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.

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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?

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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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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

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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

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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

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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

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“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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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

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2. Similarity between the fossils of extinct species and the living species in that same area

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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

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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.

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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

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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.

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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.

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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.

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8.8 Genetic drift is a random change in allele frequencies

in a population.

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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.

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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.

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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.

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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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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.

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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.

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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.

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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.

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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.

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Disruptive SelectionIndividuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the

lowest.

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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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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

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8.18 The fossil record documents the process of natural selection.

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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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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.

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DNA Similarities and Differences

Related vs. unrelated individuals

The more distantly you and another

individual are related…

…the more your DNA differs.

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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.

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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.

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Changes in the Frequencyof the Various Alleles

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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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