1. Population and Evolutionary Genetics
Active Lecture PowerPoint® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino
Chapter 23
Population and Evolutionary Genetics
Copyright © 2010 Pearson Education, Inc.

2. How traits are passed from one generation to the next
The nature of genes
How traits are passed from one generation to the next
Three main branches of Genetics
Genetic compositions of groups/populations - how they change and why…

3. Population Genetics
Study of genetic variation in groups or populations and how it changes over time.
A population is a group of individuals with a common set of genes that lives in same geographic area and interbreeds
We look at populations or groups instead of individuals.

4. 23.1 Genetic variation is present in most populations & species
This variation reflects the variation in the alleles distributed among populations of a species
A population's gene pool is all of the alleles present in that population
Due to population dynamics, the gene pool can change over time
Populations are dynamic. They expand and contract through changes in birth and death rates, migration or contact with new groups. Some individuals in the population will produce more offspring than the others, contributing disproportionate amount of their alleles to the next generation. Thus the dynamics of a population can over time lead to changes in the gene pool.
The primary goal of population genetics is to understand the processes that shape a population’s gene pool.

5. Genetic variation in Lady-bird beetles
There is an enormous reservoir of genetic variability within most populations. At the DNA level most, and perhaps all genes exhibit diversity from individual to individual. Alleles representing these variations are distributed among members of the population.

6. Detecting genetic variation by artificial selection
Dog example
All dogs are descendents from wolves
Dramatic change in phenotype occurred in about 15,000 years.
Broad array of sizes, shapes, colors and behaviors seen in dogs all arose from variation present in wild wolves.
If you looked just at the wild wolf population, you would not think there is such a great genetic variation present, until… they were domesticated and bred through artificial selection

7. Figure 23-1 Alcohol dehydrogenase (Adh) gene in Drosophylla

8. Figure 23-2 DNA sequence variation in Adh gene in Drosophylla
There is an enormous reservoir of genetic variability within most populations. At the DNA level most, and perhaps all genes exhibit diversity from individual to individual. Alleles representing these variations are distributed among members of the population.

9. 23.2 The Hardy–Weinberg Law
Describes what happens to alleles and genotypes in an “ideal” population assuming
No selection
No mutation
No migration
Large population
Random mating
Two scientists, British mathematician Godfrey Hardy & German physician Wilhelm Weinberg developed a mathematical model to explain what happens to genotype and allele frequencies in a population under these conditions.
The law is actually a mathematical model that evaluates the effect of reproduction (segregation of alleles in gamete formation and the combining of alleles in fertilization) on genotype and allele frequencies.

10. The Hardy–Weinberg Laws
Frequency of alleles in the gene pool does not change over time
After one generation of random mating, genotype frequencies for two alleles can be calculated as
p2 + 2pq + q2 = 1
where p = frequency of allele A
and q = frequency of allele a
If these assumptions are met, the law says ……..
This is for a theoretical population. It is rare for a real population to conform to the Hardy-Weinberg model and for all allele and genotype frequencies to remain unchanged at all loci for generation after generation.

11. Allele vs. Genotype Frequency
Alleles
Freq of allele A =0.7 freq of allele a =0.3
Genotypes
AA, Aa, aa
HW law explains the relationship between allele and genotype frequency in a population
Mendels principle of segregation says that each individual organism possesses two alleles at a locus and that each allele has an equal probability of passing into a gamete. Thus the frequency of alleles in gametes will be same as the frequency of alleles in parents.
We can show this in a Punnett square.

12. Allele frequencies after one generation under HW Law
Parental generation
Freq of allele A =0.7 freq of allele a =0.3
How do you calculate allele frequencies after one generation?
Mendels principle of segregation says that each individual organism possesses two alleles at a locus and that each allele has an equal probability of passing into a gamete. Thus the frequency of alleles in gametes will be same as the frequency of alleles in parents.
We can show this in a Punnett square.

13. = 1.0
FIGURE 22-1
Calculating genotype frequencies from allele frequencies. Gametes represent samples drawn from the gene pool to form the genotypes of the next generation. In this population, the frequency of the A allele is 0.7, and the frequency of the a allele is 0.3. The frequencies of the genotypes in the next generation are calculated as 0.49 for AA, 0.42 for Aa, and 0.09 for aa. Under the Hardy-Weinberg law, the frequencies of A and a remain constant from generation to generation.

