Types of Natural Selection

What is the importance of natural selection?

Through natural selection, favourable traits are inherited across generations. It is one of the processes driving evolution and helps explain the diversity of organisms on earth. Diversity is essential; without genetic variation, a population wouldn’t evolve in response to the changing environment, which may increase the risk of extinction.

What are the three types of natural selection?

There are three main types of natural selection, each of which affects the characteristics of the population in different ways. They are:

• Stabilising selection, which preserves the average phenotype
• Directional selection, which favours one extreme phenotype
• Disruptive selection, which favours more than one extreme phenotype

We will go through examples of each of these in turn.

What is stabilising selection?

Stabilising selection selects for the average phenotypes and against more extreme phenotypes. This type of selection occurs in populations where environmental conditions remain constant for long periods. Therefore, this selection form keeps allele frequencies relatively steady over generations and maintains (keeps stable) an intermediate phenotype.

Fig. 2 - Stabilising selection

What is directional selection?

For traits that display continuous variation, populations are often generally distributed around an optimum phenotype. However, when the environment changes, the optimum value for survival might also change. Individuals on either side of the mean might possess alleles that result in the new optimal phenotype. As a result, a directional selection pressure might occur, resulting in the mean phenotype moving to either the left or right of its original position. Directional selection would lead to one extreme phenotype being selected for rather than the other.

Fig. 3 - Directional selection

Directional selection in peppered moths

Directional selection can be seen in peppered moths. The peppered moth has a lighter morph and a darker melanic morph. Against lichen-covered trees, lighter moths can blend in well, whereas melanic moths stand out, making them vulnerable to predation from insectivorous birds. As a result, the population underwent directional selection favouring lighter moths; thus, melanic moths only made up a small fraction of the population in the early 1800s.

However, about 50 years later, many more melanic moths were recorded in industrial areas where trees and buildings were blackened by soot. Here, melanic moths can blend in and light moths cannot, making the latter much more vulnerable. This time, the population underwent selection in the opposite direction, favouring darker moths. By the end of the century, the overwhelming majority of the pepper moth population was composed of melanic moths.

Fig. 4 - The lighter morph and melanic morph peppered moths

What is disruptive selection?

Disruptive selection, also known as diversifying selection, favours several extreme phenotypes rather than intermediate phenotypes and is the least common form of selection. This type of selection occurs when the extreme phenotypes are more successful than intermediate forms within the same habitat, as depicted below.

Fig. 5 - Disruptive selection

What is polymorphism?

Disruptive selection can lead to polymorphism in a population, wherein many (poly) genetically distinct forms (morphs) of a species still interbreed and exist in the same population.

Disruptive selection can also create polymorphism in mating strategies. For example, in several species of fish, males can employ different mating strategies. Large, territorial males can win mates by fighting with each other, while relatively small males can ‘sneak’ in with the females and score mates in this manner. Intermediate sized males are at a disadvantage here as they are not large enough to compete with the territorial males but not small enough to blend in with the females.