The 5 Types of Selection

Darwin's finches
Darwin's finches. Print Collector/Hulton Archive/Getty Images

Charles Darwin was not the first scientist to explain evolution, or that species change over time. However, he gets most of the credit simply because he was the first to publish a mechanism for how evolution happened. This mechanism is what he called Natural Selection.

As time passed, more and more information about natural selection and its different types was discovered. With the discovery of Genetics by Gregor Mendel, the mechanism of natural selection became even clearer than when Darwin first proposed it. It is now accepted as fact within the scientific community. Below is more information about 5 of the types of selection known today (both natural and not so natural).

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Directional Selection

A graph of directional selection. Graph By: Azcolvin429 (Selection_Types_Chart.png) [GFDL]

The first type of natural selection is called directional selection. It derives its name from the shape of the approximate bell curve that is produced when all individuals' traits are plotted. Instead of the bell curve falling directly in the middle of the axes on which they are plotted, it skews either to the left or the right by varying degrees. Hence, it has moved one direction or the other.

Directional selection curves are most often seen when one coloring is favored over another for a species. This could be to help them blend into an environment, camouflage themselves from predators, or to mimic another species to trick predators. Other factors that may contribute to one extreme being selected for over the other include the amount and type of food available. 


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Disruptive Selection

A graph of disruptive selection. Chart By: Azcolvin429 (Selection_Types_Chart.png) [GFDL]

Disruptive selection is also named for the way the bell curve skews when individuals are plotted on a graph. To disrupt means to break apart and that is what happens to the bell curve of disruptive selection. Instead of the bell curve having one peak in the middle, disruptive selection's graph has two peaks with a valley in the middle of them.

The shape comes from the fact that both extremes are selected for during disruptive selection. The median is not the favorable trait in this case. Instead, it is desirable to have one extreme or the other, with no preference over which extreme is better for survival. This is the rarest of the types of natural selection. 

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Stabilizing Selection

A graph of stabilizing selection. Graph By: Azcolvin429 (Selection_Types_Chart.png) [GFDL

The most common of the types of natural selection is stabilizing selection. In stabilizing selection, the median phenotype is the one selected for during natural selection. This does not skew the bell curve in any way. Instead, it makes the peak of the bell curve even higher than what would be considered normal.

Stabilizing selection is the type of natural selection that human skin color follows. Most humans are not extremely light skinned or extremely dark skinned. The majority of the species fall somewhere in the middle of those two extremes. This creates a very large peak right in the middle of the bell curve. This is usually caused by a blending of traits through incomplete or codominance of the alleles. 


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Sexual Selection

A peacock showing his eyespots. Getty/Rick Takagi Photography

Sexual Selection is another type of Natural Selection. However, it tends to skew the phenotype ratios in the population so they do not necessarily match what Gregor Mendel would predict for any given population. In sexual selection, the female of the species tend to choose mates based on traits they show that are more attractive. The fitness of the males is judged based on their attractiveness and those who are found more attractive will reproduce more and more of the offspring will also have those traits. 

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Artificial Selection

Domestic Dogs. Getty/Mark Burnside

Artificial selection is not a type of natural selection, obviously, but it did help Charles Darwin obtain data for his theory of natural selection. Artificial selection mimics natural selection in that certain traits are chosen to be passed down to the next generation. However, instead of nature or the environment in which the species lives being the deciding factor for which traits are favorable and which are not, it is humans that do the selecting of traits during artificial selection.

Darwin was able to use artificial selection on his birds to show that desirable traits can be chosen through breeding. This helped back up the data he collected from his trip on the HMS Beagle through the Galapagos Islands and South America. There, Charles Darwin studied native finches and noticed those on the Galapagos Islands were very similar to the ones in South America, but they had unique beak shapes. He performed artificial selection on birds back in England to show how the traits changed over time.

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Scoville, Heather. "The 5 Types of Selection." ThoughtCo, Oct. 25, 2017, Scoville, Heather. (2017, October 25). The 5 Types of Selection. Retrieved from Scoville, Heather. "The 5 Types of Selection." ThoughtCo. (accessed January 23, 2018).