1. Modes of Natural Selection

2. Modes of Natural Selection
Directional Selection
Favors individuals at one end of the phenotypic range
Most common during times of environmental change or when moving to new habitats
Disruptive selection
Favors extreme over intermediate phenotypes
Occurs when environmental change favors an extreme phenotype

3. Directional Selection

4. Disruptive Selection

5. Modes of Natural Selection
Stabilizing Selection
Favors intermediate over extreme phenotypes
Reduces variation and maintains the current average
Example: Human birth weight

7. Variations in Populations

8. Geographic Variations
Variation in a species due to climate or another geographical condition
Populations live in different locations
Example: Finches of Galapagos Islands & South America

10. Heterozygote Advantage
Favors heterozygotes (Aa)
Maintains both alleles (A,a) instead of removing less successful alleles from a population
Sickle cell anemia
> Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age.
> Heterozygotes are less susceptible to malaria

11. Sickle Cell and Malaria

13. Other Sources of Variation
Mutations
In stable environments, mutations often result in little or no benefit to an organism, or are often harmful
Mutations are more beneficial (rare) in changing environments  (Example:  HIV resistance to antiviral drugs)
Genetic Recombination
source of most genetic differences between individuals in a population
Co-evolution
-Often occurs between parasite & host and flowers & their pollinators

14. Coevolution

15. Hardy-Weinberg Principle

16. The Hardy-Weinberg Principle
Used to describe a non-evolving population.
Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool. 
 Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

17. The Hardy-Weinberg Principle
Deviation from Hardy-Weinberg equilibrium usually results in evolution
Understanding a non-evolving population, helps us to understand how evolution occurs
                        .

18. 5 Assumptions of the H-W Principle
Large population size - small populations have fluctuations in allele frequencies (e.g., fire, storm).
No migration - immigrants can change the frequency of an allele by bringing in new alleles to a population.
No net mutations - if alleles change from one to another, this will change the frequency of those alleles

19. 5 Assumptions of the H-W Principle
Random mating - if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles.
No natural selection - if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

20. Traits Selected for Random Mating

21. The Hardy-Weinberg Principle
The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change)
The Hardy-Weinberg Equation: 
               1.0 = p2 + 2pq + q2
 Where:
p2 = frequency of AA genotype
2pq = frequency of Aa
q2 = frequency of aa genotype

22. The Hardy-Weinberg Principle
Determining the Allele Frequency using Hardy-Weinberg: 
              
1.0 = p + q
 Where:
p = frequency of A allele
q = frequency of a allele

23. Allele Frequencies Define Gene Pools
500 flowering plants
480 red flowers
20 white flowers
320 RR
160 Rr
20 rr
As there are 1000 copies of the genes for color,
the allele frequencies are (in both males and females):
320 x 2 (RR) x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R
160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r