1. Evolution of Populations
18.1 , 18.2 and 18.3

2. 5/24 Objective: Explain how populations evolve
DMA: What is the term used for measuring an organisms ability to survive and reproduce in its environment?

3. The Evolution of Populations

4. Genetic variation in a population increases the chance that some individuals will survive.
Gene pool – the combined alleles of all of the individuals in a population.
Different combinations of alleles are made when individuals mate and have offspring.
Allele Frequency – a measure of how common a certain allele is in the population.

5. Calculating Allele Frequency

6. Genetic variation comes from 2 main sources:
Recombination
Mutation
Random change in the DNA that can be passed to the next generation
New allele combinations form during sexual reproduction.

7. Question: If a certain trait’s allele frequency is 100%, describe the genetic variation for that trait in the population.

9. Natural selection acts on distributions of traits.
Normal distribution: the frequency is highest near the mean value and decreases toward each extreme end of the range.

10. Natural selection can change the distribution of a trait in one of 3 ways:
1. Directional Selection 2. Stabilizing Selection 3. Disruptive Selection

11. Directional Selection
Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

12. Stabilizing Selection
Individuals near the center of the curve have higher fitness than individuals at either end.

13. Disruptive Selection
Phenotypes at both the upper and lower ends of the curve have higher fitness than individuals near the middle.

14. Other factors that lead to the evolution of a population….
Gene Flow
This is the movement of alleles from one population to another.

15. Video Explaining Genetic Drift

16. Genetic Drift
Due to chance, some allele frequencies will decrease or become eliminated.
Happens in small populations.
Ex: The last green-eyed person in a small town dies, leaving only brown-eyed and blue-eyed people.

17. Genetic Drift is commonly caused by ….
Bottleneck Effect- an event greatly reduces the size of a population.

18. Founder Effect
Allele frequencies change as a result of the migration of a small subgroup of a population

19. 2. Why is genetic drift more likely to occur in smaller populations?
Question:
2. Why is genetic drift more likely to occur in smaller populations?

20. Sexual Selection- occurs when certain traits increase mating success.

21. Sexual Selection videos