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In nature, evolution takes place through a process called selection. In a group of organisms of the same species, differences occur in how prominent particular traits are. The offspring of organisms with traits that facilitate survival in the environment they are in have a higher chance of surviving. This process of selection gradually increases the number of organisms with particular traits and the species evolves.
Directional selection is said to occur when one of the extremes of a trait distribution is eliminated. For instance, consider the trait length of a plant's root. If the plant is in an environment where the ground water level has fallen, only the plants that have long roots are able to reach the water and survive. The plants with shorter roots are not able to survive and this changes the genetic profile of future generations of the plant such that the length of their roots increases.
For any trait of organisms belonging to a particular species, there are some that have extreme values of the relevant trait. Disruptive selection is said to occur when these organisms survive and those that do not lie at the extremes are eliminated. For example, consider a species of animal with different height. If organisms that are extremely short and those that are extremely tall are in a more favorable position to survive than those with height lying in the middle, there is a disruptive selection as organisms that lie in the middle of the trait are eliminated.
Directional selection occurs when one extreme of a trait is favored. For instance, if a white coat was favored over both a black and gray colored coat. This would skew the population's genetic make up so that if you were to graph it, most of the population would fall under a bell curve at one end of the graph.
Disruptive selection is where any two extremes of a trait are favored. (Perhaps a flower being either red or white was beneficial over being pink). Then the population would fall under two seperate bell curves creating a dip in the middle of the graph.
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