1. Population Genetics

2. Overview: The Smallest Unit of Evolution
Natural selection acts on individuals
On phenotypes
only populations evolve
Genetic variations contribute to evolution
Microevolution
change in the genetic makeup of a population from generation to generation

3. The Modern Synthesis Population genetics
study of how populations change genetically over time
integrates Mendelian genetics with the Darwinian theory of evolution by natural selection

4. Gene Pools and Allele Frequencies
gene pool is the total aggregate of genes in a population at any one time
gene pool consists of all gene loci in all individuals of the population

5. LE 23-3 Beaufort Sea Porcupine herd range NORTHWEST TERRITORIES
MAP
AREA
CANADA
ALASKA
Beaufort Sea
Porcupine
herd range
NORTHWEST
TERRITORIES
Fairbanks
Fortymile
herd range
Whitehorse
ALASKA
YUKON

6. The Hardy-Weinberg Theorem
describes a population that is not evolving
frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation
provided that only Mendelian segregation and recombination of alleles are at work
Mendelian inheritance preserves genetic variation in a population

7. LE 23-4 Generation 1 X CRCR CWCW genotype genotype Plants mate
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

8. 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
p2 and q2 represent the frequencies of the homozygous genotypes
2pq represents the frequency of the heterozygous genotype
p + q = 1
p represents the frequency of one allele (dominant)
q represents the frequency of the other allele (recessive)

9. 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

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

11. variation that makes evolution possible
Mutation
changes in the nucleotide sequence of DNA
cause new genes and alleles to arise
point mutation is a change in one base in a gene
usually harmless but may have significant impact on phenotype
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
Gene duplication is nearly always harmful
Mutation rates are low in animals and plants
one mutation in every 100,000 genes per generation
Mutations are more rapid in microorganisms
sexual recombination
far more important than mutation in producing the genetic differences that make adaptation possible

12. Deletion
Duplication
Inversion
Translocation

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

14. 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

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

16. Genetic Variation
Genetic variation occurs in individuals in populations of all species
It is not always heritable

17. Map butterflies that emerge in spring: orange and brown
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white

18. Phenotypic polymorphism
a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable
Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups

19. 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

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

21. Directional selection
favors individuals at one end of the phenotypic range
Disruptive selection
favors individuals at both extremes of the phenotypic range
Stabilizing selection
favors intermediate variants and acts against extreme phenotypes

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

23. The Preservation of Genetic Variation
Diploidy
maintains genetic variation in the form of hidden recessive alleles
Balancing selection
occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
leads to a state called balanced polymorphism
Heterozygote Advantage
maintains two or more alleles at that locus
Ex. sickle-cell allele
causes mutations in hemoglobin
also confers malaria resistance

24. 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%

25. In frequency-dependent selection
fitness of any morph declines if it becomes too common in the population

26. 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

27. Sexual Selection Sexual selection natural selection for mating success
can result in sexual dimorphism
marked differences between the sexes in secondary sexual characteristics
Intrasexual selection
competition among individuals of one sex for mates of the opposite sex
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