1. Microevolution: Population Genetics and Hardy-Weinberg Equilibrium
Chapter 16
Microevolution: Population Genetics and Hardy-Weinberg Equilibrium
Mrs. Stewart
AP Biology

2. Bell Work
List the evidence (at least 3) that supports the theory of evolution.

3. Standard
CLE Explain how genetic variation in a population and changing environmental conditions are associated with adaptation and the emergence of new species.

4. Objective
List examples of the forces that act upon populations to alter the allele frequencies
Explain how these forces affect allele frequencies in a population
Calculate Hardy-Weinberg problems accurately

5. Variation Within a Population
Populations show variations in their phenotypes:
Different shaped beaks
Different colors
Different athletic abilities
Different immune system responses

6. Sources of Variation Within a Population
Variations in the genotypes of a population arise by:
mutation – changes in genes that occur either naturally or influenced by environment
Passed to offspring if occurs in gametes
Recombination – the law of independent assortment (chromosomes) and crossing over during meiosis
random pairing of gametes (sexual reproduction) – organisms produce large numbers of gametes, so the union of a particular pair is strictly by chance.

7. Population Genetics
The study of evolution from a genetic point of view
Microevolution: change in the allele frequencies of a population

8. Reminder: Phenotypes = physical appearance of traits
Genotypes = Which “variations” (alleles) a person inherited
Genes = the section of DNA that codes for a protein. Expression of these genes means those proteins are produced, which directly leads to the physical appearance of the trait (phenotype)

9. Review: Remember: Genes are alleles R = non-red r = red
Question: What color hair would a Rr person have?

10. Review:
Classify each of the following examples a phenotype or a genotype.
Huntington’s disease
Able to taste PTC (bitter)
Carrier of Cystic Fibrosis

11. Genotype Expression Review:
F = freckles, f = no freckles
If a gene is inherited as complete dominance, and a person has FF, what is the only protein expressed?
If a person has Ff, what protein(s) are expressed?

12. Genotype Expression Review Part II
L = normal lactase; l = abnormal lactase
Why is a person who is Ll still able to break down lactose?

13. Question:
How do we know if a person showing the dominant phenotype has two dominant alleles or if they are heterozygous?

14. The Gene Pool
A collection of all the alleles in a population is called the GENE POOL.
In other words: All of the possible alleles (variations) that are present, for each gene, within a population
Frogs in a pond
Trees in the forest
People in a town

15. Allele Frequency
Allele frequency is the number of times a specific allele occurs in the gene pool
This is in comparison to how often the other alleles occur too

16. Relative Allele Frequencies
determined by dividing the total number of a certain allele by the total number of alleles of all types in the population
Total number of a certain allele___
total number of all alleles in a population
Expressed as a percentage or a decimal.

17. Calculate allele frequency

18. Hardy – Weinberg Allele Frequencies
If all diploid organisms have 2 alleles, then we can calculate their frequencies as such:
p = frequency of dominant allele
q = frequency of recessive allele
The frequency of all alleles in a population will add up to 1
p + q = 1

19. Microevolution is measured by any change in the relative frequency of alleles in a population.
Remember: Populations, not individual organisms, evolve over time.

20. Hardy - Weinberg Genetic mutations Gene flow Genetic drift
Allele frequencies in the gene pool do not change unless acted upon by certain forces.
Genetic mutations
Gene flow
Genetic drift
Nonrandom mating
Natural selection

21. Hardy – Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

22. Five conditions that affect the relative frequency of alleles
This is known as the Hardy Weinberg Genetic Equilibrium model
used to determine and understand the forces that act upon genetic equilibrium

23. 1. Mutations
Mutations are changes in the DNA.

24. 2. Gene Flow The flow of genes between populations
Emigration and immigration cause gene flow between populations and can thus affect gene frequencies.

25. Immigration vs. Emigration
Immigration: Gene flow INTO a population
Emigration: Gene flow OUT of a population

26. Human Evolution from Gene Flow

27. 3. Genetic Drift
Genetic drift is a change in allele frequencies due to random events.
Genetic drift operates most strongly in small populations.

28. Think – pair - share
What are some other random events that could affect allele frequencies in a population?

29. Random Mating
Do humans randomly mate?
No.
Random mating: happens more by chance and not by choice (has less effect on allele frequencies)

30. 4. Nonrandom Mating: Sexual Selection
Mating is nonrandom whenever individuals may choose partners.
Sexual selection occurs when certain traits increase an individual’s success at mating.
Sexual selection affects the allele frequencies of a population.
Courtship ritual

31. 5. Natural Selection
The ongoing process in nature where the presence or absence of certain factors in the environment “select” which traits/variations within a population are most successful
Most traits are polygenic = many variations

32. Without Natural Selection, polygenic traits maintain a bell curve

33. With Natural Selection…
Three general patterns
Stabilizing Selection
favors the formation of average traits.
Disruptive Selection
favors extreme traits rather than average traits.
Directional Selection
favors the formation of one of the extreme traits.

35. Think – Pair – Share
How does evolution by natural selection depend on variation within a population?

36. Bellwork – which natural selection pattern is represented?

37. HW Calculations
If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then…
p2 + 2pq + q2 = 1
p2 = # homozygous dominant individuals
q2 = # homozygous recessive individuals
2pq = # heterozygous individuals

38. Applying the HW principle:
We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that:
The PKU gene mutation rate is low
Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

39. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
The population is large
Migration has no effect as many other populations have similar allele frequencies

40. PKU Problem
If the number of babies born with PKU is 1 out of 10,000, then how many people in the population are heterozygous for this trait?
p + q = 1
p2 + 2pq + q2 = 1

41. What do you know?
The problem gave you a phenotype which we know is the recessive phenotype.
So, 1 in 10,000 = the homozygous recessive = q2

42. Practice: The occurrence of PKU is 1 per 10,000 births q2 = 0.0001
The frequency of normal alleles is
p = 1 – q = 1 – 0.01 = 0.99
The frequency of carriers is
2pq = 2 x 0.99 x 0.01 =
or approximately 2% of the U.S. population

43. Red Hair Practice R = non red, r = red hair
If 2% of humans on the planet have red hair, then what percent of humans are heterozygous for this trait?

44. Red Hair Red hair = q2 q2 = .02 … q = .14
p + q = 1 … p = = .86
2pq = 2 (.86)(.14) = .24 or 24%

45. Practice problems

46. HW Goldfish Lab Each group should obtain the following items:
10 pretzel goldfish
10 cheddar goldfish

47. Activity Discover the allele frequency for brown (pretzel) fish.
Add up the total number of all alleles in your population of fish
Brown fish (pretzel) = BB
Orange fish (regular) = Bb
Yellow fish (parmesan) = bb
Divide the number of “brown” alleles by the total number of alleles.

48. Activity Immigrate: 10 yellow fish move in from a neighboring pond
Emigrate: 5 brown fish move out to another pond
Recalculate the brown allele frequency
Question: How does immigration or emigration affect allele frequencies in a gene pool?

49. Activity Genetic Drift: Perform a random act to your population
Recalculate the brown allele frequency
Question: How does genetic drift affect allele frequencies in a gene pool?

50. Activity
What if every fish in your “pond” only wants to mate with an brown fish?
What would that do to your allele frequencies?