biol 102 chp 23: the evolution of populations
DESCRIPTION
This is a lecture presentation for my BIOL 102 General Biology II students on Chapter 23: The Evolution of Populations (Biology 8E by Campbell et al, 2008). Rob Swatski, Assistant Professor of Biology, Harrisburg Area Community College - York Campus, York, PA. Email: [email protected] Please visit my website, BioGeekiWiki, for more biology learning resources: http://robswatskibiology.wetpaint.com Visit my Flickr photostream for anatomy model photographs! http://www.flickr.com/photos/rswatski/ Thanks for looking!TRANSCRIPT
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The Evolution The Evolution of Populationsof Populations
BIOL BIOL 102: 102: General Biology IIGeneral Biology II
Chapter Chapter 2323
Rob Rob SwatskiSwatski Associate Professor Associate Professor of Biologyof Biology
HACCHACC--YorkYork
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Overview of Overview of Natural Natural
SelectionSelection
Natural selection acts on individuals,
but only populations evolve
Evolution occurs through genetic
variations in populations
Ex: Medium ground finch &
beak size during drought
2 Medium ground finchMedium ground finch
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MicroevolutionMicroevolution
Microevolution: changes in a
population’s allele frequencies over
generations
Three mechanisms cause allele frequency
change:
1) Natural selection, 2) Genetic drift, 3)
Gene flow
3
1976 (similar to the prior 3 years)
1978 (after
drought)
Ave
rage
be
ak d
ep
th (
mm
)
10
9
8
0
3
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AllelesAlleles
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Two main sources of Two main sources of gene pool variation:gene pool variation:
Mutation Sexual
Reproduction
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Harold and Maude (1971)Harold and Maude (1971)
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Genetic Genetic VariationVariation
Variation in individual genotype leads to variation in
individual phenotype
Natural selection can only act on variation
with a genetic component
Not all phenotypic variation is heritable
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Moth caterpillars raised on oak flower diet resemble oak flowers
NonheritableNonheritable VariationVariation
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Moth caterpillar siblings raised on oak leaves resemble oak twigs 13
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Population variation is the Population variation is the result of:result of:
Discrete characters
Are classified as “either/or”
Quantitative characters
Vary along a continuum within
a population
Phenotype is often influenced
by 2 or more genes 14
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Discrete Discrete CharactersCharacters
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Quantitative CharactersQuantitative Characters
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GenotypesGenotypes
Homozygous
Individual having 2 of the same
alleles for a given locus
Heterozygous
Individual having 2 different alleles for a given locus
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Average Average HeterozygosityHeterozygosity
A measure of gene variability
Measures the average % of loci that are heterozygous in a
population
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Nucleotide Nucleotide VariabilityVariability
Measured by comparing the differences between DNA sequences of pairs
of individuals
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Drosophila Drosophila melanogastermelanogaster
Average heterozygosity
13,700 genes in genome
= 14% (1,920 loci)
Nucleotide variability
180 million nucleotides in genome
= 1% (1.8 million)
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Geographic Geographic VariationVariation
Differences between gene pools of separate
populations or population subgroups
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13.17 19 XX 10.16 9.12 8.11
1 2.4 3.14 5.18 6 7.15
9.10
1 2.19
11.12 13.17 15.18
3.8 4.16 5.14 6.7
XX
Geographic Variation in Isolated Mouse Geographic Variation in Isolated Mouse Populations on MadeiraPopulations on Madeira
Isolated populations have differences in fused chromosomes 24
karyotypes
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ClineCline
A graded change in a trait along a
geographic axis
Ex: lactate dehydrogenase
frequency is higher in cold water (allows faster swimming in
fish)
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1.0
0.8
0.6
0.4
0.2
0
46 44 42 40 38 36 34 32 30
Georgia Warm (21°C)
Latitude (Latitude (°°N)N) Maine Cold (6°C)
Mummichog
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MutationMutation
A change in the nucleotide
sequence of DNA
Causes new genes & alleles to arise
Only mutations in gamete-producing cells can be passed
to offspring 27
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Point Mutation:Point Mutation: a change in 1 base in a gene
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Effects of Effects of Point Point
MutationsMutations
Mutations in noncoding regions of DNA are often
harmless due to redundancy
Mutations resulting in a change in protein
production are often harmful
Mutations may also be beneficial & increase an
organism’s fit into its environment
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Types of MutationsTypes of Mutations
Deletions
More harmful
Disruptions
More harmful
Rearrangements
More harmful
Duplication
Less harmful
Genes can take on new
functions
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Mutation Mutation RatesRates
Low in animals & plants: avg 1 mutation in every 100,000 genes
per generation
Often higher in prokaryotes & viruses
Prokaryotes & viruses have short generation times so mutations can
quickly produce genetic variation
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Sexual Sexual ReproductionReproduction
Shuffles existing alleles into new combinations
Recombination
More important than mutation in
producing genetic differences …Why?
