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Population Genetics
Chapter 18
Levels of Organization
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Atoms - CHNOPS
Molecules – Carbs, Proteins, Lipids, Nucleic Acids
Organelles – Nucleus, Ribsomes, ER, Golgi, vessicles etc
Cells – smallest level for life
Tissues
Organs
Organ Systems
Organism - Individual
Population – smallest unit of evolution
Community
Ecosystem
Biosphere
Population
• Group of
organisms that
can interbreed to
produce fertile
offspring
• Gene pool – total
alleles in a
population
Hardy-Weinberg Equilibrium
• Evolution does NOT
occur if the gene
pool remains
constant from one
generation to the
next.
• Outside forces must
act on a population
for there to be
change
Conditions
NO Evolution = genetic
equilibrium
• No Mutations
• Extremely Large population
size – no genetic drift
• No gene flow – no migration
• Random Mating
• No Natural selection
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Evolution = no genetic
equilibrium
Mutations
Small populations – genetic
drift
Gene flow – migration
Non Random mating
Natural Selection
Equation - Hardy-Weinberg
Equilibrium
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p2 + 2pq + q2 = 1
p+q=1
p = dominant allele
q = recessive allele
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive
Example Problem #1
• In a population the frequency of the recessive
allele is 0.4.
– What is the frequency of the dominant allele?
– What frequency of the population will be
homozygous dominant, heterozygous, and
homozygous recessive?
– What frequency of the population will
demonstrate the dominant phenotype?
Example problem #2
• In a population 25% of the individuals
demonstrate the recessive phenotype.
– What is the frequency of the recessive allele?
– What is the frequency of the dominant allele?
– What frequency will be homozygous dominant in
the population?
– What frequency will demonstrate the dominant
phenotype?
Violations to H-W Equilibrium –
cause evolution
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Mutations
Small populations – genetic drift
Non random mating
Natural Selection
Migration – gene flow
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23_08EvolutionaryChanges_A.swf
1. Mutations
• Change in DNA’s nucleotide sequence.
• Raw source for new genes and alleles
• Most mutations are somatic cell mutations and do
not affect offspring
• Only gametic mutations affect a gene pool.
• Mutation rates
– Lower in organisms with a longer generation span
• Plants and animals – 1/100000 genes
– Higher in organisms with a shorter generation span
• Bacteria and viruses
1. Mutations
• Point Mutations – alter
one nucleotide base only
– Usually neutral
– Sickle cell anemia
• Chromosomal Mutations –
alter many regions or loci
of the entire chromosome
– Gene duplication
• Usually harmful, but when
beneficial act as an important
source of variation in a
population
2. Nonrandom Mating – sexual
selection
• Creates sexual recombination – joining of
different alleles in a gene pool
– Huge source of variation in a population
• Gametes from different organisms contribute
different alleles to the next generation.
3. Natural Selection
• Differential success in reproduction based on
variation in a population.
• Better suited organisms in an environment tend
to produce more offspring than less suited
organisms.
– Fitness
• Types
– Directional
– Disruptive
– stabilizing
– Sexual
– Artificial
4. Genetic Drift
• Random fluctuation of allele frequencies from one
generation to the next.
• Greater occurrence in smaller populations b/c one
organism who is homozygous recessive breeding more
than expected can create a large increase in the frequency
of the recessive allele, if the pop is small
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
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
Figure 23.7
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
4. Genetic Drift
• Bottleneck effect
– Sudden environmental change can drastically reduce the size of a
population – only some survive
• Fire, flood, human influence etc.
– Reduces the genetic variation
in a population
– Ex: An example of a bottleneck:
Northern elephant seals have reduced genetic variation probably because of a
population bottleneck humans inflicted on them in the 1890s. Hunting reduced
their population size to as few as 20 individuals at the end of the 19th century.
Their population has since rebounded to over 30,000—but their genes still
carry the marks of this bottleneck: they have much less genetic variation than a
population of southern elephant seals that was not so intensely hunted.
4. Genetic Drift
• Founder effect
– Occurs when a few individuals become isolated
from the original population
– The smaller population may not have the same
gene pool as the original and speciation is likely
5. Migration – Gene Flow
• Addition or loss of alleles
to or from a gene pool.
• Caused by the movement
of fertile individuals to or
from a population –
migration, a deer goes to
live with another
population
• Scientists are studying the
effects of cultivated crop’s
(artificially selected crops)
gene flow on wild
populations
Genetic Variation
• Differences in phenotypes between members of a
population
• Inherited in genotype
• The raw source for natural selection within a
population
Figure 23.1
Sources of Genetic Variation
• Mutations
• Sexual Reproduction
– Crossing over
– Independent
assortment
– Random fusion of
gametes
• Diploidy – recessive
allele does not show
• 23_04SexualRecombination_A.swf
Sources of Genetic Variation
• Outbreeding – mating with
unrelated partners
• Balanced polymorphism
– Heterozygote advantage –
sickle cell carriers
– Hybrid vigor – plant species
– Frequency – dependent
selection – least common
phenotypes have an
advantage