11 2 Natural Selection in Populations KEY CONCEPT

11. 2 Natural Selection in Populations KEY CONCEPT Populations, not individuals, evolve.

11. 2 Natural Selection in Populations Natural selection acts on distributions of traits. • A normal distribution graphs as a bell-shaped curve. – highest frequency near mean value (MEAN VALUE IS THE AVERAGE VALUE) – frequencies decrease toward each extreme value v SO…EXTREME VALUES LIE TOWARD THE BOTTOM OF EACH SIDE OF THE TOP • Traits not undergoing MIDDLE MEAN/AVERAGE natural selection have a VALUES normal distribution.

11. 2 Natural Selection in Populations • Microevolution is evolution within a population from generation to generation – observable change in the allele frequencies – can result from natural selection

11. 2 Natural Selection in Populations Natural selection can change the distribution of a trait in one of three ways. 1) Directional Selection 2) Stabilizing Selection 3) Disruptive Selection These types of selection shifts cause a certain phenotype trait to become more common in the population

11. 2 Natural Selection in Populations 1) Directional selection favors phenotypes at one extreme of a trait’s range causing it to shift in that direction.

11. 2 Natural Selection in Populations 2) Stabilizing selection favors the intermediate phenotype leading to a decline of extreme phenotypes and possibly extinction.

11. 2 Natural Selection in Populations 3) Disruptive selection favors both extreme phenotypes. • Favoring both extreme phenotypes can lead to development of new species thru speciation