biol 108 chp 8 - evolution and natural selection
TRANSCRIPT
Evolution andNatural Selection
BIOL 108 Intro to Bio Sci
Chapter 8
Rob SwatskiAssoc Prof Biology
HACC-York
Learning Goals
Be able to explain
evolution in action.
Be able to explain
Darwin’s journey to an
idea.
Be able to describe and explain the
four mechanisms that can give
rise to evolution.
Be able to explain how
populations of organisms can adapt to their environment
through natural
selection.
Be able to explain how the evidence
for the occurrence of
evolution is overwhelming.
2
8·1
Evolution is an ongoing
process.
Evolution in Action8.1 We can see evolution occur right before us.
Could you breed fruit flies who could live longer than 20 hours on average?
4
5
When these eggs hatch, do you think the flies in this new generation will live
longer than 20 hours without food?
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7
What Happened?
Evolution
A genetic change in the population
Natural Selection
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
population8
Does Evolution Occur?
The answer is an unambiguous YES
We can watch it happen in the lab
whenever we want.
9
Evolution
How does evolution occur?
What types of changes can
evolution cause in a population?
Five primary lines of evidence
Evolution by natural selection
10
Take-Home Message 8.1
The characteristics of individuals in a population can
change over time.
We can observe such change in nature and can even cause such
change to occur. 11
8.2–8.5
Darwin journeyed
to a new idea.
13
Darwin’s Journey to an Idea
8.2 Before Darwin, most people believed that all species had been
created separately and were unchanging. 14
15
16
Jean-Baptiste Lamarck
Early 1800s: Species evolve
through the use & disuse of body
parts
Characteristics acquired during life are inherited
Unsupported by evidence
17
Lamarck’s Hypothesis
18
19
Aren’t you glad Lamarck was wrong?
20
Charles Lyell
Principles of Geology (1830)
Charles Lyell
Early 1800s geologist:
Principles of Geology (1830)
Geological forces had shaped the earth and were continuing to do
so.
Gradual but constant change
21
22
“The mechanisms of
change are constant over
time”
Uniformitarianism
Take-Home Message 8.2
People used to think that the earth was
6,000 years old and that species were
unchanging.
We can observe such change in nature and can even cause such
change to occur.
These changes helped shape Darwin’s
thinking.
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24
Darwin’s Research
Darwin always had a deep interest in science & nature
Was a student of medicine & theology at Cambridge
5-year voyage on the Beagle after
graduation
8.3
Don’t Listen to Your Parents
“You care for nothing but
shooting, dogs, and rat-catching. You
will be a disgrace to yourself and your
family.” - DR. ROBERT DARWIN25
Voyage of the Beagle
Collected specimens of South American plants & animals
Observed adaptations to many diverse environments
Became interested in the geographic
distribution of species after a stop
at the Galápagos Islands 26
27
Take-Home Message 8.3
After initially training in medicine and theology, Charles Darwin studied the natural world on a
‘round-the-world voyage.28
8.4 Observing geographic similarities and differences among fossils and living plants and animals, Darwin
developed a theory of evolution.
29
Two important and unexpected patterns:1. Traits exhibited by species
30
2. Similarity between the fossils of extinct species and the living species in that same area
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32
Thomas Malthus
An economist who had a major influence
on Darwin
Malthus wrote about the potential for
human population to increase faster than
food supplies & other resources
33
34
Take-Home Message 8.4
Darwin noted unexpected patterns among fossils he
found and living organisms he observed while on the
voyage of the Beagle.
Fossils resembled but were not identical to the living
organisms in the same area.
Finch species on the Galapagos Islands differed
in small but significant ways.
These observations helped him to develop his theory
of how species might change over time.
35
If you had an idea thatwas going to outrage
society…
would you keep it to yourself?36
AlfredRussellWallace
37
8.5 In 1859, afterdecades of mullingand procrastinating,Darwin published histhoughts on naturalselection.
38
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Take-Home Message 8.5
After putting off publishing his thoughts on natural selection for more than 15 years, Darwin did
so only after Alfred Russel Wallace independently came up
with the same idea.
They published a joint presentation on their ideas in 1858 and Darwin published a
much more detailed treatment in The Origin of Species in 1859,
sparking wide debate and discussion of natural selection.
41
8.6–8.11
Four mechanisms can
give rise to evolution.