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FlowerFlower SymmetrySymmetry inin AntirrhinumAntirrhinum SpeciesSpecies
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Recombination During MeiosisRecombination During Meiosis
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Gene PoolGene Pool: all the alleles for all loci in a population
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Porcupine herd
PorcupinePorcupine herd herd rangerange
Beaufort Sea
MAP AREA
FortymileFortymile herd herd rangerange
Fortymile herd
overlapoverlap
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Calculate the Frequency of an Allele in a Calculate the Frequency of an Allele in a Population:Population:
Total # of alleles at a locus = total # of individuals x 2
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Total # of Dominant or Total # of Dominant or Recessive Alleles at a LocusRecessive Alleles at a Locus
2 alleles for each homozygous
dominant or recessive individual plus…
1 allele for each heterozygous
individual
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If there are 2 alleles at a locus, p & q are used to represent their frequencies
The frequency of all alleles in a population will add up to 1
p + q = 1
p q
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HardyHardy--Weinberg Weinberg PrinciplePrinciple
Describes a hypothetical population that is not
evolving
In real populations, allele & genotype
frequencies change over time
If a population does not meet H-W criteria, then
the population is evolving
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HardyHardy--Weinberg Weinberg
EquilibriumEquilibrium
Allele & genotype frequencies in a population
remain constant from generation to generation
In a population where gametes randomly
contribute to the next generation, allele
frequencies will not change
Mendelian inheritance preserves genetic variation in
a population
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CRCR
CWCW
CRCW
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Frequencies of alleles
Alleles in the Alleles in the populationpopulation
Gametes produced
Each egg:
Each sperm:
80% chance
80% chance
20% chance
20% chance
q = frequency of
p = frequency of
CR allele = 0.8
CW allele = 0.2
equilibrium
random
Selecting Alleles at Random from a Selecting Alleles at Random from a Gene PoolGene Pool
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p2 & q2 are the frequencies of the homozygous genotypes
2pq is the frequency of the heterozygous genotype
If p & q represent the relative frequencies of the only two possible alleles in a population at a
particular locus, then:
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SpermSperm CR
(80%)
80% CR (p = 0.8)
CW (20%)
20% CW (q = 0.2)
16% (pq)
CRCW
4% (q2)
CW CW
64% (p2)
CRCR
16% (qp)
CRCW
Parent Generation:
F1:
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Gametes of this generation:
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Genotypes in the next generation:
With random mating, these gametes will result in the same mix of genotypes
64% CRCR, 32% CRCW, and 4% CWCW F1:
64% CRCR, 32% CRCW, and 4% CWCW plants F2:
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No mutations Random mating
No natural selection
Extremely large
population size
No gene flow
The 5 H-W conditions for nonevolving populations are rarely met in nature:
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3 Major Factors of 3 Major Factors of Evolutionary ChangeEvolutionary Change
Natural Selection
Genetic Drift
Gene Flow
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Natural Natural SelectionSelection
Differential reproductive
success
Results in certain alleles being
passed to the next generation in
greater proportions than
other alleles
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Genetic Genetic DriftDrift
Allele frequencies can fluctuate
unpredictably from one generation to
the next
Reduces genetic variation through
loss of alleles
The smaller a sample, the greater
the chance of deviation from a predicted result 51
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Generation 1Generation 1
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3
Generation 2Generation 2
CR CW CR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5 q = 0.5