14. Allele frequencies in the next generation
AA - 49% Aa - 42% aa - 9% Frequency of allele A = = 0.70 Frequency of allele a = = 0.30 Allele frequency has not changed from the previous generation

15. p2 + 2pq + q2 = 1
FIGURE 22-2
The general case of allele and genotype frequencies under Hardy-Weinberg assumptions. The frequency of allele A is p, and the frequency of allele a is q. After mating, the three genotypes AA, Aa, and aa have the frequencies p2, 2pq, and q2, respectively.
Frequencies for multiple alleles can be calculated using the Hardy-Weinberg equation by adding more variables…
For instance, in a situation involving three alleles (p + q + r = 1), the frequencies of the genotypes are given by
(p + q + r)2 = p2 + q2 + r2 + 2pq + 2pr + 2qr = 1.

16. The Hardy–Weinberg Law
The Hardy-Weinberg law allows the frequency of heterozygotes in a population to be estimated
In general, the frequencies of all three genotypes can be estimated once frequency of either allele is known and Hardy-Weinberg assumptions are invoked

17. The Hardy–Weinberg Law
Problem 1:
Human blood types, MM, MN and NN are controlled by two alleles (M & N) of a single gene. In a population of 100 individuals, 49% are of the NN blood type.
What percentage is expected to be MN assuming Hardy-Weinberg equilibrium conditions?
q2=49% or 0.49, q=0.7 p=0.3
Heterozygots =2pq=2x0.7x0.3=0.42 or 42%

18. The Hardy–Weinberg Law
Problem 2:
In a population of 100 that meets the Hardy-Weinberg equilibrium assumptions, 81 individuals are homozygous for a recessive allele.
What percentage of the individuals would be expected to be homozygous for the dominant allele in the next generation?
q2=81% = 0.81, q= 0.9, p=0.1,
Heterozygots are 2pq=2x0.1x0.9=0.18=18% of the population
Homozygous for the dominant allele=p2=0.01=1%
Make sure you get all genotypes added up to 100 (AA=1, Aa=18, aa=81)

19. The Hardy–Weinberg Law Assumes
There is no selection
That no new alleles arise from mutation
That there is no migration into or out of the population
That the population is infinitely large
That random mating occurs
By specifying the conditions under which allele frequencies will not change, the law identifies the forces that can drive evolution in a population.
Change in allele frequency is evolution!

20. Change in allele frequency is evolution!!!!
By specifying the conditions under which allele frequencies will not change, the law identifies the forces that can drive evolution in a population.
Change in allele frequency is evolution!

21. 23.3 The Hardy–Weinberg Law can be applied to human populations
CCR5 gene – Chemokine co-receptor 5
Normal allele –CCR5-1
Δ32 mutation – 32 bp deletion in the gene
In a population of 100 people
Homozygous for CCR5 - 1/1 79
Heterozygous / Δ32 29
Homozygous for Δ32 - Δ32/ Δ32 1
CCR5 Δ32 allele makes one resistant to infection by HIV-1.

22. Forces Driving Allele Frequency Change (p 491)
Natural selection
Mutation
Migration
Genetic drift in small populations
Non-random mating
Natural selection is a major force

23. Natural Selection
Natural selection takes place when individuals with adaptive trait produce a greater number of offspring than that produced by others in a population
In the next generation, the offspring with the adaptive trait appear in greater frequency
What are some examples?
Skin color in polar bears
Bacterial resistance to antibiotics. Natural selection of Staphylococcus aureus.
Type II Diabetes genes
Mosquitos resistant to insecticides
HIV virions resistant to AZT

24. Natural Selection
If individuals are subject to natural selection and do not have equal rates of survival and reproductive success, allele frequencies may change from one generation to the next
Natural selection is the principal force that shifts allele frequencies within large populations

25. Population with varied inherited traits1
Population with varied inherited traits
This figure is not from the Genetics textbook.
An example of natural selection in action. 2
Elimination of individuals with certain traits 3
Reproduction of survivors

26. FIGURE 22-8 part 1
Change in the frequency of a lethal recessive allele, a. The frequency of a is halved in two generations, and halved again by the sixth generation. Subsequent reductions occur slowly because the majority of a alleles are carried by heterozygotes.

27. The frequency of the Acetylcholine esterase, AceR allele, which confers resistance to the insecticide chlorpyrifos, is usually higher in house mosquito populations exposed to chlorpyrifos.
The frequency of an allele for an enzyme unrelated to chlorpyrifos metabolism (aspartate amino transferase 1) shows no such pattern.
FIGURE 22-12
The effect of selection on allele frequencies in natural populations. (a) The frequency of the AceR allele, which confers resistance to the insecticide chlorpyrifos, is usually higher in house mosquito populations exposed to chlorpyrifos. (b) The frequency of an allele for an enzyme unrelated to chlorpyrifos metabolism (aspartate amino transferase 1) shows no such pattern.

28. acts on polygenic traits too
Natural selection
acts on polygenic traits too
FIGURE 22-13
The impact of directional, stabilizing, and disruptive selection. In each case, the mean of an original population (green) and the mean of the population following selection (red) are shown.

