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

2. Concept 23.1: Population genetics provides a foundation for studying evolution
Microevolution is change in the genetic makeup of a population from generation to generation

3. The Modern Synthesis
MAP
AREA
CANADA
ALASKA
Beaufort Sea
Porcupine
herd range
NORTHWEST
TERRITORIES
Fairbanks
Fortymile
Whitehorse
YUKON
Population genetics is the study of how populations change genetically over time

4. The Hardy-Weinberg Theorem
Generation 3
25% CRCR 4
50% CRCW
25% CWCW
50% CW
gametes
50% CR
come together at random
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions 2 1
CRCR
CWCW
genotype
Plants mate
All CRCW
(all pink flowers) X
The Hardy-Weinberg theorem describes a population that is not evolving

5. Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes a population where allele frequencies do not change
p2 + 2pq + q2 = 1
And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

6. 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

7. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population

8. The five conditions for non-evolving populations are rarely met in nature:
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection

9. Population Genetics and Human Health
We can use the Hardy-Weinberg equation to estimate the percentage of a population carrying the allele for an inherited disease

10. Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible
Two processes, mutation and sexual recombination, produce the variation in gene pools that contributes to differences among individuals

11. Point Mutations A point mutation is a change in one base in a gene
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

12. Sexual Recombination
Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible
What are the sources of genetic variation in sexual recombination?

13. Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition
Three major factors alter allele frequencies and bring about most evolutionary change:
Natural selection
Genetic drift
Gene flow

14. Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

15. Animation: Causes of Evolutionary Change
Genetic Drift
The smaller a sample, the greater the chance of deviation from a predicted result
allele frequencies fluctuate unpredictably from one generation to the next
reduces genetic variation through losses of alleles
Animation: Causes of Evolutionary Change

16. 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

17. The Bottleneck Effect
The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population
Original
population
Bottlenecking
event
Surviving

18. The Founder Effect
The founder effect occurs when a few individuals become isolated from a larger population
It can affect allele frequencies in a population

19. Gene Flow
Gene flow consists of genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes

20. Map butterflies that emerge in spring: orange and brown
LE 23-9
Genetic variation occurs in individuals in populations of all species
It is not always heritable
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white

21. Cline: 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

22. Directional, Disruptive, and Stabilizing Selection
Selection favors certain genotypes by acting on the phenotypes of certain organisms
Three modes of selection:
Directional
Disruptive
Stabilizing

23. LE 23-12 Original population Frequency of individuals Original
Evolved
population
Phenotypes (fur color)
Directional selection
Disruptive selection
Stabilizing selection

24. Heterozygote Advantage
Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes
Natural selection will tend to maintain two or more alleles at that locus

25. 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%

26. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

27. Intersexual selection occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex
Selection may depend on the showiness of the male’s appearance