1. Types of Natural Selection

2. Natural Selection and Genetic Equilibrium
Synthetic Theory of Evolution: Darwin’s ideas about natural selection plus our modern understanding of genetics.
According to the Synthetic Theory of Evolution, natural selection disturbs genetic equilibrium.
As a result, allele frequencies will change.
In this way, natural selection determines which adaptations are beneficial to a species.
There are three main types of natural selection

3. Directional Selection
Directional Selection: An extreme phenotype (very long or very short etc.) is a favorable adaptation.
Ex. Neck length in giraffes, very long necks are favorable.
Selects in the direction of one extreme phenotype

5. Stabilizing Selection
Stabilizing Selection: Average phenotype is a favorable adaptation. Extreme phenotypes are unfavorable.
Operates most of the time in populations
Ex. Mouse size: too big use too much energy to keep warm; too small can’t burrow. Average is just right
Limits evolution by keeping allele frequencies relatively constant

6. Disruptive Selection
Disruptive Selection: Most rare type of natural selection. Two opposite extreme phenotypes are favorable over average phenotype
Ex. Some crabs show a continuous range of color from light tan to dark brown. The light tan provides camouflage in the sand. Dark brown provide camouflage in mud. Colors in between – no camouflage
Over time this could lead to speciation

8. Summary