• Natural selection can cause three different forms of microevolution, depending on which phenotypes in a population are favored. • There are three forms of selection: 1. In stabilizing selection, the extremes are selected against, and the average is selected for. 2. In directional selection, one extreme is selected for, and the opposite extreme is selected against. 3. In diversifying selection, the extremes are selected for and the average is selected against. • Sexual selection influences evolution by propagating traits deemed favorable by the opposite sex, not necessarily the environment.

stabilizing selection

- Under stabilizing selection, the extremely large and small galls would be selected against, and the average size gall would be favored. The width of the resulting curve is narrower than the curve of the original population

directional selection

- Under directional selection, an overall shift toward one extreme phenotype occurs as the population shifts away from the opposite extreme. In this example, large galls are selected over small galls. The resulting curve is shifted to the right, relative to the original curve.

diversifying selection

- Under diversifying selection, the extremes are selected for, and the average size is selected against. In this example, large and small galls are favored over medium-size galls. The resulting curve is bimodal, and the highest points occur at either end of the spectrum

- Review: Polymorphism is the existence of two or more inherited forms in a population. - Varieties will be selected for, or against, based on their ability for survival in a particular environment at a given moment. - A case study of the gallfly illustrates three different modes of selection. - The natural history of the gallfly is as follows. The fly lays eggs in the growing tip of a goldenrod plant. The goldenrod produces a gall because of an overproduction of growth hormones. The gall protects the larva during the winter. A number of predators prey upon the larvae of the gallfly, including certain species of wasps, beetles, woodpeckers, and chickadees. - Within the population of gallflies, a continuum in the size of the galls is genetically possible. The bell-shaped curve on the left shows a normal distribution in gall size. The size of the gall is on the x axis, and the frequency of individuals with a given size gall is on the y axis. - Natural selection can favor certain gall sizes over others. Selection will change the bell-shaped curve of the population.

Diversifying selection

- does not favor the average phenotype.

In the phrase “evolution by natural selection”, natural selection refers specifically to changes in allelic frequencies due to

- nonrandom reproductive success

Directional selection

- shifts the allelic frequency in a steady, consistent direction.

10.3.2 Modes of Selection Flashcards by Irina Soloshenko

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10.3.2 Modes of Selection Flashcards

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