Transcript Document

Hardy-Weinberg Principle
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Hardy-Weinberg - original proportions of
genotypes in a population will remain
constant from generation to generation
– Sexual reproduction (meiosis and
fertilization) alone will not change allelic
(genotypic) proportions.
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Hardy-Weinberg Equilibrium
Population of cats
n=100
16 white and 84 black
bb = white
B_ = black
Can we figure out the allelic frequencies of individuals BB and Bb?
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Hardy-Weinberg Principle
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Necessary assumptions
Allelic frequencies would remain constant
if…
– population size is very large
– random mating
– no mutation
– no gene input from external sources
– no selection occurring
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Hardy-Weinberg Principle
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Calculate genotype frequencies with a
binomial expansion
(p+q)2 = p2 + 2pq + q2
p2 = individuals homozygous for first allele
2pq = individuals heterozygous for alleles
q2 = individuals homozygous for second allele
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Hardy-Weinberg Principle
p2 + 2pq + q2
and
p+q = 1 (always two alleles)
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16 cats white = 16bb then (q2 = 0.16)
This we know we can see and count!!!!!
If p + q = 1 then we can calculate p from q2
Q = square root of q2 = q
√.16 q=0.4
p + q = 1 then p = .6 (.6 +.4 = 1)
P2 = .36
All we need now are those that are heterozygous
(2pq) (2 x .6 x .4)=0.48
.36 + .48 + .16
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Hardy-Weinberg Equilibrium
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Five Agents of Evolutionary Change
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Mutation
– Mutation rates are generally so low they
have little effect on Hardy-Weinberg
proportions of common alleles.
 ultimate source of genetic variation
Gene flow
– movement of alleles from one population
to another
 tend to homogenize allele frequencies
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Five Agents of Evolutionary Change
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Nonrandom mating
– assortative mating - phenotypically similar
individuals mate
 Causes frequencies of particular
genotypes to differ from those predicted
by Hardy-Weinberg.
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Five Agents of Evolutionary Change
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Genetic drift – statistical accidents.
– Frequencies of particular alleles may
change by chance alone.
 important in small populations
 founder effect - few individuals found
new population (small allelic pool)
 bottleneck effect - drastic reduction in
population, and gene pool size
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Genetic Drift - Bottleneck Effect
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Five Agents of Evolutionary Change
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Selection – Only agent that produces adaptive
evolutionary change
– artificial - breeders exert selection
– natural - nature exerts selection
 variation must exist among individuals
 variation must result in differences in
numbers of viable offspring produced
 variation must be genetically inherited
 natural selection is a process, and
evolution is an outcome
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Five Agents of Evolutionary Change
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Selection pressures:
– avoiding predators
– matching climatic condition
– pesticide resistance
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Measuring Fitness
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Fitness is defined by evolutionary biologists
as the number of surviving offspring left in
the next generation.
– relative measure
 Selection favors phenotypes with the
greatest fitness.
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Interactions Among Evolutionary Forces
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Levels of variation retained in a population
may be determined by the relative strength
of different evolutionary processes.
Gene flow versus natural selection
– Gene flow can be either a constructive or
a constraining force.
 Allelic frequencies reflect a balance
between gene flow and natural
selection.
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Natural Selection Can Maintain Variation
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Frequency-dependent selection
– Phenotype fitness depends on its frequency
within the population.
 Negative frequency-dependent selection
favors rare phenotypes.
 Positive frequency-dependent selection
eliminates variation.
Oscillating selection
– Selection favors different phenotypes at
different times.
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Heterozygote Advantage
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Heterozygote advantage will favor
heterozygotes, and maintain both alleles
instead of removing less successful alleles
from a population.
– Sickle cell anemia
 Homozygotes exhibit severe anemia,
have abnormal blood cells, and usually
die before reproductive age.
 Heterozygotes are less susceptible to
malaria.
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Sickle Cell and Malaria
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Forms of Selection
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Disruptive selection
– Selection eliminates intermediate types.
Directional selection
– Selection eliminates one extreme from a
phenotypic array.
Stabilizing selection
– Selection acts to eliminate both extremes
from an array of phenotypes.
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Kinds of Selection
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Selection on Color in Guppies
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Guppies are found in small northeastern
streams in South America and in nearby
mountainous streams in Trinidad.
– Due to dispersal barriers, guppies can be
found in pools below waterfalls with high
predation risk, or pools above waterfalls
with low predation risk.
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Evolution of Coloration in Guppies
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Selection on Color in Guppies
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High predation environment - Males exhibit
drab coloration and tend to be relatively
small and reproduce at a younger age.
Low predation environment - Males display
bright coloration, a larger number of spots,
and tend to be more successful at defending
territories.
– In the absence of predators, larger, more
colorful fish may produce more offspring.
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Evolutionary Change in Spot Number
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Limits to Selection
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Genes have multiple effects
– pleiotropy
Evolution requires genetic variation
– Intense selection may remove variation
from a population at a rate greater than
mutation can replenish.
 thoroughbred horses
Gene interactions affect allelic fitness
– epistatic interactions
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