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MECHANISMS FOR
EVOLUTION
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Objectives
– State the Hardy-Weinburg theorem
– Write the Hardy-Weinburg equation and be
able to use it to calculate allele and genotype
frequencies
– List the conditions that must be met to maintain
Hardy Weinburg equilibrium
VOCABULARY
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POPULATION
SPECIES
GENE POOL
GENE FLOW
BOTTLENECK
EFFECT
FOUNDER EFFECT
HETEROZYGOTE
ADVANTAGE
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HYBRID VIGOR
STABILIZING
SELECTION
DIRECTIONAL
SELECTION
DIVERSIFYING
SELECTION
SEXUAL
DIMORPHISM
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POPULATION
– Localized group belonging to the same species
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SPECIES
– Naturally breeding group of organisms that
produce fertile offspring
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GENE POOL
– Total aggregate of genes in a population at any
one time
Most species are not evenly distributed over a
geographic range. Individuals are more likely
to breed with others from their population
center
HARDY – WEINBURG THEOREM
Describes a NON-EVOLVING population
 In the absence of other factors the
segregation and recombination of alleles
during meiosis and fertilization will not
alter the overall genetic make-up of a
population
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Imagine an isolated
wildflower population
with the following
characteristics
– Diploid with both pink
and white flowers
– Pink is dominant A and
white is recessive a
– There are 480 pink
flowers and 20 white
» 320 are AA
» 160 are Aa
(p + q)2 = 1
p+q=1
p2 + 2pq +q2 = 1
p2 = frequency of AA
 2pq = frequency of Aa
 q2 = frequency of aa
 Calculate q2 first
 There are 1000 alleles
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– AA -- 320 x 2/plant = 640
– Aa --160 x 1/plant = 160
800
– aa -- 20 x 2/plant = 40
– Aa – 160 x 1/plant =160
–
200
– Frequency of A = 80% and a = 20%
Condition for Hardy-Weinburg
 Large
population
 No net mutation
 Isolated population
 Random mating
 No natural selection
MICROEVOLUTION
LEADS TO
MACROEVOLUTION
CAUSES OF
MICROEVOLUTION
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GENETIC DRIFT
– BOTTLENECK EFFECT
– FOUNDER EFFECT
Bottleneck
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Population bottlenecks occur when a
population's size is reduced for at least one
generation. Because genetic drift acts more
quickly to reduce genetic variation in small
populations, undergoing a bottleneck can
reduce a population's genetic variation by a
lot, even if the bottleneck doesn't last for
very many generations.
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.
Founders
Founder effects
A founder effect occurs when a new colony
is started by a few members of the original
population. This small population size
means that the colony may have:
 reduced genetic variation from the original
population.
 a non-random sample of the genes in the
original population.
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An example of Founders Effect
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the Afrikaner population of Dutch settlers in
South Africa is descended mainly from a few
colonists. Today, the Afrikaner population has
an unusually high frequency of the gene that
causes Huntington's disease, because those
original Dutch colonists just happened to carry
that gene with unusually high frequency. This
effect is easy to recognize in genetic diseases,
but of course, the frequencies of all sorts of
genes are affected by founder events.
GENE FLOW – genetic exchange
between populations due to migration
 Mutation – a new mutation that is
transmitted in a gamete can immediately
change the gene pool
 NONRANDON MATING
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– Breed with other members of the
“neighborhood” promotes inbreeding
– Assortative mating – mate with others like
themselves
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NATURAL SELECTION
Variation within Populations
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Most heritable variation is measured by
– Quantitative characters (vary along a
continuum ie. Height) are polygenetic
– Discrete characters (pink or white) are
located on a single gene
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Polymorphism – two or more forms of a
discrete character are represented in a
population
GEOGRAPHICAL VARIATION
A cline is a graded change in some trait along a
geographical axis.
MODES OF SELECTION