Download a population

Evolution of
Populations
Chapter 23
Populations Evolve
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individuals do not evolve, populations do
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a population is a localized group of individuals
that are capable of interbreeding & producing
fertile offspring
microevolution – a change in the genetic
makeup of a population from generation to
generation
population genetics – the study of how
populations genetically change over time
Gene Pool
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the total collection of genes in a population
at any one time; consists of all the alleles in
all the individuals of that population
each allele has a frequency in the population
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for a trait controlled by two alleles, p & q are used
to represent the frequency of the alleles
usually…
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p represents the frequency of the dominant allele
q represents the frequency of the recessive allele
Calculating Allele Frequencies
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EXAMPLE – the incomplete dominant trait for flower
color is controlled by two alleles:
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in a population of 500 flowers…
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total R = RR + R from RW = (320 x 2) + 160 = 800
 q (the frequency of W) = 200/1000 = 0.2
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320 are red (RR), 160 are pink (RW), & 20 are white (WW)
because each flower has two alleles for the trait,
there are 1000 alleles in the population
 p (the frequency of R) = 800/1000 = 0.8
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R = red
W = white
total W = W from RW + WW = 160 + (20 x 2) = 200
notice that p + q = 1
Hardy-Weinberg Theorem
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the proportion of genotypes (AA, Aa, aa) in a
population will remain constant from generation to
generation if…
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the population is very large
there is no gene flow (immigration or emigration)
there are no mutations
random mating is occurring
there is no natural selection
…because under these conditions, the frequency of
the alleles (A & a) does not change
populations exhibiting these conditions are not
evolving & are said to be in Hardy-Weinberg
equilibrium
Hardy-Weinberg Equation
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p2 + 2pq + q2 = 1 where…
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p = the frequency of the dominant allele (A)
q = the frequency of the recessive allele (a)
p2 = the frequency of the homozygous dominant genotype (AA)
2pq = the frequency of the heterozygous genotype (Aa)
q2 = the frequency of the homozygous recessive genotype (aa)
uses:
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to calculate the frequencies of alleles & genotypes in a
population
to determine if a population is in Hardy-Weinberg EQ
to estimate what % of a population is carrying the allele for a
recessively inherited disease
Example #1
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What are the allele & genotypic frequencies
for a population of 500 mice in which 245 are
black (BB), 210 are brown (Bb), & 45 are
white (bb)?
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 (BB) = 245/500 = 0.49
 (Bb) = 210/500 = 0.42
 (bb) = 45/500 = 0.09
 (B) = p2 = 0.49 = 0.7
 (b) = q2 = 0.09 = 0.3
Example #2
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What are the allele & genotypic frequencies
for a population of 1000 pea plants in which
750 have purple flowers (PP) & 250 have
white flowers (pp)?
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 (pp) = 250/1000 = 0.25
 (p) = q2 = 0.25 = 0.5
recall p + q = 1,   (P) = 1 – q = 1 – 0.5 = 0.5
 (PP) = p2 = (0.5)2 = 0.25
 (Pp) = 2pq = 2(0.5)(0.5) = 0.5
Example #3
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What % of the US population carries the
allele for PKU, a homozygous recessive
disease? (NOTE: 1 in 10,000 babies born in
the US have PKU)
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 (aa) = 1/10,000 = 0.0001
 (a) = q2 = 0.0001 = 0.01
  (P) = 1 – q = 1 – 0.01 = 0.99
 (Aa) = 2pq = 2(0.01)(0.99) = 0.0198
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 percent carriers in US population is ~2%
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Factors that disrupt HW-EQ
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natural selection
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genetic drift – changes in allele frequencies due to
chance
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results in alleles being passed to the next generation in
different proportions
bottleneck effect – occurs when a population is drastically
reduced in size due to an environmental disaster (ie: fire,
flood) & the gene pool of the survivors no longer
represents that of the original population
founder effect – occurs when a small group of individuals
is isolated from the larger population & the gene pool of
this splinter population does not reflect the source
population
gene flow – the loss/gain of alleles due to
emigration/immigration
Sources of Variation
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mutations
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point mutations, chromosome rearrangements,
duplication
rate if low in animals & plants
rate is higher in viruses & bacteria b/c they have
short life spans
sexual recombination
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reshuffling of alleles can lead to phenotypic
variations
3 Modes of Natural Selection
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directional selection – favors variants at
one extreme
disruptive selection – favors variants at
both extremes
stabilizing selection – favors intermediate
variants
Final Question…
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Why doesn’t natural selection result in the
culling of all unfavorable genotypes?
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recessive alleles are carried (hidden) in
heterozygotes
heterozygote advantage (ex: sickle-cell trait)
frequency-dependent selection
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when the fitness of any one variation declines if it
becomes too common
neutral variation
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genetic variation that has no impact on reproductive
success