. It is the rarest of the three forms. Like directional selection, it happens when habitats or resources undergo a change — but here two different extremes each turn out to work, and the middle option works worst.

Because it pulls a population apart, disruptive selection can lead to the formation of an entirely new species, a process called speciation. For that reason it is sometimes called diversifying selection.

Darwin’s finches

The finches of the Galápagos Islands are one of the best known examples. Fifteen different species evolved from a single common ancestor, and multiple beak types adapted to different food sources over time.

On the island of Santa Cruz, ground finches eat mostly seeds and some arthropods, tree finches eat more fruits and arthropods, vegetarian finches feed on leaves and fruit, and warblers take mainly arthropods.

When food is abundant their diets overlap and beak shape hardly matters. When food is scarce, each specialisation lets that species compete for one particular food better than the others can — which is how each species ends up occupying its own niche.

Worked examples