Transcript Genetic drift is a change in allele frequencies due to chance.

11.1 KEY CONCEPT
A population shares a common gene pool.
 Genetic
variation leads to phenotypic variation.
 Phenotypic variation is necessary for natural selection.
 Genetic variation stored in gene pool.
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made up of all alleles in a population
Offspring = new allele combinations
 Allele
frequencies measure genetic variation.
– how common allele is in population
– can be calculated for each allele in gene pool
1. Calculate the allele
frequency for
G(Green frogs) in
the population
2. Calculate the allele
frequency for g
(brown frogs) in
the population
 Mutation
is a random change in the DNA of a gene.
– can form new allele
–How can mutations be
passed on to offspring?
• Recombination forms new combinations of alleles.
– usually occurs during meiosis, What is the process called?
– parents’ alleles
arranged in new
ways in gametes
 Hybridization
is the crossing of two different
species.
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occurs when individuals can’t find mate of own species
Much more successful/common in plants
topic of current scientific research
 Why
aren’t mutations in a skin or kidney cell
sources of genetic variation?
 If you have 10 individuals, 5 homozygous
recessive and 5 heterozygous, what is the
allele frequency for each allele?
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A normal distribution graphs as a bell-shaped curve.
– highest frequency near
mean value
– frequencies decrease
toward each extreme
value
• Why is this curve called
“normal”?
• Traits not undergoing
natural selection have a
normal distribution. Why?
 Microevolution
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is evolution within a population.
observable change in the allele frequencies
can result from natural selection
How does natural selection cause a change in allele
frequencies?
 Natural
paths.
selection can take one of three
– Directional selection favors phenotypes at one
extreme.
– Bacteria, Greyhounds
 Natural
paths.
selection can take one of three
– Stabilizing selection favors the
intermediate phenotype.
– Gall flies, Siberian Huskies
– What happens to the allele frequency
in this distribution?
 Natural
selection can take one of
three paths.
– Disruptive selection favors both
extreme phenotypes.
– What happens if the middle cuts off
completely?
 Think
of an example for each of the types of
distributions for natural selection
Use a deck of cards to represent a population of island
birds. The four suits represent different alleles for tail
shape.
 What would be the allele frequencies in the original
population?
 What are the allele frequencies of each suit when only
40 cards are chosen?
 Suppose a few birds are blown by a storm to a new
island. If we reshuffle the deck and only choose 10
alleles (how many birds does that represent?) what are
the new allele frequencies?
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 Gene
flow occurs when
individuals join new
populations and reproduce.
 Gene flow keeps neighboring
populations similar.
 Low gene flow increases the
chance that two populations
will evolve into different
species.
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What is a species?
Why would low gene flow
create new species?
bald eagle migration
 Genetic
drift causes a loss of genetic diversity.
 It is most common in small populations. Why?
 A population bottleneck can lead to genetic drift.
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It occurs when an event
drastically reduces
population size.
Example?
The bottleneck effect is
genetic drift that occurs
after a bottleneck event.
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The founding of a small population can lead to genetic drift.
– It occurs when a few individuals start a new population.
– Can you think of an example of when this might occur?
– The founder effect is genetic drift that occurs after start
of new population.
 Genetic
drift has negative effects on a population.
– less likely to have some individuals that can adapt
– Why does that occur more in small population?
– Penny Activity:
Flip the penny 3 times, record how many head and
how many tails you flipped.
Flip the penny 17 more times, record how many
heads and tails you flipped.
Which results were closer to the 1:1 ratio you expect
to get?
– harmful alleles can become more common due to
chance
 Sexual
selection occurs
due to higher cost of
reproduction for
females.
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males produce many
sperm continuously
females are more limited
in potential offspring each
cycle
Result: Females are
picky!
 There
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are two types of sexual selection.
intrasexual selection: competition among males
– Example: lions
intersexual selection: males display certain traits to
females
– Example: birds
These birds have
huge red air sacs
which make them
easier for
predators to spot.
Why would they
have evolved
these, then?
Male Irish elks,
now extinct, had
12-foot antlers.
Describe how
sexual selection
could have caused
such an
exaggerated trait
to evolve.
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Populations become isolated when there is no gene flow.
 Isolated populations adapt to their own environments.
 Genetic differences can add up over generations.
Two small, isolated
populations of dolphins in Tin
Can Bay and the Great Sandy
Strait are at risk of extinction,
and are the focus of a study by
Southern Cross University
researcher Daniele Cagnazzi.
Mr Cagnazzi has been
studying dolphins along the
Queensland coast for the last
three years.
~ABC News
 Reproductive
isolation can occur between isolated
populations.
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members of different populations cannot mate successfully
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Prezygotic: temporal isolation, behavioral isolation
Postzygotic: hybrid sterility
final step to becoming separate species
 Speciation
is the rise of two or more species from one
existing species.
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Behavioral barriers can cause isolation.
 called behavioral isolation
 includes differences in courtship or mating behaviors
 Geographic
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called geographic isolation
physical barriers divide population
 Temporal
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barriers can cause isolation.
barriers can cause isolation.
called temporal isolation
timing of reproductive periods prevents mating
 Natural
selection can have direction.
 The effects of natural selection add up over time.
What examples can you think of
of convergent evolution?
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Divergent evolution describes evolution toward different traits in
closely related species.
red fox
kit fox
ancestor
Other examples of divergent
evolution?
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Two or more species can evolve together through coevolution.
 evolutionary paths become connected
 species evolve in response to changes in each other
 Coevolution
can occur in beneficial relationships.
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Coevolution can occur in competitive relationships, sometimes called
evolutionary.
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Extinction is the elimination of a species
from Earth.
Background extinctions occur continuously
at a very low rate.
 Same rate as speciation
 Few species in small area
 caused by local changes in environment
 Mass
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extinctions are rare but much more intense.
destroy many species at global level
catastrophic events
5 in last 600 million years
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A pattern of punctuated equilibrium exists in the fossil record.
 theory proposed by Eldredge and Gould in 1972
 Episodes of speciation occur suddenly followed by periods of
little change
 revised Darwin’s idea
adaptation
 population
 vestigial structure
 homologous structure
 analogous structure
 gradualism
 catastrophism
 uniformitarianism
 natural selection
 artificial selection

gene pool
 allele frequency
 genetic drift
 founder effect
 bottleneck effect
 coevolution
 divergent evolution
 convergent evolution
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Paleozoic, Mesozoic, and Cenozoic Eras
half-life
5 types of fossils
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Charles Darwins’ observations on Galapagos Islands
Scientists and their contributions to evolution: Lamarck,
E.Darwin, Linnaeus
4 principles of natural selection and their application
(adaptation, variation, overproduction, descent with
modification
Reasons for genetic variation (mutations, recombination,
etc.)
3 types of selection (directional, disruptive, stabilizing)
and application
Isolations: reproductive, behavioral, temporal, geographic
Labs for this Unit: Beak Lab, Lethal Alleles Lab (Tigers born
without fur, two colors of M&M’s)– know the basic
procedure and what principles of evolution the lab
demonstrated, if you weren’t here for a lab, check with
someone else to get the overall idea of what we did