Generation 3Generation 3
p = 1.0 q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR CR CR
CR CR
CR CR CR CR
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Founder Founder EffectEffect
Occurs when a few individuals become
isolated from a larger population
Allele frequencies in the small founder population may differ from those in
the larger parent population
Ex: Amish
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PolydactylyPolydactyly 56
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Bottleneck Bottleneck EffectEffect
Occurs when population size is reduced due to a sudden change in the
environment
The resulting gene pool may no longer reflect the
original population’s gene pool
If the population remains small, it may be further affected by genetic drift
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Original population
Bottlenecking event
Surviving population
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Genetic Drift & Genetic Drift & the Greater the Greater
Prairie ChickenPrairie Chicken
Habitat loss caused a severe reduction in the population of greater
prairie chickens in Illinois
The surviving birds had low levels of genetic
variation
Only 50% of their eggs hatched
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Range of greater prairie chicken
Pre-bottleneck (Illinois, 1820)
Post-bottleneck (Illinois, 1993)
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Greater Prairie Greater Prairie Chicken Chicken
Research, cont.Research, cont. DNA from museum specimens used to compare genetic
variation before & after bottleneck
Results showed a loss of alleles at several loci
Introduced prairie chickens from other
states to increase gene pool diversity
Successfully introduced new alleles & increased egg hatch rate to 90%
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NumberNumber of allelesof alleles per locusper locus
Minnesota, 1998Minnesota, 1998 (no bottleneck)
Nebraska, 1998Nebraska, 1998 (no bottleneck)
Kansas, 1998Kansas, 1998 (no bottleneck)
IllinoisIllinois
1930–1960s
1993
LocationLocation PopulationPopulation
sizesize
%% of eggsof eggs hatchedhatched
1,000–25,000
<50
750,000
75,000– 200,000
4,000
5.2
3.7
93
<50
5.8
5.8
5.3 85
96
99
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Effects of Effects of Genetic Genetic
DriftDrift Significant in
small populations
Causes allele frequencies to
change at random
Can lead to a loss of genetic
variation within populations
May cause harmful alleles to
become fixed 63
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Gene FlowGene Flow
Movement of alleles among populations
Transferred through movement of fertile
individuals or gametes
Usually reduces differences between
populations over time
More likely than mutation to directly
alter allele frequencies
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Gene Flow & Gene Flow & Decreasing Decreasing
FitnessFitness
Ex: Bent grass
Alleles for copper tolerance are beneficial in populations
near copper mines, but harmful to those in other
soils
Windblown pollen moves alleles between populations
Movement of unfavorable alleles into a population
decreases the fitness between organism &
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NON- MINE SOIL
MINE SOIL
NON- MINE SOIL
Prevailing wind direction
Ind
ex o
f co
pp
er
tole
ran
ce
Distance from mine edge (meters)
70
60
50
40
30
20
10
0 20 0 20 0 20 40 60 80 100 120 140 160
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Population in which the
surviving females
eventually bred
Central
Eastern
Surv
ival
rat
e (
%)
Surv
ival
rat
e (
%)
Females born
in central
population
Females born
in eastern
population
Parus major
60
50
40
30
20
10
0
Central
population
NORTH SEA Eastern
population Vlieland,
the Netherlands
2 km
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Gene Flow & Gene Flow & Increasing Increasing
FitnessFitness Ex: Insecticide
resistance in mosquitoes
Insecticides have been used to kill mosquitoes
that carry West Nile virus & malaria
Alleles have evolved in some mosquito
populations that confer insecticide resistance
The flow of these resistance alleles into a population can increase
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Why are the phrases “survival of the fittest”
and “struggle for existence”
misleading?
