Mechanisms of Natural Selection
Introduction
There are three different types of selection that are observed in nature:
- Directional Selection
- Stabilizing Selection
- Disruptive Selection
We will explore each of these types.
Learning:
Directional Selection
In Directional Selection, individuals who possess ‘outlying’ traits – or traits that deviate significantly from the norm or average – have higher survivability and are therefore ‘favored’.
An example of directional selection is what happened to the peppered moth in 18th and 19th century England. Before this time, the normal color of the peppered moth was light-colored (circle on the lower left in picture). This allowed the moth to blend in on the light-colored tree bark. Therefore, they were not visible to predators. Due to the Industrial Revolution soot was being poured into the atmosphere and it was darkening everything, including tree trunks. The trees were covered with soot. Under these circumstances the light-colored peppered moth became more visible to their predators. This caused the phenotype of the light-colored moth to decline because those moths were more likely to be eaten. The moths that had the phenotype for the darker color were able to survive and reproduce (circle on the upper right in picture). Therefore, the ‘outlying’ darker colored phenotype become more prevalent over generations.
This graph demonstrates directional selection. The red line represents the distribution of phenotypes in the original population. The blue bell-shaped curve represents the population that has been disrupted in one direction.
Stabilizing Selection
In Stabilizing Selection, individuals who deviate far from the ‘norm’ or average are selected against.
An example of stabilizing selection can be explained by using the number of eggs found in a robin’s nest. The typical number of eggs laid by a robin is four. If more are produced the chicks may become malnourished due to the increased amount of food needed for more chicks. A smaller number of eggs may result in no viable offspring surviving. So smaller and larger number of eggs will become less frequent and only a certain number (usually the average) will survive. Therefore variability decreases and the population “stabilizes” around the average (in this case 3 eggs as shown in the picture).
This graph demonstrates stabilizing selection. The red line represents the distribution of the original population. The blue bell-shaped curve represents how the population that has been stabilized and the amount of variability has decreased.
Disruptive Selection
There are examples of when two or more distinct phenotypes could have an advantage over the intermediate type in a population. In this case the intermediate type is disrupted and less fit to survive.
An example of disruptive selection is a population of rabbits living in a rocky environment. Gray and Himalayan rabbit types would be able to blend into the rocky environment while white rabbits would not be camouflaged. The white rabbits therefore would seen by predators and that phenotype would become lower in frequency.
The graph above demonstrates disruptive selection. The lower and higher number of phenotypes would increase while the normal phenotype would decrease. This sometimes results into two distinct populations. The blue line represents the distribution of the original population which in this case was mostly all white rabbits with a minority of black rabbits and white rabbits with black markings. The red curve with two peaks represents how the population that has been disrupted. The amount of variability has increased and the rabbits are now more likely to be either black or white with black markings than simply all white.
Summary
- Directional selection: natural selection in which a single phenotype is favored, causing the allele frequency to shift towards that favored phenotype.
- Stabilizing Selection: individuals who deviate far from the ‘norm’ or average are not favored and are selected against.
- Disruptive Selection: individuals who exhibit extreme traits are favored over the intermediate.
Sources:
“Adaptive Evolution.” By OpenStax Biology 2e. Retrieved from: https://openstax.org/books/biology-2e/pages/19-3-adaptive-evolution/ Licensed under: CC-By: Attribution
Genetic Distribution.svg, Image of the different types of genetic selection. June 2, 2015. https://upload.wikimedia.org/wikipedia/commons/6/62/Genetic_Distribution.svg: Licensed under: CC-By: Attribution
American Robin Nest with Eggs.jpg. Image of American Robin’s Nest with Eggs. July 14, 2011. https://commons.wikimedia.org/wiki/File:American_Robin_Nest_with_Eggs.jpg : Licensed under: CC-By: Attribution