KEY CONCEPT Populations, not individuals, evolve.

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 SO…EXTREME VALUES LIE TOWARD THE BOTTOM OF EACH SIDE OF THE TOP-MIDDLE MEAN/AVERAGE VALUES Traits not undergoing natural selection have a normal distribution.

Microevolution is evolution within a population from generation to generation observable change in the allele frequencies can result from natural selection

1) Directional Selection 2) Stabilizing Selection 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

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

2) Stabilizing selection favors the intermediate phenotype leading to a decline of extreme phenotypes and possibly extinction.

3) Disruptive selection favors both extreme phenotypes. Favoring both extreme phenotypes can lead to development of new species thru speciation