29. Mutation
Mutation is the only process that creates new alleles in a gene pool
Mutation creates new alleles on which natural selection acts upon
Change in allele frequency due to mutation depends on the fitness they confer and the action of natural selection
Independent assortment and recombination produce new combinations, but not new alleles.
Mutation creates new alleles randomly. Natural selection works on these mutations.
If the mutation rate is known, the extent to which mutation can cause allele frequencies to change from one generation to the next can be estimated.

30. FIGURE 22-16
Replacement rate of an allele by mutation alone, assuming an average mutation rate of 1.0 x 10-5.
In general, although mutation provides the raw material for evolution, mutation by itself plays a relatively insignificant role in changing allele frequencies

31. Migration or Gene Flow
Migration occurs when individuals move between the populations
Migration may have a large effect on allele frequency if:
the rate of migration is large or
if the allele frequency of the migrant population differs greatly from that of the population to which it is moving
When a species divides into populations that are separated geographically, the allele frequencies in these new populations may differ over time due to migration.
Migration tends to keep populations homogenious in their allele frequencies.
Migration adds genetic variation to the population

32. Migration – B allele of ABO locus
FIGURE 22-17
Migration as a force in evolution. The B allele of the ABO locus is present in a gradient from east to west. This allele shows the highest frequency in Central Asia, and the lowest is in northeastern Spain. The gradient parallels the waves of Mongol migration into Europe following the fall of the Roman Empire and is a genetic relic of human history.

33. Genetic Drift
Genetic drift occurs when # of reproducing individuals in a population is too small to ensure all alleles in the gene pool will be passed on to next generation in their existing frequencies
Genetic drift may result in one allele becoming fixed and one allele disappearing in a population
Genetic Drift Causes Random Changes in Allele Frequency in Small Populations.
Only when populations are large will the gametes carry genes that represent the the parental gene pool.
In small populations, significant fluctuations in allele frequency is possible by chance deviation. Smaller the sample, greater the deviation.

34. Nonrandom Mating
Nonrandom mating can change the genotype frequency in a population
But not the allele frequency
The fifth assumption is that members of a population mate at random.
Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency
For a given allele, inbreeding increases the proportion of homozygotes in the population, and a completely inbred population theoretically will consist only of homozygotes.

35. Forms of Nonrandom Mating
Positive assortive mating in which similar genotypes are more likely to mate than dissimilar ones
Negative assortive mating in which dissimilar genotypes are more likely to mate than similar ones
Inbreeding in which mating individuals are related
The fifth assumption is that members of a population mate at random.
Nonrandom Mating Changes Genotype Frequency but Not Allele Frequency
For a given allele, inbreeding increases the proportion of homozygotes in the population, and a completely inbred population theoretically will consist only of homozygotes.

36. Inbreeding
One consequence of inbreeding is an increased chance that an individual will be homozygous for a recessive deleterious allele
The significance of this fact is that inbred populations often have a lowered mean fitness, called inbreeding depression
Mating between related individuals, first cousins increases the risk of spontaneous abortions, neonatal deaths, congenital deformities and recessive genetic disorders
If members of two inbred lines are mated, the offspring often display hybrid vigor. Hybrid vigor is highest in the F1 generation and generally declines thereafter.
Inbreeding may increase the efficiency with which selection removes a deleterious allele from a population.

37. Inbreeding in plants Self-fertilization is a form of inbreeding
Continuous self-fertilization can lead to fully homozygous plants called inbred lines
If members of two inbred lines are mated, the offspring often display hybrid vigor.
Hybrid vigor is highest in the F1 generation and generally declines thereafter.
Mating between related individuals, first cousins increases the risk of spontaneous abortions, neonatal deaths, congenital deformities and recessive genetic disorders
If members of two inbred lines are mated, the offspring often display hybrid vigor. Hybrid vigor is highest in the F1 generation and generally declines thereafter.
Inbreeding may increase the efficiency with which selection removes a deleterious allele from a population.

38. Self-fertilization is a form of inbreeding common in plants.
The rate of homozygotes in a self-fertilizing population rapidly increases over a few generations, but the overall allele frequency does not change
FIGURE 22-22
Reduction in heterozygote frequency brought about by self-fertilization. After n generations, the frequencies of the genotypes can be calculated according to the formulas in the bottom row.

39. Inbreeding: First-cousin marriage
A coefficient of inbreeding can be calculated to give the probability that two alleles of a given gene in an individual are identical because they are descended from the same single copy of the allele in an ancestor
FIGURE 22-23
Calculating the coefficient of inbreeding (F) for the offspring of a first-cousin marriage.

40. TABLE 22.5
Mortality in Offspring of Inbred Zoo Animals