# How Populations Evolve

```How Populations Evolve
Gene pool
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All genes present in population
microevolution
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Change in relative frequencies of alleles
in a population over time
Hardy-Weinberg Theorum: in absence
of selection, the allele frequencies
within a population will remain constant
from one generation to the next
Hardy-Weinberg Theory
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5 conditions
Large population
No migration
No net changes in gene pool due to
mutation
Random mating
Equal reproductive success of each
genotype
Hardy Weinberg equation
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P and q represent proportions of the two alleles
within a population
Combined frequencies of the alleles must equal
100% of the genes for that locus within a
population p + q = 1
P2 + 2pq + q2 = 1
P from mom p from dad p2
P from mom q from dad pq
P from dad q from mom pq
2pq
Q from mom q from dad q2
Example 1
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If p = .7 (allele A) then q = .3 (allele a)
Then P2 + 2pq + q2 = 1
P2 = AA =.49
2pq = 2Aa = .42
q2 = aa = .09
P = frequency of dominant allele A
Example 2
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If a pop has the folloing genotype
frequencies, AA = .42, Aa = .46,
aa=.012, what are the allele
frequencies?
A) A = 0.42, a=0.12
B) A=0.88, a = 0.12
C) A=0.65, a = 0.35
D) A= 0.6, a = 0.4
Example 2 Solution
Frequency of A = .42 + 1/2 (.46) = .65
1/2 = .35 OR
 Frequency of a = .12
 Frequency of a = 1-.65 = .35
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Example 3
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In a population with two alleles, B and b,
then allele frequency of B is 0.8. What
would be the frequency of heterozygotes if
the population is in Hardy-Weinberg
equilibrium?
A) .8
B) .16
C) .32
D) .64
Example 3 Solution
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Population of Bb = 2(.8)(.2) = .32
Example 4
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In a population that is Hardy-Weinberg
equilibrium, 16% of the population
shows a recessive trait. What percent
is homozygous dominant for the trait?
A) 6%
B) 36%
C) 48%
D) 84%
Example 4 Solution
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aa = .16 a = .4; then A = .6
AA = .36
(and Aa = 2*.4*.6 = .48)
Example 5
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In a random sample of a population of
Shorthorn cattle, 73 animals were red (CRCR),
63 were roan (CRCW –a mixture of red and
white), and 13 were white (CWCW). Estimate
the allele frequencies of CR and CW and
determine whether the population is in HardyWeinberg equilibrium.
Example 5 Solution
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Frequency of CW = [13/(73+63+13)]1/2
CW=(0.09)1/2 = .3
Frequency of CR = 1-.3 = .7
This genotypic ratio is what would be
predicted from these frequencies if the
population were in Hardy-Weinberg
equilibrium.
Example 6
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In a study of population of field mice, you find that
48% of the mice have a coat color that indicates that
they are heterozygous for a particular gene. What
would be the frequency of the dominant allele in this
population?
A) .4
B) .5
C) .7
D) you cannot estimate allele frequency from this
information
Example 6 Solution
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Frequency of heterozygous = 2pq,
therefore it would not be possible to
estimate the frequency of either p or q
Causes of Microevolution
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5 potential agents of microevolution
Small populations
Migration or emigration
Spontaneous mutations-point mutations
Nonrandom mating
Some genotypes are not equally successful
reproductively
Genetic Drift
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Chance change in a gene pool of a small
population; it is not related to the fitness of
the individuals
Bottleneck effect occurs if a catastrophic
event reduces the population size and the
survivors are not representative of the
original population
Founder effect is when a few individuals
colonize a new area; unlikely to be
representative of parent population
Gene flow
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Migration of individuals or transfer of
gametes between populations may
result in gain or loss of alleles
Eg. Pilot whale populations – pods
intermingle and mate at upwellings;
transfer of gametes
Mutation
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Mutations are the main method of
diversity in prokaryotes, but of little
importance in microevolution of
eukaryotes
Mutation rates for most gene loci is one
mutation in every 105 or 106 gametes
Mutation is the original source of
genetic variation, consequently, it is
central to evolution
Variation within populations
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Individual variation is what natural
selection acts on-on the phenotype
Polygenic traits that vary provide
variation
Polymorphism provide variation (blood
types)
2 flies in a Drosophila pop may vary at
25% of their loci-individual differences
Geographic variations
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Regional differences in allele
frequencies among the populations of a
species
Variations may be due to differing
environmental selection factors or
genetic drift
If parameter changes gradually across a
distance then a cline may develop
Origin of Species
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Speciation is the basis of evolution of
biological diversity
Anagenesis (phyletic evolution) is the
transformation of an entire population into a
different enough form that it is renamed a
new species
species arise from a parent species that
continues to exist
species
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Reproductive and genetically isolated
group of individuals
Limitation of this concept is it can’t
apply to asexually reproducing
organisms
Reproductive barriers
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Prezygotic barriers: before formation of zygote
Postzygotic barriers: prevention of
Prezygotic barriers
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Mechanical isolation- parts don’t fit
Geographical isolation – never meet
Temporal isolation – breed at different times
Behavioral isolation – wrong courtship dance,
wrong pheromones
Gametic isolation – gametes will not fuse to
form zygote-can’t line up or wrong molecular
recognition mechanism of egg and sperm
Postzygotic barriers
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Hybrid inviability – hybrid zygote fails to survive
embryonic development
Hybrid sterility – viable hybrid is sterile (usually
gametic problem)
Hybrid breakdown – hybrids are viable and fertile but
their offspring are defective or sterile
Exception may be introgression when offspring may
be able to mate w parent species variation in gene
pool without sacrificing species
Biogeography of speciation
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Allopatric speciation: gene pool of population
is segregated geographically from other
populations (opposite sides of river)
Parapatric speciation: genes pools of both
populations diverge without the dilution of
genes from their neighbors (theoretical)
Sympatric speciation: subpopulation becomes
reproductively isolated within parent
population (plants,wasps)
evolution
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numerous species from one parent
population – like Darwin’s Galapagos
finches
Convergent evolution is the formation
of homologous structures due to
environmental conditions
evolution
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microevolution species continue to evolve
over long periods of time
Punctuated evolution (Gould & Eldredge) long
period of stasis are punctuated by episodes of
relative rapid change and speciation in a few
thousand years vs millions of years –
Cambrian explosion of species
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