Mechanisms of Evolution Notes

1. Gene Pool

2. Directional Selection

Example: there was a selection pressure against short giraffe necks, since giraffes with short necks could not reach as many leaves to feed. As a result, the distribution of neck length shifted to favor individuals with long necks

3. Stabilizing Selection

EX. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease.

4. Disruptive Selection

Example: imagine a plant that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would lean toward both short and tall, but not medium height plants

5. Genetic Drift

6. Bottleneck Affect

7. Founders Affect Example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency.

8. Gene Flow

9. Speciation

10. Reproductive Isolation

11. Behavioral Isolation

12. Geographical Isolation

13. Temporal Isolation

Closely related species fireflies mate at different times of night

Plants frequently have differing flowering seasons

14. Mimicry

15. Camouflage

16. Resistance