Types of Natural Selection 

Directional Selection 

The directional selection theory is when one of the extreme characteristics or traits are favored over another extreme trait and average traits. An example of directional selection is giraffe neck lengths. The giraffes’ environment created positive selection pressure which favored giraffes with longer necks who could reach more food in the trees. The giraffes with longer necks had an advantage over the giraffes with shorter necks because they could reach food in taller places with more efficiency. At the same time, there was negative selection pressure against giraffes with shorter necks. The giraffes with shorter necks were at a disadvantage because they had more trouble reaching food in higher places compared to the giraffes with longer necks. Over time, the long neck gene overtook and dominated the short neck gene due to the genetic advantage of the longer neck giraffe. (“Directional Selection, Stabilizing Directional and Disruptive Selection,” 2017). 

Another example of directional selection is the peppered moth. Prior to the industrial revolution, in England the peppered moth was found exclusively to have a light colored body with black spots. The light colored moth had an advantage over the dark colored moths because they could blend in with the light colored lichen of the trees. However, once the industrial revolution began, the sulfur dioxide being released started to kill the light colored lichen and left behind only the dark colored bark of the trees. Now the dark peppered moths had an advantage over the light colored moths and began to grow in population over time (“Natural Selection and Mutation - The Case of the Peppered Moth,” n.d.). 

Stabilizing Selection 

Stabilizing selection is any selective force or forces which move a population toward an average or median trait. Stabilizing selection is what happens to an individual trait when the extremes of the trait are not selected, but pushed towards the average. This increases the frequency of the median trait in the population. An example of stabilizing selection is robin eggs. Robins are not able to raise more than 4 chicks with much success. The 4 chicks as offspring are the average offspring that the robins produce. This is due in part because of the robin’s size and the amount of food that two adult robins can provide. (“Stabilizing Selection,” 2018). 

Another example of stabilizing selection is human birth weight in underdeveloped countries with little to no medicine. Human birth weight is pushed toward a median trait of weight because if a baby’s weight is too small it will be weak and experience health problems, while if the baby is too large it will have difficulty passing through the birth canal of the mother. This has improved in the human population with the increase in medicine to help smaller babies and medical procedures such as c-section operation to be able to remove larger babies (Scoville, 2019).

Diversifying / Disruptive selection

Diversifying / Disruptive selection is the type of natural selection theory that favors both of the extreme traits in a population. For example, in a population of plants, some pollinators visit the tallest plants, a different species visit medium-height plants, and a third species that prefers the shortest plants. If the pollinator species that prefer medium-height plants are removed, natural selection would select against medium-height plants and the overall plant population would move toward the two extreme traits of only tall and short plants. (“Directional Selection, Stabilizing Directional and Disruptive Selection,” 2017). Another example of diversifying / disruptive selection is the shape of the Mexican spadefoot toad tadpoles. Spadefoot tadpoles have higher populations in the extremes of their shape instead of the median. The more omnivorous vegetarian tadpoles are round-bodied, while more carnivorous meat eating are narrow-bodied (Scoville, 2019). 

Frequency Dependent selection

Frequency-dependent selection is when the fitness of a gene depends on the species-environment. If a frequency-dependent selection is positive, the fitness of a phenotype or genotype increases as it becomes more common because it provides the species with an advantage and helps it adjust to its environment. An example of a positive frequency-dependent selection is warning coloration in certain species. Predators are more likely to remember a common color pattern that they have already hunted for frequently compared to one that is rare. Therefore the uncommon warning coloration is a positive frequency-dependent selection because it helps the species avoid predators in its environment. Another example of positive frequency-dependent selection is the intimidation of the warning coloration of the scarlet kingsnake. The scarlet kingsnake is a type of snake that is harmless that copies the coloration of the eastern coral snake, which is a very dangerous snake that is found in the same geographical region. This copying of the coloration of the eastern coral has helped protect the scarlet kingsnake from predators and has common more common in the scarlet kingsnake population because it serves as an environmental advantage (“19.3C: Frequency-Dependent Selection,” 2020)

