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Natural selection in which a single phenotype is favored is Directional, in which extreme values of trait are favored over intermediate is disruptive and which favors stabilized phenotype is stabilizing selection.
Directional Selection
On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life, the seminal 1859 work by the English biologist and scientist Charles Darwin, was the first to identify directional selection as a type of natural selection.
According to the directional selection theory, an extreme phenotype—a set of features or characteristics—is preferred to other phenotypes, and as a result, the allele frequency—the frequency with which a gene variant appears in a population—shifts over time in favor of the extreme phenotype. In other words, a feature will manifest itself in the population with the highest frequency if it is favorable.
Giraffe neck lengths serve as an illustration of directed selection. Because they could reach more food in the trees, giraffes with longer necks were favored by the environment's selection pressure. At the same time, giraffes with shorter necks faced selection pressure. Both long and short necks are extreme phenotypes, but due to selection pressure over time, the long neck phenotype predominated; that is, this characteristic in giraffes moved in favor of long necks.
Stabilizing Selection
Stabilizing selection is also referred to as "middle-of-the-road" selection since it favors a non-extreme characteristic over one of the two extreme ones. Plant height is an illustration of this. Short plants may not receive enough sunlight in a population of plants, and towering plants may sustain wind damage. This causes a decrease in the number of very tall and very short plants and an increase in medium-height plants. Stabilizing selection is thought to be the most prevalent method of natural selection because most features do not change significantly over time.
Human birthweight and bird egg production are two other instances of stabilizing selection (clutch size). Human babies' birth weights keep within a range because newborns with extremely low birth weights have a lower chance of surviving and those with extremely high birth weights can cause difficulties during delivery that endanger both the mother and the baby's lives. There is a maximum number of eggs that can be laid by a certain bird species in a clutch. The clutch should contain just the right number of eggs to protect it against disease and/or predators, but not too many that the parent(s) will have trouble feeding.
Selective Disruption
Natural selection of this kind favors extreme features in a population and is bimodal. For instance, some pollinators in a population of plants will visit the tallest plants, while another species will visit the medium-height plants, and a third species will favor the lowest plants. Natural selection would favor tall and short plants, the two extreme phenotypes, and remove the pollinator that favors medium-height plants, causing medium-height plants to become less common.
The size of the beaks of finches on the Galapagos Islands, which Darwin observed, is a more well-known illustration of disruptive selection. Finches with large and small beaks (no finches with medium-sized beaks) were preferred on certain of the islands because the bulk of seeds discovered there was either huge or small.
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