Natural selection describes the change in a population’s allele frequency (gene pool) in favour of individuals that are better adapted to survive selection pressures. These organisms harbour advantageous alleles and, therefore, a favourable phenotype that increases their chances of survival. As a result, they will live on to reproduce and create offspring, which inherit the advantageous allele. Here, we will learn about the three different types of selection that influence a population’s gene pool.
Fig. 1 - Process 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
In humans, characteristics such as height and weight undergo stabilising selection.
The number of eggs laid by birds also undergoes stabilising selection. When birds lay too many eggs, they may not be able to feed all of the chicks sufficiently, leading to malnourished offspring. In addition, laying too many eggs can be a significant drain on the mother’s resources, reducing the number of eggs she can lay in the future. On the other hand, laying too few eggs might result in no viable offspring. In this case, an intermediate value would be selected.
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.
Continuous variation is a characteristic that changes gradually over a range of values, e.g., height and weight.
Discontinuous variation is a characteristic of any species with only a limited number of possible values, e.g., sex and eye colour.
Fig. 3 - Directional selection
An example of this would be directional selection due to climate change. For instance, a population of polar bears is usually distributed around average body weight. Individuals with larger body sizes tend to survive better in colder climates, while individuals in warmer climates are more likely to survive with smaller bodies. An increase in global temperatures might shift the optimum body mass to the left of the original optimum, thus creating directional selection towards smaller polar bears.
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
Disruptive selection is seen in rabbits. In an environment with black and white rocks, black and white rabbits will be camouflaged, whereas grey rabbits will be visible and eaten by the predator.
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.
You can see the effects of polymorphism on organisms’ body colour. In some oyster populations, both light and dark morphs can exist; lighter oysters can blend in successfully against light sand, while darker oysters can blend into the shadows under dark rocks. Intermediate coloured oysters would stand out against either backdrop.
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.
Types of Natural Selection - Key takeaways
There are three main types of selection: stabilising, directional, and disruptive.
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.
Directional selection favours one extreme phenotype and usually occurs when changes in the environment shift the optimum phenotype to the left or right of the original value.
Disruptive selection favours extreme rather than intermediate phenotypes and is the least common form of selection.
Polymorphism in a population occurs when many genetically distinct forms (morphs) of a species still interbreed in the same population.
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