1. Ch. 22/23 Warm-up List 5 different pieces of evidence for evolution.
(Review) What are the 3 ways that sexual reproduction produces genetic diversity?
What is 1 thing you are grateful for today?

2. Ch. 23 Warm-up
In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb).
The allele frequencies of this population are:
B allele: ___ b allele: ___
The genotype frequencies are:
BB: ___ Bb: ___ bb: ___
Use the above info to determine the genotype frequencies of the next generation:
B (p): ___ b (q): ___
BB (p2): ___ Bb (2pq): ___
bb (q2): ___

3. The Evolution of Populations
Chapter 23

4. What you must know:
How mutation and sexual reproduction each produce genetic variation.
The conditions for Hardy-Weinberg equilibrium.
How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving.

5. Smallest unit of evolution
Microevolution: change in the allele frequencies of a population over generations

6. Darwin did not know how organisms passed traits to offspring
Mendel published his paper on genetics
Mendelian genetics supports Darwin’s theory  Evolution is based on genetic variation

7. Sources of Genetic Variation
Point mutations: changes in one base (eg. sickle cell)
Chromosomal mutations: delete, duplicate, disrupt, rearrange  usually harmful
Sexual recombination: contributes to most of genetic variation in a population
Crossing Over (Meiosis – Prophase I)
Independent Assortment of Chromosomes (during meiosis)
Random Fertilization (sperm + egg)

8. Population genetics: study of how populations change genetically over time
Population: group of individuals that live in the same area and interbreed, producing fertile offspring

9. Gene pool: all of the alleles for all genes in all the members of the population
Diploid species: 2 alleles for a gene (homozygous/heterozygous)
Fixed allele: all members of a population only have 1 allele for a particular trait
The more fixed alleles a population has, the LOWER the species’ diversity

10. Populations & gene pools
Concepts
a population is a localized group of interbreeding individuals
gene pool is collection of alleles in the population
remember difference between alleles & genes!
allele frequency is how common is that allele in the population
how many A vs. a in whole population

11. Evolution of populations
Evolution = change in allele frequencies in a population
hypothetical: what conditions would cause allele frequencies to not change?
non-evolving population
REMOVE all agents of evolutionary change
very large population size (no genetic drift)
no migration (no gene flow in or out)
no mutation (no genetic change)
random mating (no sexual selection)
no natural selection (everyone is equally fit)

12. Hardy-Weinberg Principle
Hardy-Weinberg Principle: The allele and genotype frequencies of a population will remain constant from generation to generation …UNLESS they are acted upon by forces other than Mendelian segregation and recombination of alleles Equilibrium = allele and genotype frequencies remain constant

13. Hardy-Weinberg equilibrium
Hypothetical, non-evolving population
preserves allele frequencies
Serves as a model (null hypothesis)
natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on a population
measuring evolutionary change
G.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?"
G.H. Hardy
mathematician
W. Weinberg
physician

14. Conditions for Hardy-Weinberg equilibrium
No mutations.
Random mating.
No natural selection.
Extremely large population size.
No gene flow.
If at least one of these conditions is NOT met, then the population is EVOLVING!

15. Hardy-Weinberg Principle
Allele Frequencies:
Gene with 2 alleles : p, q
p = frequency of dominant allele (A) q = frequency of recessive allele (a)
Note:
1 – p = q
1 – q = p
p + q = 1

16. p2 + 2pq + q2 = 1 Hardy-Weinberg Equation Genotypic Frequencies:
3 genotypes (AA, Aa, aa)
p2 = AA (homozygous dominant)
2pq = Aa (heterozygous)
q2 = aa (homozygous recessive)
p2 + 2pq + q2 = 1

17. Hardy-Weinberg theorem
Counting Alleles
assume 2 alleles = B, b
frequency of dominant allele (B) = p
frequency of recessive allele (b) = q
frequencies must add to 1 (100%), so:
p + q = 1 BB Bb bb

18. Hardy-Weinberg theorem
Counting Individuals
frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1 BB Bb bb

19. H-W formulas B b BB Bb bb BB Bb bb Alleles: p + q = 1
Individuals: p2 + 2pq + q2 = 1 B b BB Bb bb BB Bb bb

20. Using Hardy-Weinberg equation
population: 100 cats
84 black, 16 white
How many of each genotype?
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): = 0.6
p2=.36
2pq=.48
q2=.16 BB Bb bb
Must assume population is in H-W equilibrium!
What are the genotype frequencies?

