Ch. 23 (cont): The Evolution of Populations

Genetic Drift- Chance events that cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations.

-Two examples that can shrink populations downt to a small zie is the are known as the bottleneck effect and the founder effect.
1)The Bottleneck Effect- a sudden change in the environment (natural disaster) that may drastically reduce the size of the population. By chance alone certain alleles may establish a new population whose gene pool differs from the source population. (ex. heterozygote advantage)

2)The Founder Effect- a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population.

Effects of Genetic Drift:

1. Genetic drift is significant in small populations

2. Genetic drift can cause allele frequences to change at random

3. Genetic Drift can lead to a loss of genetic cariation within populations

4. Genetic drift can cause harmful alleles to become fixed

Natural Selection- differential success in survival and reproduction. It is the only mechanism that consistently causes adaptive evolution.

-Natural selection can affect the frequency of a heritable trait in a population in three different ways, depending on which phenotypes in a varying population are favored. These three trends are directional, diversifying, and stabalizing selection.

• Directional Selection- occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, there by shifting the frequency curve for the phenotypic character in one direction or another. (extreme)
• Disruptive Selection- occurs when conditions favor individuals at both extremes of a phenotype range over individuals with intermediate phenotypes. (bimodial)
• Stabalizing Selection- acts against both extreme phenotypes and favors intermediate variants. (intermediate)

Sexual Dimorphism- is the systematic difference in form between individuals of different sex in the same species. (ex. courship displayed in fights, color, size, body parts)