Which of the following can result in a population looking more similar over time and can be described as selection against both extremes of a trait? a. directional selection b. stabilizing selection c. disruptive selection d. species selection

The correct answer would be B. stabilizing selection.

It is a type of selection in which intermediate trait is selected over both the extreme values of the traits.

Due to this selection, the intermediate trait becomes more common or stable in the population whereas the extreme traits are reduced or lost from the population.

For example, the weight of an infant at the time of birth in humans. Too high or too less weight are both dangerous or inappropriate for the survival of the infant. In contrast, the intermediate weight of an infant is considered as ideal for the mother as well as an infant at the time of delivery.

When an average phenotype or character of a population is selected over both the extreme forms of a trait, the selection is said to be stabilizing selection.  

Further Explanation:

In a population where selection pressure works against the extreme phenotypes but favors the average phenotypic character, such population results in stabilizing selection. The stabilizing selection makes the population look more or less similar over time due to the selection of the average character. This results in the reduction of genetic variability in the population. For example, a short heightened plant cannot extract more sunlight as compared to the other tall plants. On the contrary, certain plants are quite tall which can be more susceptible to damage through the wind. When the selection pressure based on the extraction of sunlight and resistance to wind damage is selected, the plant with average or medium height is able to survive. Therefore, under such selection pressure medium height plants will increase in number compared to the tall and short plants which will make the population appear less diverse.

When the selection pressure favors any one of the two extreme traits in a population, the allele frequency increases in a particular direction. This is called directional selection. For example, in the moth population, the moths with high melanin content were able to survive in an area affected by air pollution. This is because of the dark coloration of the moth which camouflages them against the sooty tree. Therefore, the frequency of dark-colored moth increases in one extreme direction compared to the average or light-colored moth.

In disruptive selection, the two extreme phenotypes are more fit under the selection pressure that they outrun the average individuals. Such selection is seen in animal populations where males opt for multiple mating strategies, for example, lobsters. In the alpha male territory, the small males chose furtive copulations with the females while the dominant, large male population use brute force for mating. In such a scenario, the medium-sized males cannot compete and overtake the small and large male population for mating. Therefore, the extreme populations on both ends are selected.

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Answer Details:

Grade: Secondary School

Chapter: Types of Natural Selection

Subject: Biology

Keywords:

Natural selection, stabilizing selection, disruptive selection, directional selection, selection pressure, phenotypes, traits, average phenotype, extreme phenotype, genitive diversity.

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