branch EVOLUTION
Show
=Stabilizing selection= Within a population most variation can be fitted to a bell curve. For instance, in the image below, a group of people have been arranged based on height with the shortest on the left and the tallest on the right. You can clearly see that the majority of individuals are clustered somewhere in the middle and fewer individuals at each end of the range. [image:http://i.imgur.com/q54HGwV.png] [image:http://i.imgur.com/B5ZSgsP.jpg?1] Stabilising selection is probably the most common type of natural selection; it favours the most common phenotype as the best adapted. Stabilising selection reduces variation by selecting against alleles that produce more extreme phenotypes at either end of the phenotypic range. The resulting bell shaped curve is narrower. A good example is birth weight. Babies that are too light are often under-developed and therefore have a reduced chance of survival. Babies that are too heavy are usually larger and therefore they may be an increased risk of complications during birth. =Directional selection= [image:http://i.imgur.com/9MtATdN.jpg?1] This type of natural selection is most common during periods of '''environmental change'''. Directional selection favours alleles that produce phenotypes at one extreme of a phenotypic range. Selection reduces variation at one extreme of the range while favouring variants at the other end. The resulting bell shaped curve shifts in the direction of the selection. For instance a population of snails may exhibit some variation in shell colour. Directional selection might act against the lightest coloured individuals, reducing the frequency of alleles that code for lighter colours. =Disruptive selection= [image:http://i.imgur.com/gYKA0mZ.jpg?1] Disruptive selection favours alleles that code for phenotypes at both extremes of a phenotypic range. The bell shaped curve acquires two peaks. Disruptive selection may occur when environmental conditions are varied or when the population covers a large area. For instance, a population of snails might live in a region that has areas with white rocks and areas with black rocks. Disruptive selection would act against the individuals with intermediate colours (grey or beige individuals), reducing the frequency of alleles that code for these colours. Disruptive selection can result in two distinct groups and if they become adapted to a different way of life (niche) they could eventually evolve into separate species (speciations). 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 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. Which type of selection favors individuals with traits at both extremes of the bell curve and works against individuals who are in the middle?Stabilizing Selection
This 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.
Which type of selection favors both extremes of an individuals trait?Diversifying (or disruptive) selection: Diversifying selection occurs when extreme values for a trait are favored over the intermediate values. This type of selection often drives speciation. Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum.
Which mode of natural selection would be represented by a bell shape curve?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.
What type of selection are both extremes selected for?When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. For example, plant height might be acted on by stabilizing selection.
|