8.6 Evolution occurs when the allele frequencies in a population change.
43
Witnessing Evolution
Alter the population
Increase the white phenotype
through breeding
As the generations go
by…
Higher proportion of white tigers
Evolution =
Change in the allele frequencies
of a population44
Individuals do not evolve.
Populations evolve.
45
Natural Selection
An efficient mechanism of
evolution…
… and a powerful force in adapting
populations to their environment
Evolution and natural selection are not the same
thing.46
Agents of Evolutionary Change
1. Mutation
2. Genetic
drift
3. Migration
4. Natural
selection
47
48
Take-Home Message 8.6
Evolution is a change in allele frequencies
within a population.
It can occur by four different mechanisms: mutation, genetic drift, migration, and natural
selection. 49
8.7 Mutation—a direct change in the DNA of an individual—is the ultimate
source of all genetic variation.
50
51
MutationAn alteration of the base-pair
sequence of an individual’s DNA
When this alteration occurs
in a gene…
…the change in the DNA sequence
may change the allele.
53
54
Causes of Mutations
The process of cells dividing can go awry
Environmental phenomena:
mutagens
Mutations are random
Mutation is the ultimate source of genetic
variation in a population.
Nearly all mutations reduce an organism’s fitness.
Suppose that you have written a ten-page
paper.
Randomly select one
letter in the paper and
change it to another
letter
Is the change more likely
to make your paper better
or worse?
56
Take-Home Message 8.7
Mutation is an alteration of the base-pair sequence in
an individual’s DNA.
This constitutes evolution if it changes an allele the
individual carries.
Mutations can be caused by high-energy sources or
chemicals in the environment and also can
appear spontaneously.
Mutation is the only way that new alleles can be
created within a population, and so generates the
variation on which natural selection can act.
57
8.8 Genetic drift is a random change in allele frequencies
in a population.
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59
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.
60
The impact of genetic drift is much greater in small populations than in
large populations.
61
62
FixationGenetic 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.
Two special cases of genetic drift, the founder effect and population
bottlenecks, are important in the evolution of populations.
63
64
Founder Effect
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.
65
66
Take-Home Message 8.8 Genetic drift is a random
change in allele frequencies within a population,
unrelated to the alleles’ influence on reproductive
success.
Genetic drift is a significant agent of evolutionary
change primarily in small populations.
67
8.9 Migration into or out of a population may change allele frequencies.
Migration (gene flow) is the movement of some individuals of a
species from one population to another.68
Take-Home Message 8.9
Migration, or gene flow, leads to a change in allele
frequencies in a population as individuals move into or
out of the population.69
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.
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71
Condition 1: Variation for a Trait
Variation is all around us and is the raw material on which evolution feeds.
72
Condition 2: HeritabilityWe call the transmission of traits from parents to
their children through genetic information inheritance or heritability.
73
Condition 3: Differential Reproductive Success
1. There are more
organisms born than can
survive.
2. Organisms are
continually struggling for
existence.
3. Some organisms are more likely to
win this struggle &
survive and reproduce.
74
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. 75
Most agricultural pests evolve resistance to pesticides.
How does this happen?
78
Take-Home Message 8.10 Natural selection is a mechanism of evolution that occurs when there is heritable variation for a trait, and
individuals with one version of the trait have greater reproductive success than individuals with a different version of the trait.
It can also be thought of as the elimination of alleles from a population that reduce the
reproductive rate of individuals carrying them relative to the
reproductive rate of individuals who do not carry the alleles.
79
8·12–8·17 Through natural selection, populations of organisms can become adapted to their environments.
8.12 Traits causing some individuals to have more offspring
than others become more prevalent in the population.
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“Survival of the Fittest”
Reproductive success
Fitness
A measure of the relative amount of reproduction of an individual with a
particular phenotype, as compared with the reproductive output of individuals with
alternative phenotypes
Fruit Fly Example
One fly carries the genes for a
version of a trait that allows it to survive a long time without
food.
The other fly has the genes for a
different version of the trait that
allows it to survive only a
short while without food.
Which fly has the greater
fitness?
83
The alleles carried by an individual with high fitness will increase their market
share in a population over time and the population will evolve.
84
3 important elements to an organism’s fitness:
1. An individual’s fitness is
measured relative to other
genotypes or phenotypes in
the population.
2. Fitness depends on the
specific environment in
which the organism lives.
3. Fitness depends on an
organism’s reproductive
success compared to
other organisms in the
population.
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Take-Home Message 8.12 Fitness is a measure of the
relative amount of reproduction of an individual with a particular
phenotype, as compared with the reproductive output of individuals with alternative
phenotypes.