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Relative Relative FitnessFitness
Reproductive success is generally more subtle &
depends on many factors
The contribution an individual makes to the gene pool of the next
generation…
…relative to the contributions of other
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3 Types of Selection3 Types of Selection
Directional Disruptive Stabilizing
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Directional SelectionDirectional Selection
Favors individuals at one extreme of the phenotypic range
Original population
Phenotypes (fur color)
Evolved population
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Disruptive SelectionDisruptive Selection
Favors individuals at both extremes of the phenotypic range
Original population
Phenotypes (fur color)
Evolved population
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Stabilizing SelectionStabilizing Selection
Favors intermediate variants & acts against extreme phenotypes
Original population
Phenotypes (fur color)
Evolved population
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Natural Natural Selection & Selection &
Adaptive Adaptive EvolutionEvolution
Natural selection increases the frequencies of alleles that enhance survival &
reproduction
Adaptive evolution occurs as the match between an
organism & its environment increases
Because the environment can change, adaptive
evolution is a continuous dynamic process
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Bones shown in
green are movable.
Ligament
Movable jaw bones in snakes 86
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Sexual Sexual SelectionSelection
Natural selection for mating success
May result in sexual dimorphism
Can lead to significant differences between
secondary sexual traits 87
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Types of Sexual Types of Sexual SelectionSelection
Intrasexual selection
Intersexual selection
(Mate choice)
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Competition between individuals of one sex (often males) for mates of the opposite sex
IntrasexualIntrasexual SelectionSelection
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Occurs when individuals of one sex (usually females) are more choosy in selecting their mates
Intersexual Selection (Mate Choice)Intersexual Selection (Mate Choice)
Sooty Sooty Grouse Grouse
mating ritualmating ritual
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Male showiness can increase his chances of attracting a female, but also decrease his overall chances of survival 95
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Good Genes Good Genes HypothesisHypothesis
One explanation for the evolution of female
preference
If a trait is related to male health, selection should favor both the
male trait & the female preference for that trait
Ex: Gray tree frog mating call
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Significance of Significance of Call Duration on Call Duration on
Mate ChoiceMate Choice
Long-Calling (LC) & Short-Calling (SC)
Does call duration indicate the male’s
overall genetic quality?
Do females choose mates based upon this
trait?
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SC male
Female gray tree frog
LC male
SC sperm Eggs LC sperm
Offspring of LC father
Offspring of SC father
Fitness of these half-sibling offspring compared
EXPERIMENTEXPERIMENT
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RESULTSRESULTS
Time to metamorphosis
Larval survival
Larval growth
NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.
Offspring Performance 1995 1996
LC better NSD
NSD
LC better (shorter)
LC better (shorter)
LC better
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The Preservation of Genetic VariationThe Preservation of Genetic Variation
Diploidy Balancing selection
Heterozygote advantage
Frequency-dependent selection
Neutral variation
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DiploidyDiploidy Maintains genetic variation in the form of hidden recessive alleles
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Biston betularia morpha typica Biston betularia morpha carbonaria
Balancing Balancing SelectionSelection
Natural selection maintains stable frequencies of 2 or more phenotypic
forms in a population
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Heterozygote Heterozygote AdvantageAdvantage
Heterozygotes have a higher fitness than both
homozygotes
Natural selection will tend to maintain 2 or more
alleles at that locus
The sickle-cell allele causes mutations in hemoglobin, but also provides malaria
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Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote)
Key
Frequencies of the sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
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FrequencyFrequency--Dependent Dependent SelectionSelection
The fitness of a phenotype decreases if it becomes
too common in the population
Selection can favor the least common phenotype
in a population
Ex: scale-eating fish (Perissodus)
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“Left-mouthed”
P. microlepis
“Right-mouthed”
P. microlepis
1.0
0.5
0 1981
Sample year ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Fre
qu
en
cy o
f
“le
ft-m
ou
thed
” i
nd
ivid
uals
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Neutral Neutral VariationVariation
Genetic variation that appears to provide no selective advantage or
disadvantage
Ex: Variations in noncoding regions of DNA
Ex: Variations in proteins that have little effect on function or reproductive
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Selection can act only on existing
variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, & the
environment interact
Why Natural Selection Cannot Fashion Why Natural Selection Cannot Fashion “Perfect” Organisms“Perfect” Organisms
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CreditsCredits by Rob Swatski, 2013
http://robswatskibiology.wetpaint.com
Visit my website for more Anatomy study resources!Visit my website for more Anatomy study resources!
http://www.flickr.com/photos/rswatski
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