If the frequency-dependent selection is negative, the fitness of a gene decreases because it harms the species.  An example of negative frequency-dependent selection is plant inability to reproduce genes. When two plants share the same incompatibility genes, they are unable to mate and produce offspring. Because of this, a plant with a  gene that can mate can dominate and spread its gene quickly through the population and produce offspring that can spread it to future generations (“Frequency-dependent selection,” 2020). Another example of negative frequency-dependent selection is the interaction between the human immune system and infectious bacteria and viruses. As humans become infected from a common strain of bacteria or virus, eventually most of the human population becomes immune it that strain. However, mutations occur in that strain and make new strains of uncommon bacteria or viruses that can infect humans. These new strains are more deadly than the previous stain because they are less common, so it will be more difficult for humans to fight them off and develop immunity (“19.3C: Frequency-Dependent Selection,” 2020).

Sexual Selection 

Sexual selection theory is when certain physical traits in animals make them more attractive and have a higher chance of obtaining a mate and producing offspring (Ayala, 2019). An example of sexual selection theory is when the male peacock has a lot of showy feathers. Peacocks use their feathers to attract female peacocks known as peahens. The peacocks with the showier feathers can attract mates, so they are the ones that have offspring, and pass on the attractive fancy-feather genes to the next generation. (“Sexual Selection,” n.d.). Another example of sexual selection is the large antlers on male deer (Odocoileus). The larger the antlers, the more attractive the male deer are to female deer (“Sexual Selection,” n.d.).

Work Cited

Ayala, F. J. (2019, August 8). Sexual selection. Retrieved October 24, 2020, from 

https://www.britannica.com/science/sexual-selection

Editors, B. (2017, November 05). Directional Selection, Stabilizing Directional and Disruptive 

Selection. Retrieved October 24, 2020, from 

https://biologydictionary.net/directional-selection-stabilizing-directional-disruptive-select

ion/

Editors, B. (2018, June 20). Stabilizing Selection. Retrieved October 24, 2020, from 

https://biologydictionary.net/stabilizing-selection/

Frequency-dependent selection. (2020, September 27). Retrieved October 24, 2020, from

 https://en.wikipedia.org/wiki/Frequency-dependent_selection

Libretexts. (2020, August 15). 19.3C: Frequency-Dependent Selection. Retrieved November 17, 

2020, from 

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book:_General

_Biology_(Boundless)/19:_The_Evolution_of_Populations/19.3:_Adaptive_Evolution/19

.3C:_Frequency-Dependent_Selection

Natural Selection and Mutation - The Case of the Peppered Moth. (n.d.). Retrieved November 

13, 2020, from 

https://globalchange.umich.edu/globalchange1/current/labs/Lab5_PepperedMoth/Peppere

dMoth.htm

Scoville, H. (2019, January 19). What Is Stabilizing Selection in Evolution? Retrieved November

 17, 2020, from 

https://www.thoughtco.com/types-of-natural-selection-stabilizing-selection-1224583

Scoville, H. (2019, February 16). What Is Disruptive Selection? Retrieved November 17, 2020, 

from https://www.thoughtco.com/what-is-disruptive-selection-1224582

Sexual Selection. (n.d.). Retrieved October 24, 2020, from https://necsi.edu/sexual-selection

GrrlScientist. (2017, March 9). Why Are Robin Eggs Blue? Forbes. https://www.forbes.com/sites/grrlscientist/2016/07/25/why-are-robins-eggs-blue/?sh=630b3bd16408. 

GuideCentral. (n.d.). How to Tell the Difference Between a King Snake and a Coral Snake. wikiHow. https://www.wikihow.com/Tell-the-Difference-Between-a-King-Snake-and-a-Coral-Snake. 

Nicholls, H. (2016, June 30). Giraffes may not have evolved long necks to reach tall trees. BBC.http://www.bbc.com/earth/story/20160629-giraffes-did-not-evolve-long-necks-to-reach-tall-trees. 

Spurr, K. (2020, March 31). The Frog Family. College Arts & Sciences Magazine. https://magazine.college.unc.edu/news-article/the-frog-family/.