An individual’s fitness can vary, depending on the environment in
which the individual lives.
87
8.13 Organisms in a population can become better matched to their environment through natural
selection.
88
Take-Home Message 8.13
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.89
8.14 Natural selection does not lead to perfect organisms.
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Organisms aren’t “perfect”
Evolution in general, and natural selection
specifically, do not guide organisms toward “better-ness” or
perfection.
If the environment changes, the alleles
causing the traits favored by natural
selection may change, too.
Why doesn’t natural selection lead to the
productionof perfect organisms?
Factors that Prevent Populations from Progressing Inevitably toward Perfection
1. Environments
change quickly.
2. Variation is needed as the raw material of
selection.
3. There may be multiple
different alleles for a trait, each
causing an individual to
have the same fitness.
92
Take-Home Message 8.14
Natural selection does not lead to organisms perfectly adapted to
their environment because:
1. Environments can change more quickly
than natural selection can adapt organisms to
them.
2. All possible alleles are not produced by
mutation.
3. There is not always a single optimum
adaptation for an environment.
93
8.15 Artificial selection is just a special case of natural selection.
94
Take-Home Message 8.15 Animal breeders and farmers utilize natural selection when
they modify their animals and crops because the three
conditions for natural selection are satisfied.
Because the differential reproductive success is
determined by humans and not nature, this type of
natural selection is also called artificial selection.
95
8.16 Natural selection can change the traits seen in a population in several ways.
96
Directional SelectionIndividuals with one extreme from the range of variation in the population have higher fitness.
97
Stabilizing Selection
Individuals with intermediate phenotypes are most fit.
98
Disruptive SelectionIndividuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the
lowest.
99
Take-Home Message 8.16
Natural selection can change populations 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
100
8.17 Natural selection can cause the evolution of complex traits and behaviors.
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103
Often, structures appear because they
serve some other purpose.
Functional Shifts
Take-Home Message 8.17
Natural selection can change allele frequencies
for genes involving complex physiological processes and
behaviors.
This sometimes involves a trait that has been selected
for one function being modified at a later time to
serve a completely different function.
105
8.18–8.22
The evidence for
evolution is
overwhelming.
Five primary lines of evidence:
1. The fossil record
2. Biogeography
3. Comparative anatomy & embryology
4. Molecular biology
5. Laboratory & field
experiments
107
108
8.18 The fossil record documents the process of natural selection.
109
Take-Home Message 8.18
Radiometric dating confirms that the earth is very old and makes it possible to determine the age of
fossils.
Analysis of fossil remains enables biologists to reconstruct what
organisms looked like long ago, learn how organisms were related to each other, and understand how groups of
organisms evolved over time.112
8-19 Geographic patterns of species’ distributions reflect their evolutionary
histories.
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115
History Matters
Who arrived first?
Are numerous different habitats
available?
116
Take-Home Message 8.19
Observing geographic patterns of species distributions—noting similarities and differences among species living close together but in very different habitats
and among species living in similar habitats but located far from one
another—helps us understand the evolutionary histories of populations.
117
8.20 Comparative anatomy and embryology reveal common evolutionary origins.
118
Homologous Structures
119
VestigialStructures
120
Convergent Evolution
Analogous structures all developed from different original structures. 121
Take-Home Message 8.20
Similarities in how organisms look and develop shows their
common evolutionary origins.
122
8.21 Molecular biology reveals that common genetic sequences link all
life forms.
The genetic code provides our fourth line of evidence that evolution occurs.
123
124
DNA Similarities and Differences
Related vs. unrelated individuals
The more distantly you and another
individual are related…
…the more your DNA differs.
125
DNA Similarity between Two
Species
Compare their DNA sequences for
individual genes.
In Rhesus monkeys, 138 amino acids are the same as those found in human
hemoglobin.
126
Recency of Common Ancestry
Estimates of evolutionary
relatedness are made from:
Comparative anatomy,
embryology, & the fossil record
“Molecular clocks”
Take-Home Message 8.21
All living organisms are share the same genetic code.
The degree of similarity in the DNA of different species can
reveal how closely related they are and the amount of time
that has passed since they last shared a common ancestor.
127
8.22 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.
128
Changes in the Frequencyof the Various Alleles
129
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131
Take-Home Message 8.22
Replicated, controlled laboratory selection
experiments and long-term field studies of natural populations
allow us to watch and measure evolution as it occurs.
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