21. Using Hardy-Weinberg equation
p2=.36
2pq=.48
q2=.16
Assuming H-W equilibrium BB Bb bb
Null hypothesis
p2=.20
p2=.74
2pq=.10
2pq=.64
q2=.16
q2=.16
Sampled data 1:
Hybrids are in some way weaker.
Immigration in from an external population that is predomiantly homozygous B
Non-random mating... white cats tend to mate with white cats and black cats tend to mate with black cats.
Sampled data 2:
Heterozygote advantage.
What’s preventing this population from being in equilibrium. bb Bb BB
Sampled data
How do you explain the data?
How do you explain the data?

22. Allele frequencies

23. Genotypic frequencies

24. Strategies for solving H-W Problems:
If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles.
If you know phenotypes, then use “aa” to find q2, and then q. (p = 1-q)
Use p2 + 2pq + q2 to find genotype frequencies.
If p and q are not constant from generation to generation, then the POPULATION IS EVOLVING!

25. Hardy-weinberg practice problem #1
The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths.
AA = Aa = 138 aa = 5
Allele Frequencies:
A = a =
Genotypic Frequencies:
AA =
Aa =
aa =

26. Hardy-weinberg practice problem #2: PTC Tasters
Taster = AA or Aa Nontaster = aa
Tasters = ____ Nontasters = ___
q2 = q =
p + q = 1 p = 1 – q =
p2 + 2pq + q2 = 1

27. Causes of evolution

28. Conditions for Hardy-Weinberg equilibrium
No mutations.
Random mating.
No natural selection.
Extremely large population size.
No gene flow.
If at least one of these conditions is NOT met, then the population is EVOLVING!

29. Minor Causes of Evolution:
#1 - Mutations
Rare, very small changes in allele frequencies
#2 - Nonrandom mating
Affect genotypes, but not allele frequencies
Major Causes of Evolution:
Natural selection, genetic drift, gene flow (#3-5)

30. 1. Mutations
Mutations can change the frequency of alleles in a population, but this is a very slow effect in humans. Bacteria - this is fast!
Mutation is one of the sources of genetic
variation that leads to natural selection.
Humans – 1/million
Because microorganisms have very short generation times, mutation generates genetic variation rapidly.
In an AIDS patient, HIV generates 1010 new viruses per day.
With its RNA genome, mutation rate is higher than DNA genomes.
This combination of mutation and replication rate will generate mutations in the HIV population at every site in the HIV genome every day.
In the face of this high mutation rate, single-drug treatments are unlikely to be effective for very long and the most effective treatments are multiple drug “cocktails.”
It is far less probable that mutations against all the drugs will appear in individual viruses in a short time.
Chromosomal mutations, including rearrangements of chromosomes, affect many genes and are likely to disrupt proper development of an organism.
However, occasionally, these dislocations link genes together such that the phenotype is improved.
Duplications of chromosome segments, whole chromosomes, or sets of chromosomes are nearly always harmful.
However, when they are not harmful, the duplicates provide an expanded genome.
These extra genes can now mutate to take on new functions.
Most point mutations, those affecting a single base of DNA, are probably harmless.
Most eukaryotic DNA does not code for proteins and mutations in these areas are likely to have little impact on phenotype.
Even mutations in genes that code for proteins may lead to little effect because of redundancy in the genetic code.
However, some single point mutations can have a significant impact on phenotype.
Sickle-cell disease is caused by a single point mutation.

31. 2. Nonrandom Mating Inbreeding and assortive mating cause
an increase in homozygotes.
Allele frequencies will not change, but
genotype frequencies will.

32. Nonrandom Mating M/M M/N N/N M/M M/N N/N 0.835 0.156 0.009 0.150 0.700
Eskimo
Egyptian
0.278
0.489
0.233
0.332
0.486
0.182
Chinese
The Hardy-Weinberg equilibrium was derived on the assumption of "random mating," but we must carefully distinguish two meanings of that process. First, we may mean that individuals do not choose their mates on the basis of some heritable character. Human beings are random mating with respect to blood groups in this first sense, because they generally do not know the blood type of their prospective mates, and, even if they did, it is unlikely that blood type would be used as a criterion for choice. In the first sense, random mating will occur with respect to genes that have no effect on appearance, behavior, smell, or other characteristics that directly influence mate choice.
The second sense of random mating is relevant when there is any division of a species into subgroups. If there is genetic differentiation between subgroups so that the frequencies of alleles differ from group to group and if individuals tend to mate within their own subgroup (endogamy), then, with respect to the species as a whole, mating is not at random and frequencies of genotypes will depart more or less from Hardy-Weinberg frequencies. In this sense, human beings are not random mating, because ethnic and racial groups differ from one another in gene frequencies and people show high rates of endogamy not only within major races, but also within local ethnic groups. Spaniards and Russians differ in their ABO blood group frequencies, Spaniards marry Spaniards and Russians marry Russians, so there is unintentional nonrandom mating with respect to ABO blood groups. Table 24-8 shows random mating in the first sense and nonrandom mating in the second sense for the MN blood group. Within Eskimo, Egyptian, Chinese, and Australian subpopulations, females do not choose their mates by MN type, and, thus, Hardy-Weinberg equilibrium exists within the subpopulations. But Egyptians do not mate with Eskimos or Australian aborigines, so the nonrandom associations in the human species as a whole result in large differences in genotype frequencies and departure from Hardy-Weinberg equilibrium.
Australian
.024
0.304
0.672
OBSERVED
EXPECTED from Hardy-Weinberg Theorem

