Stabilizing selection. If more then four eggs are produced then the baby chicks are malnourished while smaller egg numbers result in no viable (able to live) baby chicks. The first type of natural selection is called directional selection. Selective pressures are environmental factors that lead to directional selection. For instance, if the smallest organisms get eaten, and larger organisms are totally protected, the population will tend to get much larger. Directional Selection Definition. 30 seconds . Q. Robins typically lay fours eggs. A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Therefore when looking at a distribution of traits in a population, a graph tends to lean more to one side: Example: Giraffes with the longest necks are able to reach more leaves to each. Directional selection is a type of natural selection in which the phenotype (the observable characteristics) of the species tends toward one extreme rather the mean phenotype or the opposite extreme phenotype. It derives its name from the shape of the approximate bell curve that is produced when all individuals' traits are plotted. The key difference between directional and disruptive selection is that the directional selection prefers and selects only one extreme trait among the two extreme traits whereas the disruptive selection favours both extreme traits together.. A classic example of this type of selection is the evolution of the peppered moth in eighteenth- and nineteenth-century England. SURVEY . When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation. Discover natural selection as a mechanism of evolution with the Amoeba Sisters. Phenotypes are the physical features of animals and humans. You would expect that one allele will provide higher fitness than the other, and therefore would outcompete the other allele. Directional selection is a type of natural selection that occurs when one side of the spectrum of a certain trait is favored over the other. When the environment changes, populations will often undergo directional selection, which selects for phenotypes at one end of the spectrum of existing variation. Directional Selection This type of natural selection occurs when selective pressures are working in favour of one extreme of a trait. Directional Selection. Disruptive selection . The theories of directional and disruptive selection came into limelight with the introduction of the theory of Natural Selection … This is an example of Balancing selection means that two alleles are maintained in the population because of natural selection. Directional Selection. Directional selection. The 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. Directional selection is one of three widely studied types of natural selection, in addition to stabilizing selection and disruptive selection. Tags: Question 9 . Natural selection is the driving force behind evolution - the gradual change in organisms' genetic makeup over time. Directional Selection. While some traits are discrete and have specific variations (think eye color), other traits are continuous, and exists as a wide range of nearly infinite values (think height).If a discrete trait goes through a directional selection… 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. 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