Download Evolution of Populations

Flies in the Barnyard:
On a sheet of
paper write 1-2 sentences explaining why you
think this happened 
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A farmer was working with dairy cattle at an agricultural
experiment station. The population of flies in the barn where the
cattle lived was so large that the animals’ health was affected. So
the farmer sprayed the barn and the cattle with a solution of
insecticide A. The insecticide killed nearly all the flies.
Sometime later, however, the number of the flies was again large.
The farmer again sprayed with the insecticide. The result was
similar to that of the first spraying. Most, but not all of the flies
were killed.
Again within a short time the population of flies increased, and
they were again sprayed with the insecticide. This sequence of
events was repeated five times; then it became apparent that
insecticide A was becoming less effective in killing the flies
Evolution
Chapters 15 - 17
Important People in Evolution

James Hutton (1785) –
Proposed that Earth was
shaped by geological forces
that took place over
extremely long periods of
time. He estimates the
Earth to be millions of
years old.

Charles Lyell (1833) wrote Principles of
Geology
Thomas Malthus (1798)
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Predicted the human population would
grow faster than the space and food
supplies needed to sustain them.
Jean-Baptiste Lamark (1809)
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Proposed that
organisms changed
over time due to use
and disuse of certain
organs/parts. The
traits that were “used”
in an organism’s
lifetime could be
passed on to their
offspring.
His theory was flawed!
Lamarck v/s Darwin
Alfred R. Wallace (1858)

Darwin’s old assistant;
challenged Darwin’s
thinking, and wrote his
“own” essay on the theory
of evolution.

HE was the reason Darwin
reported his research!!!
All of these ideas influenced
Darwin’s thinking…
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Charles Darwin (1831) – Set
sail on the H.M.S. Beagle.
Voyage provided him with
much of the data and
evidence that lead to his
theory of evolution.
Darwin’s “Theory of
Evolution” would turn the
scientific world upside
down!!!!
What did Darwin Observe?
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Extreme diversity
existed among plant and
animal species around
the world.
The organisms found
within a certain area
were well suited to
survive in that
environment.
Living organisms he
observed greatly
resembled fossils that
he found on his voyage.

This tiny group of islands
influenced Darwin the
most!
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Although the islands were
close together, the climates
and environments differed
greatly.
Similar organisms found on
different islands had varying
characteristics.
Examples:
 Giant Galapagos Tortoises
with varying shell shape.
 Darwin’s Finches – beak
shape differed among the
birds.
The
Galapagos
Islands
Darwin Presents his Theory of
Evolution…
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1859 – Darwin published his work:
On the Origin of Species
What is a species? A group of organisms that can
interbreed and produce fertile offspring.
What is natural variation? Differences among
individuals of a species.
Humans had been employing “artificial selection” for
many years!

Selection by humans for breeding of useful traits from the
natural variation among the organisms (remember selective
breeding )
Adaptations &
Darwin’s Evidence
Day 2
Evolution by Natural Selection

Struggle for Existence

Survival of the “Fittest” – Fitness results from
adaptations that give an organism advantages for
survival. The most “fit” organisms will survive and
reproduce; passing along the advantageous
characteristics to their offspring. These changes
can only be seen after many generations!

Adaptation – a trait that makes living things better
able to survive in their surroundings.
Three Types of Adaptations

Physiological Adaptations
- traits that involve the
internal functions or
chemistry of an organism
(ex: weeds resistant to
pesticides, bacteria resistant
to antibiotics, etc.)

Think…”Chemical” or
“enzyme”
Structural Adaptations

Structural Adaptations –
traits that involve the physical
structure or anatomy of an
organism (ex: quills of
porcupine; mimicry,
camouflage, etc
Some other structural adaptations
are subtle…..MIMICRY
Mimicry enables one species to resemble
another species.
THE MOTH, LEAF INSECT, & STICK BUG ARE GREAT
EXAMPLES OF MIMICRY!!!
Behavioral Adaptations

Behavioral Adaptations – inherited traits
that help an organism survive and
reproduce in a given environment; a
behavior, response, or action (ex: living in
hunting groups)
Descent with Modification

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As organisms change over time; they
become different, resulting in many varied
species. This illustrates “common descent.”
All living things have a common ancestor.
Ex: Owl  White Mouse  Black mouse!
So…
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All species – living and extinct –were derived
from common ancestors.
This concept is called COMMON DESCENT.
Descent with modification + common descent
leads to the conclusion that there is a single
“tree of life” that links all living things on
Earth”
So, What is the proof all this
happens?

Darwin argued that living things have been
evolving on Earth for millions of years and the
Evidence Used to Support his Evolution
Theory was based on:
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Fossil Record
Geographic Distribution of living species
Homologous Body Structures
Similarities in early development
Biochemical Evidence
1.
Fossil Record dead remains of an
organism that has
left an imprint in
sedimentary rock
and/or clay.
2.
Geographic
Distribution of
Living Species Species evolve
differently based on
where they live.
3.
5.
Homologous Body
Structures – traits
from a common
embryo (one of the
first stages of
development).
Biochemical
Evidence – common
DNA sequences that
relate to GENES!!!
Similarities in
Early Development
4.
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Vestigial Organs –organ
or structure you no longer
need (or has no known
function  ex: appendix).
Homologous Structures vs.
analogous structures (add to your
notes…sorry I forgot to add in )
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Homologous structures are
similar structures occurring in
different species that are
believed to be derived from a
common ancestor.
Analogous structures are
similar structures occurring in
different species that are
believed to be the result of
convergent evolution.
The Three Types of
Natural Selection
Evolution & Genetics
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Darwin had a disadvantage when he
developed his theory of evolution…he did not
understand the mechanisms of heredity.
Today, we understand how genes, heredity,
and evolution all tie together.
Single-Gene Traits vs. Polygenic Traits
 Both lead to evolution, but polygenic traits
(wide variety of phenotypes possible) lead to
a more complex process of natural selection.
Three effects of selection on a
characteristic:
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1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
Directional Selection
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When individuals at one end of the population
curve have higher fitness than individuals in the
middle or at either end of the population curve.
Causes entire curve to move as character trait
changes
Ex:
beak size of Galapagos finches
peppered moths
 antibiotic
resistance
Directional Selection
Section 16-2
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Stabilizing Selection
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When individuals near the center of the curve
have higher fitness than individuals at either end
of the curve
Intermediate forms of a trait are favored and
alleles that specify extreme forms are eliminated
from a pop.
Counteracts the effects of mutation, gene flow, and
genetic drift – preserves the most common
phenotypes.
Ex. Weight of human babies at birth
Stabilizing Selection
Section 16-2
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Disruptive Selection
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When individuals at the upper and lower ends of
the curve have higher fitness than individuals near
the middle.
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Forms at both ends of the range of variation are
favored and intermediate forms are selected against
– selection creates two, distinct phenotypes
Ex: Bird beak size – no middle sized seeds, only
large seeds and small seeds; thus, small and large
beaks are favored
Disruptive Selection
Section 16-2
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
EVOLUTION & RESISTANCE
Evolution of Insecticide
Resistance
1. By spraying crops with poisons to kill
insects, humans have unwittingly
favored the reproductive success of
insects with inherent resistance to
poisons.
2. Resistant individuals survive and
reproduce, passing the gene for
resistance to offspring.
3. Additional applications of the same
insecticide will be less effective, and the
frequency of resistant insects in the
population will grow.
The Evolution of Drug
Resistant HIV
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Researchers developed numerous drugs to
combat HIV, but using these medications
selects for viruses resistant to the drugs
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A few drug-resistant viruses may be present by
chance at the beginning of treatment
The drug-resistant pathogens are more likely to
survive treatment and pass on the genes that
enable them to resist the drug to their offspring
As a result, the frequency of drug resistance in
the viral population rapidly increases
Antiviral Resistance
Evolution & Genetics
Day 4
Genes & Variation
While developing his theory of evolution,
Darwin did not know how heredity worked
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Without understanding heredity, Darwin was
unable to explain 2 important factors:
1. The source of variation central to his theory
2. How hereditable traits were passed from one
generation to the next
Today, genetics, molecular biology, and
evolutionary theory work together to explain
how evolution takes place
Gene Pools
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A gene pool is the combined genetic
information of all the members of a
particular population
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Recall that a population is a collection of
individuals of the same species in a given area
which share a common group of genes
The relative frequency of an allele is the
number of times that allele occurs in a
gene pool compared to the number of times
other alleles occur
For example…
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Gene pool
would be a
collection of all
of the options.

R.F. would be
how often a
particular gene
shows up
Sources of Genetic Variation
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The two main sources of genetic variation are
mutations and the genetic shuffling that
results from sexual reproduction
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A mutation is any change in a sequence of DNA
Most inheritable differences are the result of gene
shuffling that occurs during sexual reproduction
Genetic Drift
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In addition to natural selection, genetic drift is a way
by which allele frequencies can change
In the real world, population sizes fluctuate
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Because populations fluctuate in size, sometimes there can
be changes in allele frequencies due to random chance
These changes are called random genetic drift
In small populations, individuals that carry a particular
allele may leave more descendants than other
individuals, just by change

Over time, a series of chance occurrences of this type can
cause an allele to become common in a population
The Power of Genetic Drift
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Genetic drift is a powerful force when a
population size is very small
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Can and does lead to allele fixation
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Depends on starting frequency (which allele
becomes fixed)
Consequences of Genetic Drift
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Consequences of genetic drift:
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Can and does lead to fixation of alleles
Effect of chance is different from population to
population
Small populations are effected by genetic drift more
often than larger ones
Given enough time, even in large populations genetic
drift can have an effect
Genetic drift reduces variability in populations by
reducing heterozygosity
Real World Examples of Genetic Drift
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The Bottleneck Effect
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Occurs when only a few
individuals survive a
random event, resulting in
a shift in allele
frequencies within the
population
Small population sizes
facilitate inbreeding and
genetic drift, both of which
decrease genetic
variation
Real World Examples of Genetic Drift
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The Founder Effect
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Occurs when individuals
from a source population
move to a new area and
start a new population
This new population is
often started by relatively
few individuals that do
not represent the
population well in terms
of all alleles being
represented
So…
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What determines which variants survive
the event or get to the new location?
Random chance
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Genetic drift has the largest effect on small
populations (10-100 individuals)
Speciation
Day 6
Speciation
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Isolating Mechanisms = Reproductive
Isolation = Formation of a Species
In other words, Isolating mechanisms are objects
that separate species and prevent them from
reproducing.
By isolating a species  a NEW species can be
formed!!!
3 Major Types of Isolation:

Behavioral Isolation – Two separate
species are capable of interbreeding, but they
do not because of behavioral differences.
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Example  Meadowlarks,
different songs to attract mates;
courtship (dating)
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Geographic Isolation –
When one species
becomes separated by
some barrier (river,
mountain, road, mall,
etc.) they may develop
into two distinct species
over time.
Example  Squirrel
population split by
Colorado River 10,000
years ago
Temporal Isolation…
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This mechanism occurs
when species
reproduce at different
times (different mating
times) or seasons.
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Example Orchids in
the rainforest, only
release pollen 1 day!