1. The Evolution of Populations
Chapter 23
The Evolution of Populations

2. Population Genetics Darwin and Mendel Gene pool and Allele frequency
fixed
Heterozygous
Mind your p’s and q’s!

3. Calculate allele frequencies: 500 total flowers 320 red 160 pink
Generation 1 X
CRCR
CWCW
genotype
genotype
Calculate allele frequencies:
500 total flowers
320 red
160 pink
20 white
Freq CR:
FreqCW:
Plants mate
Generation 2
All CRCW
(all pink flowers)
50% CR
50% CW
gametes
gametes
come together at random
Generation 3
25% CRCR
50% CRCW
25% CWCW
50% CR
50% CW
gametes
gametes
come together at random
Generation 4
25% CRCR
50% CRCW
25% CWCW
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions

4. The Hardy-Weinberg Theorem
Allele frequencies in a population remain unchanged
Basis for understanding long-term evolutionary changes
H-W equilibrium – p2 + 2pq + q2 = 1
p + q = 1

5. Gametes for each generation are drawn at random from the gene pool
LE 23-5
Gametes for each generation are
drawn at random from the gene pool
of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm CR
(80%) CW
(20%) p2 pq
(80%) CR
Eggs
64%
CRCR
16%
CRCW
(20%) CW
16%
CRCW 4%
CWCW qp q2

6. Conditions for Hardy-Weinberg
Large population size
No gene flow
No mutations
Random mating
No natural selection

7. Sources of Variation Mutations Sexual recombination Point mutation
Alter gene number or sequence
duplication
Mutation rates
Sexual recombination

8. Altering Populations Gene Pool: The Big Players
Natural selection
Genetic Drift
Bottleneck effect
Founder effect
Gene flow

9. LE 23-7 CRCR CRCR CWCW CRCR CRCR CRCW CRCW CRCR CRCR CWCW CRCR CRCR
Only 5 of
10 plants
leave
offspring
CRCW
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CWCW
CRCR
CRCR
CWCW
CRCR
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CWCW
CRCR
CRCR
CRCR
CRCW
CRCW
CRCW
CRCR
CRCR
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0

10. LE 23-8
Original
population
Bottlenecking
event
Surviving
population

11. Natural Selection and Adaptive Evolution
Genetic variation
Polymorphism
Geographic variation
Closer look at Natural Selection
Evolutionary fitness
Types of selection
Preservation of genetic variation
Sexual Selection

12. LE 23-10 1
2.4
3.14
5.18 6
7.15
8.11
9.12
10.16
13.17 19 XX 1
2.19
3.8
4.16
5.14
6.7
9.10
11.12
13.17
15.18 XX

13. Mean height (cm) Altitude (m) Sierra Nevada Range Great Basin Plateau
LE 23-11
Heights of yarrow plants grown in common garden
100
Mean height (cm) 50
3,000
Altitude (m)
2,000
Sierra Nevada
Range
Great Basin
Plateau
1,000
Seed collection sites

14. Frequency of individuals
LE 23-12a
Original population
Frequency of
individuals
Phenotypes (fur color)

15. LE 23-12b Original population Evolved population Directional selection
Disruptive selection
Stabilizing selection

16. Plasmodium falciparum (a protozoan)
LE 23-13
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%

17. LE 23-14 On pecking a moth image the blue jay receives a food reward.
If the bird does not
detect a moth on
either screen, it pecks
the green circle to
continue a new set
of images (a new
feeding opportunity).
Parental population sample
0.6
Experimental group sample
0.5
Phenotypic
variation
0.4
Frequency-
independent control
0.3
0.2 20 40 60 80
100
Generation number
Plain background
Patterned background