The 3 Types of Natural Selection https://schooltutoring.com/help/wp-content/uploads/sites/2/2019/04/girafe-2692014_960_720.jpg 960 663 Teaching Staff Teaching Staff https://secure.gravatar.com/avatar/d96b825901af08f4b20fdfa2d056868f?s=96&d=mm&r=g April 17, 2019 April 17, 2019 Show
Natural selection is defined as a process or a “force” that allows for organisms better adapted to their environment to better survive and produce more offspring. The theory of natural selection was first founded by Charles Darwin. The process of natural selection is important and is a driving force for evolution. For organisms to evolve, there needs to be differences in traits between organisms that provide certain advantages or disadvantages, and it is these traits that natural selection acts upon. When it comes to natural selection, there are three different types of selection that can occur. These types include the following: Stabilizing SelectionThis type of natural selection occurs when there are selective pressures working against two extremes of a trait and therefore the intermediate or “middle” trait is selected for. If we look at a distribution of traits in the population, it is noticeable that a standard distribution is followed:
Example: For a plant, the plants that are very tall are exposed to more wind and are at risk of being blown over. The plants that are very short fail to get enough sunlight to prosper. Therefore, the plants that are a middle height between the two get both enough sunlight and protection from the wind. Directional SelectionThis type of natural selection occurs when selective pressures are working in favour of one extreme of a trait. Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side:
Disruptive SelectionThis type of natural selection occurs when selective pressures are working in favour of the two extremes and against the intermediate trait. This type of selection is not as common. When looking at a trait distribution, there are two higher peaks on both ends with a minimum in the middle as such:
Example: An area that has black, white and grey bunnies contains both black and white rocks. Both the traits for white and black will be favored by natural selection since they both prove useful for camouflage. The intermediate trait of grey does not prove as useful and therefore selective pressures act against the trait. SchoolTutoring Academy is the premier educational services company for K-12 and college students. We offer tutoring programs for students in K-12, AP classes, and college. To learn more about how we help parents and students in Hastings, Nebraska visit: Tutoring in Hastings, Nebraska. Directional selection, stabilizing selection and disruptive selection are three types of natural selection. They are also examples of adaptive evolution. Natural selection is the mechanism of evolution which favors organisms that are better adapted to their environments. Such organisms tend to survive longer and produce more offspring. Selection pressures act against organisms that do not have favorable traits and they are removed from the population. As a result, natural selection plays a major role in the creation of new species over time. Directional SelectionThe English naturalist and scientist Charles Darwin was the first to describe directional selection as a form of natural selection in his foundational 1859 work On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life. An example of directional selection is giraffe neck lengths. The environment created a selection pressure which favored giraffes with longer necks who could reach more food in the trees. At the same time, there was selection pressure against giraffes with shorter necks. Both long and short necks are extreme phenotypes, but over time, the long neck phenotype dominated due to selection pressure, i.e., this trait in giraffes shifted toward the direction of long necks. Stabilizing SelectionStabilizing selection can be thought of as “middle-of-the-road” selection, meaning a non-extreme trait is favored instead of one of the two extreme traits. An example of this is plant height. In a population of plants, those that are short may not get enough sunlight, but those that are tall may be subjected to wind damage. This results in an increase in the number of medium-height plants and a decrease in very tall and very short plants. Because most traits do not change drastically over time, stabilizing selection is considered to be the most common mechanism for natural selection. Other examples of stabilizing selection are the birth weight of humans and the number of eggs a bird lays (clutch size). The birth weight of human babies stays within a certain range because babies that have a very low birth weight have less chance of survival and those with a very high birth weight can cause complications during delivery which threaten the life of the mother and the child. The clutch size of bird species is limited to a certain number of eggs. There must be enough eggs so that the clutch can survive predation and/or disease, but not so large that there are too many chicks for the parent(s) to feed. Disruptive SelectionThis type of natural selection is bimodal and favors both extreme traits in a population. For example, in a population of plants, there are some pollinators that visit the tallest plants, a different species of pollinator visits medium-height plants and a third species of pollinator that prefers the shortest plants. If the pollinator that prefers medium-height plants is removed, natural selection would select against medium-height plants and the overall plant population would move toward having only tall and short plants, the two extreme phenotypes. A more classic example of disruptive selection is the beak size of finches on the Galapagos Islands that was studied by Darwin. Because the majority of seeds found on some of the islands were either large or small, finches with large and small beaks (no medium-sized beaks) were favored on those islands. Comparison Chart
 The image above shows the three patterns of natural selection using an allele that determines fur color. References
Cite This ArticleMLAAPAChicago Biologydictionary.net Editors. "Directional Selection, Stabilizing Directional and Disruptive Selection." Biology Dictionary, Biologydictionary.net, 05 Nov. 2017, https://biologydictionary.net/directional-selection-stabilizing-directional-disruptive-selection/. Biologydictionary.net Editors. (2017, November 05). Directional Selection, Stabilizing Directional and Disruptive Selection. Retrieved from https://biologydictionary.net/directional-selection-stabilizing-directional-disruptive-selection/ Biologydictionary.net Editors. "Directional Selection, Stabilizing Directional and Disruptive Selection." Biology Dictionary. Biologydictionary.net, November 05, 2017. https://biologydictionary.net/directional-selection-stabilizing-directional-disruptive-selection/. Subscribe to Our NewsletterWhat is the type of selection extreme phenotype is favored?In population genetics, directional selection, is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.
Which type of natural selection occurs when one extreme phenotype is favored?Disruptive selection is a type of natural selection in which the extremes of a phenotype are favored. This means that there is active selection against intermediates. It increases genetic diversity and is the opposite of stabilizing selection.
Which type of selection favors one of the extremes?Directional selection
occurs when natural selection favors one extreme of continuous variation. Over time, the favored extreme will become more common and the other extreme will be less common or lost.
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What type of selection occurs when one extreme population phenotype is favored?
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