33. Major Causes of Evolution
#3 – Natural Selection
Individuals with variations better suited to environment pass more alleles to next generation

34. Major Causes of Evolution
#4 – Genetic Drift
Small populations have greater chance of fluctuations in allele frequencies from one generation to another
Examples:
Founder Effect
Bottleneck Effect

35. 4. Genetic Drift = change in allele frequency due to CHANCE
4. Genetic Drift = change in allele frequency due to CHANCE. Ex: Billy goat determines which plants survives by randomly chewing off some flowers. So the allele frequency may be not 0.5 R and 0.5r in each generation.
2 types of drift….

36. Genetic Drift

37. The Bottleneck Effect (skewed representation of alleles after disasters) can lead to genetic drift. ‘Bottle neck’ is the disaster! Alleles left after disaster may not be 0.5R and 0.5r….Ex: cheetahs and hunting
Cheetahs in Africa – hunted, disease = very small population (only 3 small populations in the wild) = lab mice in genetic uniformity

38. Northern elephant seals hunted nearly to extinction in California
Bottleneck Effect
Sudden change in environment drastically reduces population size
Northern elephant seals hunted nearly to extinction in California

39. Polydactyly in Amish population
Founder Effect
A few individuals isolated from larger population
Certain alleles under/over represented
Polydactyly in Amish population

40. The Founder Effect—a small number of
individuals colonize a new, isolated area—
this can lead to genetic drift. Ex: eye disease alleles have a high frequency in the founders of a colony
                                                                      <>
Tristan da Cunha – colonized by 15 people!

41. Major Causes of Evolution
#5 – Gene Flow
Movement of fertile individuals between populations
Gain/lose alleles
Reduce genetic differences between populations

42. 5.Gene Flow - Migration If populations aren’t completely
isolated, individuals can migrate and introduce alleles into another population. Ex: wind… pollinators…
This may cause a change in allele
frequency in the next generation.

43. How does natural selection bring about adaptive evolution?

44. Fitness : the contribution an individual makes to the gene pool of the next generation
Natural selection can alter frequency distribution of heritable traits in 3 ways:
Directional selection
Disruptive (diversifying) selection
Stabilizing selection

45. 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

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

47. Directional Selection: eg
Directional Selection: eg. larger black bears survive extreme cold better than small ones
Disruptive Selection: eg. small beaks for small seeds; large beaks for large seeds
Stabilizing Selection: eg. narrow range of human birth weight

48. Sexual selection
Form of natural selection – certain individuals more likely to obtain mates
Sexual dimorphism: difference between 2 sexes
Size, color, ornamentation, behavior

49. Sexual selection
Intrasexual – selection within same sex (eg. M compete with other M)
Intersexual – mate choice (eg. F choose showy M)

50. Preserving genetic variation
Diploidy: hide recessive alleles that are less favorable
Heterozygote advantage: greater fitness than homozygotes
eg. Sickle cell disease

51. 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

52. Sickle Cell and Malaria

54. HHMI Video: Natural Selection in Humans
Running Time: 14:03 min

55. Natural selection cannot fashion perfect organisms.
Selection can act only on existing variations.
Evolution is limited by historical constraints.
Adaptations are often compromises.
Chance, natural selection, and the environment interact.

56. Sample Problem
Define the following examples as directional, disruptive, or stabilizing selection:
Tiger cubs usually weigh 2-3 lbs. at birth
Butterflies in 2 different colors each represent a species distasteful to birds
Brightly colored birds mate more frequently than drab birds of same species
Fossil evidence of horse size increasing over time