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Chapter 23: The Evolution of Populations
1. What is a population?
- Localized group of individuals of the same species
2. What is a species?
- Organisms that can mate & produce fertile offspring in nature
3. What is a gene pool?
- All of the genes (both alleles) in a population at any time
4. What is a fixed allele?
- An allele that is the same for all members of the population (rare)
5. 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
Chapter 23: The Evolution of Populations
1.
2.
3.
4.
5.
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
6. 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
Example
p+q=1
p2 + 2pq + q2 = 1
500 plants in a population
A – pink flowers
a – white flowers
20 white flowers (genotype = _____)
aa
480
How many pink flowers are there? _______
Given: 320 AA
160 Aa
20 aa
1000 = 500 x 2
How many flower color alleles in the population? ______
How many dominant (A) alleles? _____
800 = (320 x 2) + (160 x 1)
How many recessive (a) alleles? _____
200 = (20 x 2) + (160 x 1)
0.8 = 800/1000
What is the frequency of the A allele? ________
a allele? ________
0.2 = 200/1000
0.64 = p2 = (0.8)2
What is the genotypic frequency of AA flowers? ______
0.32 = 2pq = 2(0.8)(0.2)
Aa flowers? ______
aa flowers? ______
0.04 = q2 = (0.2)2
X
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
X2
0.01
0.04
0.09
0.16
0.25
0.36
0.49
0.64
0.81
}
biggest issues
Chapter 23: The Evolution of Populations
1.
2.
3.
4.
5.
6.
7.
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
8. What happens if any one of these criteria is not met?
- No more equilibrium & microevolution occurs
9. What are the causes of microevolution?
- Genetic drift – gene pool changes of a small population due to chance
Figure 23.7 Genetic drift
CW CW
CRCR
CRCR
CRCW
CW CW
CRCR
CRCR
CRCW
Only 5 of
10 plants
leave
offspring
CRCR
CW CW
CRCR
CRCW
CRCW
CRCR
CW CW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
Chapter 23: The Evolution of Populations
1.
2.
3.
4.
5.
6.
7.
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
8. What happens if any one of these criteria is not met?
- No more equilibrium & microevolution occurs
9. 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
Figure 23.8 The bottleneck effect
(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.
(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.
Original
population
Bottlenecking
event
Surviving
population
Chapter 23: The Evolution of Populations
1.
2.
3.
4.
5.
6.
7.
8.
9.
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
Chapter 23: The Evolution of Populations
1. What is a population?
2. What is a species?
3. What is a gene pool?
4. What is a fixed allele?
5. How can we determine if a population is evolving?
6. What is we want to know specific genotypes of individual in the population?
7. What are the 5 requirements for a population to be in H-W equilibrium?
8. What happens if any one of these criteria is not met?
9. What are the causes of microevolution?
10. What are polymorphisms?
- 2 or more discrete traits within a population
- ex. freckles, blood type
11. How can variation be preserved?
- Diploidy – hides less favorable alleles in Aa until natural selection
favors them as aa
12. What is a heterozygote advantage?
- Aa genotype has a selective advantage
- Sickle-cell allele & malaria resistance
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%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
7.5–10.0%
10.0–12.5%
>12.5%
Chapter 23: The Evolution of Populations
1. What is a population?
2. What is a species?
3. What is a gene pool?
4. What is a fixed allele?
5. How can we determine if a population is evolving?
6. What is we want to know specific genotypes of individual in the population?
7. What are the 5 requirements for a population to be in H-W equilibrium?
8. What happens if any one of these criteria is not met?
9. What are the causes of microevolution?
10. What are polymorphisms?
11. 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
Figure 23.12 Modes of selection
Original population
Original
population
Evolved
population
(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.
Phenotypes (fur color)
(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.