1. Evolution in Populations
Single Gene Traits
Can lead to a shift in allele frequencies.
This genetic selection can then lead to the evolution or a change in the species.
Polygenic Traits
Can lead to a shift in allele frequencies.
Changes that can alter the phenotypic make-up of a species.
Directional Selection
Stabilizing Selection
Disruptive Selection

2. Evolution in Populations Single Traits

3. Evolution in Populations Polygenic Traits

4. Evolution in Populations Polygenic Traits

5. Evolution Caused by a Genetic Change
Genetic Drift
A shift in allele frequencies in a population due to chance.
Bottlenecks
An abrupt and severe reduction in the number of individuals during the history of a species, resulting in the loss of diversity from the gene pool. The generations following the bottleneck are more genetically homogenous than would otherwise be expected. Bottlenecks often occur in consequence of a catastrophic event.
The American Heritage® Science Dictionary

6. Genetic Drift

7. Founders Effect
The frequency of the “a” allele is low in the initial population, but a small subset, in which one individual is Aa, is removed from the large population and founds a new population. The frequency of “a” is markedly higher in this new population, due to its relatively high frequency in the founders.

8. Temporal Isolation Breed at differing times of the year.
Western Spotted Skunk (Spilogale gracilis)
Breeds in late summer
Eastern Spotted Skunk (Spilogale putorius)
Breeds in late winter

9. Behavioral Isolation
Two species that are capable of interbreeding, but don’t because of courtship rituals or other similar behaviors.

10. Mechanical Isolation
When structural differences prevent successful mating. For example, in many closely related species of plants, there are certain differences, such as differences in color, that help attract different kinds of pollinators.

11. Gamete Isolation (Lock and Key Isolation)
Fertilization cannot take place if the gametes are not compatible.

12. Geographical Isolation
Individuals of a species that are physically separated by a barrier may form subspecies such as these spotted owls.