Download Population Genetics

POPULATION GENETICS
OVERVIEW: THE SMALLEST UNIT OF EVOLUTION

Natural selection acts on individuals


only populations evolve


On phenotypes
Genetic variations contribute to evolution
Microevolution

change in the genetic makeup of a population from
generation to generation
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
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
MAP
AREA
CANADA
ALASKA
LE 23-3
Beaufort Sea
Porcupine
herd range
Fairbanks
Fortymile
herd range
Whitehorse
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
population
inheritance preserves genetic variation in a
LE 23-4
Generation
1
X
CRCR
genotype
Generation
2
Plants mate
CWCW
genotype
All CRCW
(all pink flowers)
50% CW
gametes
50% CR
gametes
come together at random
Generation
3
25% CRCR
50% CRCW
50% CR
gametes
25% CWCW
50% CW
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

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)


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
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
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




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



usually harmless but may have significant impact on phenotype
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
Deletion
Duplication
Inversion
Translocation
A POPULATION’S GENETIC COMPOSITION

Three major factors alter allele frequencies
and bring about most evolutionary change:
 Natural
selection
 Genetic drift
 Gene flow
LE 23-7
CWCW
CRCR
CRCR
CRCW
Only 5 of
10 plants
leave
offspring
CRCR
CWCW
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCR
Only 2 of
10 plants
leave
offspring
CRCR
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
LE 23-8
Original
population
Bottlenecking
event
Surviving
population
GENETIC VARIATION
Genetic variation occurs in individuals in
populations of all species
 It is not always heritable

LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
POLYMORPHISM

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
LE 23-11
Heights of yarrow plants grown in common garden
Mean height (cm)
100
50
0
3,000
2,000
1,000
Sierra Nevada
Range
0
Seed collection sites
Great Basin
Plateau
SELECTION
Selection favors certain genotypes by acting
on the phenotypes of certain organisms
 Three modes of selection:

 Directional
 Disruptive
 Stabilizing

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
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
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
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%

In frequency-dependent selection
 fitness
of any morph declines if it becomes too
common in the population
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
Phenotypic
variation
Experimental group sample
0.5
0.4
Frequencyindependent control
0.3
0.2
0
Plain background
Patterned background
20
40
60
Generation number
80
100
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