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Population
• All individuals living in a particular place at
the same time
• Population gene pool:
– all alleles for all loci present in population
– Allele = 1 of 2 alternate forms of a gene
– Gene = a segment of DNA
Evolutionary Change in
Populations
Chapter 19
Chromosomes, Genes, and Alleles
Hair color
genes
Chromosomes, Genes, and Alleles
Eye color
genes
h
hh
ee
e
e
H
H
E
E
h
h
e
E
H
H
e
E
• locus – location of a particular gene on a chromosome
• alleles - different forms of gene at same locus on two
homologous
h
e
chromosomes
h
E
H
e
• dominant allele (represented by capital letter)
h
e
H
– corresponding
trait is usually detectable in the individual E
E
h
Chiasma
H
– masks
the effect
of any recessive allele that may be present
e
H
Hair color
genes
Gamete 1
Gamete 2
Gamete 3
Gamete 4
E
• recessive allele (represented by lower case letter)
– expressed only when present on both of the homologous chromosomes
– no dominant alleles at that locus
Eye color
genes
h
hh
ee
e
e
H
H
E
E
h
h
e
E
H
H
e
E
Genotype – the alleles that an individual possesses for a
particular trait
h
e
Gamete 1
homozygous alleles – two identical alleles
for a EtraitGamete 2
h
e
heterozygous alleles – different allelesHfor that gene
Gamete 3
h
h
e
E
H
E
Gamete 4
Chiasma
H
Phenotype
– an observable
trait
e
H
E
an allele is expressed if it shows in the phenotype of an
individual
Punnett square shows how alleles can be passed to
offspring.
Genotype and phenotype probabilities resulting from a mating of two heterozygous parents.
Tt female
Tt male
Heterozygous
female forms
two types
of gametes
• Genotype frequency
Heterozygous
male forms
two types
of gametes
1/
T
2
1/
T
– proportion of a genotype in the population
2
• Phenotype frequency
TT
1/
2
t
t
Homozygous
1/
4
tT
1/
1/
2
– proportion of a phenotype in the population
Tt
1/
4
tt
1/
4
4
• Allele frequency
– proportion of an allele of a given genetic locus in
the population
Possible
combinations
in offspring
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Hardy-Weinburg principle
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q2 = frequency of the homozygous recessive genotype
• Allele and genotype frequencies do not change from
generation to generation in a population at genetic
equilibrium
- no evolution is occurring
Microevolution
• Change in allele or genotype frequencies
within a population over successive
generations.
What forces might alter allele frequency in a
population?
1. Nonrandom Mating
• When individuals select mates on basis of
phenotype
• Can change genotype of a population
1. Inbreeding
– mating of genetically similar individuals (breed
with those close by vs. random from entire popn.
– not chosen at random from entire population
Hardy–Weinberg Principle
• only occurs if
–
–
–
–
–
mating is random in population
no net mutations change allele frequencies
population is large
individuals don’t migrate between populations
natural selection does not occur
Forces that change allele frequencies in populations!
(i.e., evolution is occurring)
1.
2.
3.
4.
5.
nonrandom mating
mutation
genetic drift
gene flow
natural selection
1. Nonrandom Mating
2. Assortative mating
– individuals select mates by phenotypes
• Inbreeding and assortative mating
– increase frequency of homozygous genotypes
– i.e., Reduce genetic variability
• Can cause Inbreeding depression
– inbred individuals have lower fitness than noninbred individuals
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2. Mutations
• Unpredictable changes in DNA
– source of new alleles
• Increase genetic variability
– acted on by natural selection
3. Genetic Drift
• Random change in allele frequencies of a
small population
• Decreases genetic variation in population
• Changes caused by genetic drift are usually
not adaptive
3. Genetic Drift
4. Gene Flow
• Bottleneck
– sudden decrease in population size caused by
adverse environmental factors
• Movement of alleles caused by migration of
individuals between populations
• Cheetahs
• Founder effect
– genetic drift that occurs when a small population
colonizes a new area
• Causes changes in allele frequencies
(increases variation in popn, usually)
5. Natural Selection
• Causes changes in allele frequencies that lead
to adaptation
• Operates on an organism’s phenotype
– but changes genetic composition of population in
a particular environment
– Remember the peppered moths?
• Natural selection leads to adaptive evolutionary
change
- differential reproduction in individuals with
different traits
- i.e. Natural selection preserves individuals with
favorable phenotypes
- i.e., N.S. weeds out individuals with bad
phenotypes
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Modes of Selection
Modes of Selection
• Stabilizing selection
– favors the mean over phenotypic extremes
• Directional selection
– favors one phenotypic extreme over another,
causing shift in phenotypic mean
• Disruptive selection
– favors two or more phenotypic extremes
Heterozygote Advantage
Genetic Variation
• Geographic variation
– genetic variation among different populations
within the same species
• Cline
Mean height of
yarrow plants (cm)
– gradual change in species’ phenotype and
genotype frequencies
– through a series of geographically separate
populations
100
50
0
Groveland Mather Aspen Yosemite Tenaya Tuolumne
Valley
Creek
Lake Meadows
Big Timberline Conway
Horn
Summit
Lake
Lee
Vining
Elevation (m)
4000
3000
2000
1000
0
Sierra Nevada Range
Great Basin Plateau
Fig. 19-9b, p. 424
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