1. GENETICS & EVOLUTION: population genetics
Chapter 3.3

2. Overview Microevolution and mutation
Hardy-Weinberg Principle and Equilibrium
Natural Selection
Genetic drift
Gene flow

3. Overview: The Smallest Unit of Evolution
One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but only populations evolve
Genetic variations in populations contribute to evolution
Microevolution is a change in allele frequencies in a population over generations

4. Fig. 23-1
Figure 23.1 Is this finch evolving by natural selection?

5. Mutation and sexual reproduction produce the genetic variation that makes evolution possible
Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

6. Gene Pools and Allele Frequencies
The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population
A population is a localized group of individuals capable of interbreeding and producing fertile offspring
A gene pool consists of all the alleles for all loci in a population
A locus is fixed if all individuals in a population are homozygous for the same allele

7. Porcupine herd Porcupine herd range Fortymile herd range
Fig. 23-5
Porcupine herd
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Fortymile
herd range
Figure 23.5 One species, two populations
ALASKA
YUKON
Fortymile herd

8. Porcupine herd range Fortymile herd range Beaufort Sea NORTHWEST
Fig. 23-5a
MAP
AREA
CANADA
ALASKA
Beaufort Sea
NORTHWEST
TERRITORIES
Porcupine
herd range
Figure 23.5 One species, two populations
Fortymile
herd range
ALASKA
YUKON

9. The frequency of an allele in a population can be calculated
For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2
The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles
By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies
The frequency of all alleles in a population will add up to 1
For example, p + q = 1

10. The 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

11. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population

12. Alleles in the population Frequencies of alleles
Fig. 23-6
Alleles in the population
Frequencies of alleles
Gametes produced
p = frequency of
Each egg:
Each sperm:
CR allele = 0.8
q = frequency of
80%
chance
20%
chance
80%
chance
20%
chance
CW allele = 0.2
Figure 23.6 Selecting alleles at random from a gene pool

13. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
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
where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

14. 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm (80%) CR Eggs CW (20%) CR CW
Fig
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%)
(80%) CR
Eggs
64% ( p2)
CRCR
16% ( pq)
CRCW
Figure 23.7 The Hardy-Weinberg principle
4% (q2)
CW CW
16% (qp)
CRCW CW
(20%)

15. Gametes of this generation:
Fig
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR   +    16% CR    =   80% CR = 0.8 = p
4% CW     +    16% CW   =   20% CW = 0.2 = q
Figure 23.7 The Hardy-Weinberg principle

16. Gametes of this generation:
Fig
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR   +    16% CR    =   80% CR = 0.8 = p
4% CW     +    16% CW   =   20% CW = 0.2 = q
Genotypes in the next generation:
Figure 23.7 The Hardy-Weinberg principle
64% CRCR, 32% CRCW, and 4% CWCW plants

17. Figure 23.7 The Hardy-Weinberg principle
80% CR ( p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) CR
(80%)
Eggs
64% ( p2)
CR CR
16% ( pq)
CR CW
4% (q2)
CW CW CW
(20%)
16% (qp)
CR CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
Figure 23.7 The Hardy-Weinberg principle
64% CR    +     16% CR    =   80% CR  = 0.8 = p
4% CW      +    16% CW    =  20% CW = 0.2 = q
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants

18. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population
In real populations, allele and genotype frequencies do change over time

19. The five conditions for nonevolving populations are rarely met in nature:
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

20. Applying the Hardy-Weinberg 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
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

21. The occurrence of PKU is 1 per 10,000 births
q2 =
q = 0.01
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

23. Required conditions are rarely (if ever) met
Changes in gene pool frequencies are likely
When gene pool frequencies change, microevolution has occurred
Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place
Hence, microevolution (via genetic mutations)
The raw material for evolutionary change
Provides new combinations of alleles
Some might be more adaptive than others

24. Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow

25. You should now be able to:
Explain why the majority of point mutations are harmless
Explain how sexual recombination generates genetic variability
Define the terms population, species, gene pool, relative fitness, and neutral variation
List the five conditions of Hardy-Weinberg equilibrium

26. Apply the Hardy-Weinberg equation to a population genetics problem
Explain why natural selection is the only mechanism that consistently produces adaptive change
Explain the role of population size in genetic drift

27. Allele Frequencies Red short-horned cattle are homozygous for the red allele, white cattle are homozygous for the white allele, and roan cattle are heterozygotes. Population A consists of 36% red, 16% white, and 48% roan cattle. What are the allele frequencies?
red = 0.36, white = 0.16
red = 0.6, white = 0.4
red = 0.5, white = 0.5
Allele frequencies cannot be determined unless the population is in equilibrium.
Answer: b
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

28. Hardy-Weinberg Equilibrium Determination Which of these populations are in Hardy-Weinberg equilibrium? A B
both A and B
neither A nor B
Answer: a
This question shows that two populations can have the same allele frequencies but may differ regarding whether they are in equilibrium. Also, many students have the misconception that if pp + 2pq + qq = 1, then the population is in equilibrium. Ask students with that particular misconception to calculate the outcome of random mating in each population. Population A will have the same proportions of phenotypes in the next generation, whereas population B will change to the same proportions as in population A.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

29. Cystic Fibrosis 1 The frequency of cystic fibrosis, a recessive genetic disease, is 1 per 2,500 births among Northern Europeans. Assuming random mating, what is the frequency of carriers?
1/2,500
1/50
1/25
The frequency cannot be calculated because selection violates Hardy-Weinberg assumptions.
Answer: c
The frequency of carriers is 2pq. The allele frequency, q, is 1/50 since qq = 1/2500. P is close to 1. You may want to discuss why option d does not apply.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.

30. Cystic Fibrosis 2 Until the 1950s, infants born with cystic fibrosis did not survive longer than a few months. If the frequency of carriers was 4% in the year 1900, what proportion of CF alleles was eliminated in one generation?
100%
50% 4% 2%
<0.1%
Answer: c
The frequency of CF alleles lost from the death of infants with CF is 2/2,500. The frequency of CF alleles present in heterozygotes is 1/25 or 100/2,500. Thus approximately 2/100, or 2%, of CF alleles are eliminated from the population because the homozygous infants do not survive to have progeny.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings.