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Chapter 8: Evolution and Natural Selection
Darwin’s dangerous idea: evolution by natural selection
Lectures by Mark Manteuffel, St. Louis Community College
Learning Objectives

Be able to explain evolution in action.

Be able to describe and explain the four
mechanisms that can give rise to evolution.

Be able to explain how populations of organisms
become better suited to their environment
through natural selection.

Be able to explain the scientific evidence
supporting the occurrence of evolution.
Evolution in Action
8.1 We can see evolution occur right
before us.
Could you breed fruit
flies who could live
longer than 20 hours
on average?
When these
eggs hatch,
do you think
the flies in
this new
generation
will live
longer than
20 hours
without
food?
Here we have controlled
the selective environmental
pressure (1). This is really
“artificial” selection
The genetic variability (2) is
still random.
What’s the 3rd driving force
of evolution?
What happened?




Evolution – the result
• a genetic change in the population
Natural selection – a process
• the consequence of certain individual organisms in
a population being born with characteristics that
enable them to survive better and reproduce more
than the offspring of other individuals in the
population
Artificial selection – when human beings exert
the selective pressure and control reproduction.
Does this still result in evolution?
8.14 Artificial selection is just a special case of
natural selection.
8.2 Before Darwin, most people believed that
all species had been created separately and
were unchanging.
People used to think that the earth was 6,000
years old and that species were unchanging.
In the 18th and 19th centuries, scientists
began to change their beliefs.
8.4 Observing geographic similarities and
differences among fossils and living plants
and animals, Darwin developed a theory of
evolution.
Two important and unexpected patterns:
1. Finch species on the Galapagos Islands differed in small
but significant ways.
2. Similarity between the fossils of extinct species and the
living species in that same area
Scientists are still studying evolution on Galapagos.
Four mechanisms can give
rise to evolution.
1. Mutation – one source of genetic
variability
2. Genetic drift
3. Migration – Gene Flow
4. Natural selection – whole process
Evolution is genetic change in a population.
The population is the smallest unit of
evolution.
8.6 Evolution occurs when
the allele frequencies in a
population change.
As the generations go by… Higher
proportion of white tigers
Individuals may adapt but do NOT
evolve.
 Populations
evolve
 Allele frequencies involve percentages of
dominant and recessive traits within a
population.
• The Hardy-Weinberg formula:
– Is a mathematical representation of a gene pool.
– p2 + 2pq + q2 = 1
– Adds up all of the genotypes in a population.
Natural Selection
 An
efficient mechanism of evolution
 Describes the overall process
 But there are other agents of evolutionary
change
 Mutations
 Genetic Drift
 Migration – Gene Flow
8.10 When three simple conditions are
satisfied, evolution by natural selection
occurs.
1. There must be variation for the particular trait
within a population.
2. That variation must be inheritable.
3. Individuals with one version of the trait must
produce more offspring than those with a
different version of the trait.
Condition 1: Variation for a Trait
 Variation
is all around us.
 Variation is the raw material on which
evolution feeds.
 Variation in meiosis and fertilization.
Condition 2: Heritability
We call the transmission of traits from parents to
their children through genetic information
inheritance or heritability.
Condition 3: Differential Reproductive
Success
From all the variation
existing in a
population, individuals
with traits most suited
to reproduction in
their environment
generally leave more
offspring than
individuals with other
traits.
An animal naturally-suited for its environment…
8.7
Mutationa direct
change in
the DNA
of an
individual
- is the
ultimate
source of
all genetic
variation.
What causes mutations?

The process of DNA replication or cells dividing
can go awry.

Environmental phenomena
• Mutagens (chemical or physical agents)
• i.e. Tanning beds bombard the body with ultraviolet
radiation.
• i.e. Benzene, a known carcinogen

Mutations are generally random
• Beneficial – sometimes
• Detrimental – more likely to lower the reproductive
success of the organism
8.8 Genetic drift is a random change in allele
frequencies in a population.
The important factor that
distinguishes genetic drift from
natural selection:
The change in allele frequencies is not
related to the alleles’ influence on
reproductive success.
The impact of genetic drift is much
greater in small populations
(microevolution) than in large
populations.
Genetic Drift…
Only 5 of
10 plants
leave
offspring
Generation 1
p (frequency of R) = 0.7
q (frequency of r) = 0.3
Only 2 of
10 plants
leave
offspring
Generation 2
p = 0.5
q = 0.5
Generation 3
p = 1.0
q = 0.0
Fixation
 Genetic
drift can lead to fixation for one
allele for a gene in a population.
 If
this happens, there is no more
variability in the population for this gene.
 Genetic
drift reduces the genetic
variation in a population.
Founder Effect: A special case of
Genetic Drift
A
small number of individuals may leave a
population and become the founding
members of a new, isolated population.
 The
founders may have different allele
frequencies than the original “source”
population, particularly if they are a small
sample.
Why are Amish people more likely to
have extra fingers and toes?
Bottleneck Effect: A special case of
Genetic Drift
8.9 Migration into
or out of a
population may
change allele
frequencies. Also
called Gene Flow
Gene Flow
– Causes genetic exchange
with another population
– Tends to reduce genetic
differences between
populations
– May offer stronger
combinations of genes
– This magazine cover
celebrates our changing
gene pools with a computer
generated image blending
facial features from several
different races.
Most agricultural pests evolve
resistance to pesticides.
How does this happen?
Spraying crops
with insecticides
ends up favoring
the reproductive
success of insects
with genetic
resistance to the
poison.
Will GMO’s that
are modified to
contain pesticides
in their cells favor
resistance?
Figure 13.1
8.12 Organisms in a population can become
better matched to their environment
through natural selection.
Take-home message 8.12
 Adaptation—the
process by which
organisms become better matched to their
environment and the specific features that
make an organism more fit—occurs as a
result of natural selection.
 Individuals can also adapt through
changes in behavior, but this is not a
result of natural selection.
Alex has adapted to his environment, but can he pass this
on to the next generation?
8.13 Natural selection does not lead to
perfect organisms.
Environments change quickly.
8.15 Natural selection can change the
traits seen in a population in several
ways.
1. Directional selection, in which the average
value for the trait increases or decreases
2. Stabilizing selection, in which the average
value of a trait remains the same while
extreme versions of the trait are selected
against
3. Disruptive selection, in which individuals with
extreme phenotypes have the highest fitness
Evolution occurs in only one
direction.
Stabilizing Selection
Individuals with intermediate phenotypes are most fit.
Disruptive Selection
Individuals with extreme phenotypes experience the
highest fitness, and those with intermediate phenotypes
have the lowest.
Co-evolution = mutually beneficial pressures drive evolution
Five primary lines of evidence:
1. The fossil record
2. Biogeography
3. Comparative anatomy and embryology
4. Molecular biology
5. Laboratory and field experiments
8.17 The fossil record documents
the process of natural selection.
Radiometric dating confirms that the earth is very old and
makes it possible to determine the age of fossils.
8-18. Geographic patterns of
species’ distributions reflect their
evolutionary histories.
Convergent Evolution = similar traits found in unrelated
populations in different geographical areas
Another example of convergent evolution
8.19
Comparative
anatomy and
embryology
reveal
common
evolutionary
origins.
Homologous Structures
A common vertebrate ancestor…
Vestigial Structures
Convergent Evolution
Analogous
structures all
developed from
different original
structures.
These animals are
not linked by a
common ancestor.
8.20 Molecular biology reveals
that common genetic sequences
link all life forms.
The genetic code provides our fourth
line of evidence that evolution occurs.
The more distantly you and another
individual are related, the more your DNA
differs.
8.21 Laboratory and field experiments
enable us to watch evolution in
progress.
A fifth line of
evidence for the
occurrence of
evolution comes
from multigeneration
experiments and
observations.