IMPORTANT TERMS Natural Selection some organisms are better

IMPORTANT TERMS Natural Selection- some organisms are better suited to survive and reproduce in an environment. “ Survival of the Most Fit”

Natural selection on single gene traits can lead to changes in allele frequency. (Evolution)

Polygenic trait – two or more genes control a trait Many different phenotypes are possible Ex. Height

There are three ways Natural Selection can affect the distribution of phenotypes: Frequency of Phenotype Normal Bell-shaped curve Phenotype (height)

1. Directional Selection Individuals on one side of the bell curve become more fit than the others

Directional Selection Low mortality, high fitness Food becomes scarce. High mortality, low fitness

2. Stabilizing Selection Individuals at the center of the bell curve become more fit than the others

2. Stabilizing Selection Low mortality, high fitness High mortality, low fitness Example: Human Babies Low birth weight and high birth weight babies don’t survive as well

3. Disruptive Selection Individuals at the ends of the curve become more fit than those in the middle.

3. Disruptive Selection High mortality, low fitness Number of Birds in Population Low mortality, high fitness Number of Birds in Population Largest and smallest seeds become more common. Beak Size

Genetic Drift Random change in Allele Frequency.

Genetic Drift a. Small Population

Genetic Drift a. Small Population b. Some individuals may have more descendants by chance.

Genetic Drift a. Small Population b. Some individuals may have more descendants by chance. c. Over time, an allele may become more common by chance alone

Genetic Drift Sample of Original Population

Genetic Drift Sample of Original Population Founding Population A Founding Population B

Genetic Drift Sample of Original Population Descendants Founding Population A Founding Population B

Hardy-Weinberg Principle • What happens when there is no change in a population?

Hardy-Weinberg Principle • What happens when there is no change in a population? • No change in allele frequency = genetic equilibrium

Hardy-Weinberg Principle • What happens when there is no change in a population? • No change in allele frequency = genetic equilibrium • 5 conditions must exist

Hardy-Weinberg Principle 1. Random mating 2. Large Population 3. No movement in or out

Hardy-Weinberg Principle 4. No mutations 5. No natural selection

Hardy-Weinberg Principle 4. No mutations 5. No natural selection Can this ever happen?