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Chapter 23- Evolution of populations
Chapter 24Origin of species
Blue edged slides taken from slide shows by: Kim Foglia
http://www.explorebiology.com
Individuals are selected
Populations evolve
AP Biology
2007-2008
REZNICK & ENGLER- (1980’s)
Guppy experiments
IMAGE FROM Campbell and Reece AP BIOLOGY
Changes in populations happened within 11 years
Age and size at sexual maturity change
depending on predators
Small killifish eat juvenile guppies
Large pike-cichlids eat adult guppies
Guppies in populations with pike-cichlid predators
begin reproducing at a younger age and are smaller
at maturity than guppies in populations preyed on by
killifish
IMAGE FROM Campbell and Reece AP BIOLOGY
Moving guppies to pools with different
predators changes size and age of maturity
in population
IMAGE FROM Campbell and Reece AP BIOLOGY
Changes are heritable
Changes in populations
Pesticide
molecule
Resistant
target site
Target site
Insect cell
membrane
Target site
Decreased number of target sites
Insecticide resistance
AP Biology
2005-2006
Body size & egg laying in water striders
Fitness
 Survival & Reproductive
success

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individuals with one
phenotype leave more
surviving offspring
Variation & natural selection
 Variation is the raw material for natural
selection

there have to be differences within population

some individuals must be more fit than others
AP Biology
Where does Variation come from?

random changes to DNA
 errors in mitosis & meiosis
Wet year
Beak depth
 Mutation
Dry year
Dry year
 environmental damage
1977
 Sex
mixing of alleles
 recombination of alleles
 new arrangements in every offspring
 new combinations = new phenotypes

spreads variation
 offspring inherit traits from parent
AP Biology
1980
1982
1984
11
Beak depth of
offspring (mm)

Dry year
10
9
8
Medium ground finch
8
9
10
11
Mean beak depth of parents (mm)
5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
1. Mutation & Variation
 Mutation creates variation

new mutations are constantly appearing
 Mutation changes DNA sequence
changes amino acid sequence?
 changes protein?

 changes structure?
 changes function?

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changes in protein may
change phenotype &
therefore change fitness
 A diverse gene pool is important for the
survival of a species in a changing
environment

Environments can more stable, less
stable or fluctuating
 This affects evolutionary rate and direction
 Different genetic variations can be selected
in each different generation
AP Biology
2. Gene Flow
 Movement of individuals &
alleles in & out of populations
seed & pollen distribution by
wind & insect
 migration of animals

 sub-populations may have
different allele frequencies
 causes genetic mixing
across regions
 reduce differences
between populations
AP Biology
Human evolution today
 Gene flow in human
populations is
increasing today

transferring alleles
between populations
Are we moving towards a blended world?
AP Biology
3. Non-random mating
 Sexual selection
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SEXUAL SELECTION
Favors traits with no advantage for survival
other than fact that males/females prefer them
Leads to pronounced differences between sexes
=SEXUAL DIMORPHISM
http://informalfotos.com/Fauna/Male%20Peacock%20displaying.JPG
http://www.distinctivecruises.com/AfricanSafaris/MaleFemaleLion.jpg
http://espanol.wunderground.com/data/wximagenew/i/icmoore/1013.jpg
Image from: http://www.kittens-lair.net/store/en/articles/sylvester4.jpg
KIN SELECTION
Natural selection that favors altruistic
behavior by enhancing reproductive
success of relatives
Bird that calls to warn others is in danger of being eaten,
but does it anyway.
4. Genetic drift
 Effect of chance events

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces population to
small number & then population recovers &
expands again
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 Reduction of genetic variation within a
given population can increase the
differences between populations of the
same species.
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Founder effect
 When a new population is started
by only a few individuals
some rare alleles may be at high
frequency; others may
be missing
 skew the gene pool of
new population

 human populations that
AP Biology
started from small group
of colonists
 example:
colonization of New World
Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance event

 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
AP Biology
Cheetahs
 All cheetahs share a small number of alleles
less than 1% diversity
 as if all cheetahs are
identical twins

 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
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Conservation issues
 Bottlenecking is an important
Peregrine Falcon
concept in conservation
biology of endangered
species
loss of alleles from gene pool
 reduces variation
 reduces adaptability

Breeding programs must
consciously
outcross
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Golden Lion
Tamarin
5. Natural selection
 Differential survival & reproduction due
to changing environmental conditions
 climate change
 food source availability
 predators, parasites, diseases
 toxins

combinations of alleles
that provide “fitness”
increase in the population
 adaptive evolutionary change
AP Biology
POLYGENIC traits are controlled by
two or more genes.
A bell shaped curve is
typical of polygenic traits
Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006
Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006
DIRECTIONAL SELECTION
KEY
Food becomes scarce.
Low mortality,
high fitness
High mortality,
low fitness
Individuals at one end of the curve have higher
fitness than individuals in middle or at other end.
Graph shifts as some individuals fail to survive
at one end and succeed and reproduce at other
EXAMPLE OF DIRECTIONAL SELECTION
Beak size varies in a population
Birds with bigger beaks can feed
more easily on harder, thicker
shelled seeds.
A food shortage causes small and
medium size seeds to run low.
Birds with bigger beaks would be
selected for and increase in numbers
in population.
http://www.animalbehavior.org/ABS/Stars/ONI/Podos_-_finch_graphic.jpg
Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006
STABILIZING SELECTION
Individuals in center of the curve have higher
fitness than individuals at either end
Graph stays in same place but narrows as
more organisms in middle are produced.
STABILIZING SELECTION
Section 16-2
Male birds use
their plumage to
attract mates.
Male birds with
less brilliant and
showy plumage
are less likely to
attract a mate
Male birds with
showy plumage
are more likely to
attract a mate.
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Brightness of
Selection
against both
extremes keep
curve narrow
and in same
place.
Male birds with
showier, brightly
colored plumage
also attract
predators, and
are less likely to
live long enough to
find a mate.
The most
fit is male
bird in the
middle-showy, but not
too showy.
Feather Color
Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006
EXAMPLE OF STABILIZING SELECTION
Human babies born with low birth
weight are less likely to survive.
Babies born too large have difficulty
being born.
Average size babies are selected for.
Graph from BIOLOGY by Miller and Levine; Prentice Hall Publshing©2006
DISRUPTIVE SELECTION
Individuals at extremes of the curve
have higher fitness than individuals in middle.
Can cause graph to split into two.
Selection creates two DIFFERENT PHENOTYPES
EXAMPLE OF DISRUPTIVE SELECTION
A bird population lives in area
where climate change causes medium
size seeds become scarce while large
and small seeds are still plentiful.
Birds with bigger or smaller beaks
would have greater fitness and the
population may split into TWO
GROUPS. One that eats small
seeds and one that eats large seeds.
http://www.animalbehavior.org/ABS/Stars/ONI/Podos_-_finch_graphic.jpg