1. Chapter 23: The Evolution of Populations
What is a population?
Localized group of individuals of the same species
What is a species?
Organisms that can mate & produce fertile offspring in nature
What is a gene pool?
All of the genes (both alleles) in a population at any time
What is a fixed allele?
An allele that is the same for all members of the population (rare)
How can we determine if a population is evolving?
Hardy-Weinberg Theorem….which actually describes non-evolving
populations….population is in equilibrium
Alleles & genotypes in a gene pool will remain constant unless acted on
by agents other than sexual recombination
p + q = 1
p = dominant allele
q = recessive allele
1 = ALL alleles in the gene pool

2. Chapter 23: The Evolution of Populations
What is a population?
What is a species?
What is a gene pool?
What is a fixed allele?
How can we determine if a population is evolving?
Hardy-Weinberg Theorem….which actually describes non-evolving
populations….population is in equilibrium
Alleles & genotypes in a gene pool will remain constant unless acted on
by agents other than sexual recombination
p + q = 1
p = dominant allele
q = recessive allele
1 = ALL alleles in the gene pool
What if we want to know specific genotypes of individual in the population?
(p + q)(p + q) = 1
p2 + 2pq + q2 = 1
p2 = AA – homozygous dominant
q2 = aa – homozygous recessive
2pq = Aa - heterozygotes

3. Example p + q = 1 p2 + 2pq + q2 = 1
500 plants in a population
A – pink flowers
a – white flowers
20 white flowers (genotype = _____)
How many pink flowers are there? _______
Given: 320 AA
160 Aa
20 aa
How many flower color alleles in the population? ______
How many dominant (A) alleles? _____
How many recessive (a) alleles? _____
What is the frequency of the A allele? ________
a allele? ________
What is the genotypic frequency of AA flowers? ______
Aa flowers? ______
aa flowers? ______ aa
480
1000 = 500 x 2
800 = (320 x 2) + (160 x 1)
200 = (20 x 2) + (160 x 1)
0.8 = 800/1000
0.2 = 200/1000
0.64 = p2 = (0.8)2
0.32 = 2pq = 2(0.8)(0.2)
0.04 = q2 = (0.2)2

4. X X2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 }
biggest issues
0.01
0.04
0.09
0.16
0.25
0.36
0.49
0.64
0.81

5. Chapter 23: The Evolution of Populations
What is a population?
What is a species?
What is a gene pool?
What is a fixed allele?
How can we determine if a population is evolving?
What if we want to know specific genotypes of individual in the population?
What are the 5 requirements for a population to be in H-W equilibrium?
Large population
Isolated population
No net mutations
Random mating
No natural selection
What happens if any one of these criteria is not met?
No more equilibrium & microevolution occurs
What are the causes of microevolution?
Genetic drift – gene pool changes of a small population due to chance

6. Figure 23.7 Genetic drift Generation 1 Generation 2 Generation 3
CRCR
CRCW
CWCW
Only 5 of
10 plants
leave
offspring
Only 2 of
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3

7. Chapter 23: The Evolution of Populations
What is a population?
What is a species?
What is a gene pool?
What is a fixed allele?
How can we determine if a population is evolving?
What if we want to know specific genotypes of individual in the population?
What are the 5 requirements for a population to be in H-W equilibrium?
Large population
Isolated population
No net mutations
Random mating
No natural selection
What happens if any one of these criteria is not met?
No more equilibrium & microevolution occurs
What are the causes of microevolution?
Genetic drift – gene pool changes of a small population due to chance
Bottleneck effect – a disaster reduces the population size so that the
surviving population is NOT representative of the original population
Founder effect – genetic drift in a new colony such as an island

8. Figure 23.8 The bottleneck effect
Original
population
Bottlenecking
event
Surviving
(b)
Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.
(a)
Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.

9. Chapter 23: The Evolution of Populations
What is a population?
What is a species?
What is a gene pool?
What is a fixed allele?
How can we determine if a population is evolving?
What is we want to know specific genotypes of individual in the population?
What are the 5 requirements for a population to be in H-W equilibrium?
What happens if any one of these criteria is not met?
What are the causes of microevolution?
Genetic drift – gene pool changes of a small population due to chance
Bottleneck effect – a disaster reduces the population size so that the
surviving population is NOT representative of the original population
Founder effect – genetic drift in a new colony such as an island
Gene flow – gain or loss of alleles due to immigration or emigration
Mutations – changes in an organism’s DNA can create new alleles
Non-random (selective) mating – favored alleles are selected for
Natural selection – variability within a population enables some organisms
to be better suited for survival and reproduction

10. Chapter 23: The Evolution of Populations
What is a population?
What is a species?
What is a gene pool?
What is a fixed allele?
How can we determine if a population is evolving?
What is we want to know specific genotypes of individual in the population?
What are the 5 requirements for a population to be in H-W equilibrium?
What happens if any one of these criteria is not met?
What are the causes of microevolution?
What are polymorphisms?
2 or more discrete traits within a population
ex. freckles, blood type
How can variation be preserved?
Diploidy – hides less favorable alleles in Aa until natural selection
favors them as aa
What is a heterozygote advantage?
Aa genotype has a selective advantage
Sickle-cell allele & malaria resistance

11. Figure 23.13 Mapping malaria and the sickle-cell allele
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)

12. Chapter 23: The Evolution of Populations
What is a population?
What is a species?
What is a gene pool?
What is a fixed allele?
How can we determine if a population is evolving?
What is we want to know specific genotypes of individual in the population?
What are the 5 requirements for a population to be in H-W equilibrium?
What happens if any one of these criteria is not met?
What are the causes of microevolution?
What are polymorphisms?
How can variation be preserved?
12. What is a heterozygote advantage?
13. How can natural selection change a population?
- Directional selection
- Disruptive selection
- Stabilizing selection

13. Figure 23.12 Modes of selection
(a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators.
(b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.
(c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.
Frequency of individuals
Phenotypes (fur color)
Original population
Original
population
Evolved