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FOCUS
Context
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4 12
What causes the changes in a species that
you studied in the last Focus? How do the
adaptations you studied in Focus 4.10 develop
in organisms? Why are some creatures similar
to others, such as the creatures with the
The process of natural selection
Focus 4.10 discussed adaptations and Focus 4.11 was
about evolutionary changes in a species. Scientists
have decided that both adaptations and evolutionary
changes occur by a process called natural selection.
Natural selection is the process in which some
environmental factor acts on a population and results
in some organisms in the population having more
surviving offspring than others. The environmental
factor that causes this is called the selective agent. The
selective agent usually (but not always) acts by killing
individuals that do not have features that are able to
cope with it. Those that survive pass on their features
to the next generation. So the next generation inherits
those features which can cope with the selective agent.
We say that the ones that survive have been selected.
Natural selection can only happen if there are
inherited differences between individuals in the
species. This difference is called variation. The
differences in your friends, such as height, natural
hair colour and length of legs, are variations.
Variations in different people due to inherited differences
vertebrate skeletons you looked at in Focus 4.10?
These are questions about a process first proposed
by a famous scientist called Charles Darwin. He
suggested that the changes were due to a process
which he called natural selection.
The selective agents may be a biotic factor
such as predation, bacterial or viral infections and
competition. The selective agent could also be a
physical factor such as temperature, mineral content
of the soil, wind speed or fire.
Evidence for natural selection
One of the first studies to collect evidence for
natural selection was conducted earlier last century
in England. A scientist called Kettlewell studied a
species of moth called the peppered moth. He found
that it existed in two forms. The normal colour was
white with black specks, although occasionally allblack moths were born. The different types of moths
are shown in Figure 4.12.2.
Kettlewell found that in the cities almost all the
peppered moths were black. In the country almost all
the peppered moths were white. He found that the
Fig 4.12.1
Fig 4.12.2
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Peppered moths against a dark background
reason for this difference was a selective agent acting
on the populations. The selective agent was a type of
bird that ate peppered moths.
In the cities all the buildings and tree trunks had
been blackened by centuries of industrial pollution.
Burning coal had polluted the environment. Any
white moths resting on the trees could be seen more
easily seen than the black moths. This is shown
in Figure 4.12.2. So the birds continually removed
white moths from the population and the population
eventually became mainly black. This was because the
black moths produced black offspring. So the black
form is considered an adaptation to the polluted city
environment.
In the country the white moths were harder to
see because most of the tree trunks were a light
colour. The black moths were eaten more often in this
situation so the population became nearly all white.
This is shown in Figure 4.12.3
The peppered moth on a light background
as bacteria and insects. The bacteria have
become resistant to antibiotics and many
insects are now resistant to insecticides.
Prac 1
p. 203
Homework book 4.17 Natural selection
FOCUS
4 .12
Natural selection in insects
In the middle of last century agricultural scientists
tried to reduce the crop damage due to insects. They
developed chemicals that kill insects. These are called
insecticides. The insecticides were sprayed onto insects
in the crop. The scientists thought they would kill
all the insects and that food production would rise
greatly. This happened, but only for a few years. Then
the scientists noticed that they had to use stronger and
stronger insecticides to kill the insects, and that they
did not kill them all. Eventually the sprays had no
effect. None of the insects died. The scientists called this
ability of insects to survive the poison spray resistance.
Fig 4.12.3
Spraying crops with insecticides to kill
insect pests
Kettlewell carried out experiments in which he
observed birds feeding on both forms of moth on
tree trunks. He counted the numbers of light and
dark moths eaten. The results showed that the birds
ate mostly the dark moths on the light-coloured tree
trunks. This experiment supported the results of his
earlier observations of the proportions of the light
and dark moths in the country. Since Kettlewell’s
experiments this pattern has been found in several
other moth species as well.
Natural selection highlights how much the
environment affects living organisms. It has been
shown that it can change the features of a species
over a long period of time. Natural selection in recent
times has been demonstrated in many organisms, such
Fig 4.12.4
The development of resistance to insecticides by
insects is a good example of natural selection. The
resistance was due to some insects within a species
being able to destroy the poison in their bodies. They
had inherited cell chemistry which could do this.
So there was variation in cell chemistry.
The resistant insects survived when the population
was sprayed, but the other insects all died. The
resistant insects then bred and produced offspring,
most of which were also resistant. Any new insects
moving into the area to feed were probably mainly
not resistant. So the next time the farmer sprayed
there would be a larger proportion of resistant insects
on the crops. These survived and bred. This process
happened every time the farmer sprayed. So gradually
the proportion of resistant insects in the area increased.
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Natural selection
3OME INSECTS HAVE A
NATURAL RESISTANCE TO
PESTICIDES
0LANTS ARE SPRAYED
/NLY THE RESISTANT
INSECTS SURVIVE
/THER INSECTS MOVE INTO
THE AREA 4HESE BREED WITH
THE RESISTANT ONES 3OME OF THE
OFFSPRING ARE RESISTANT
As many farmers sprayed over a large area, the insect
population quickly became all resistant. This is shown
in Figure 4.12.5. Resistant insects are shown in colour.
Natural selection in bacteria
Bacteria are small single-celled organisms, some of
which can cause infectious diseases such as cholera,
typhoid and pneumonia. Scientists found a way to kill
them when they discovered that certain fungi make
special chemicals to defend them against bacteria. The
chemicals were given the name antibiotics. The first
one was called penicillin. Many more have since been
discovered.
When you go to the doctor with an infection, you
may be given an antibiotic to kill any bacteria infecting
you. This practice has become widespread in medicine.
However, in the last few decades of the 20th century,
several of the antibiotics lost their effectiveness. The
bacteria had become resistant to them.
The bacteria became resistant by natural selection.
In the millions of cells infecting a person, some may
have inherited cell chemistry that can destroy the
antibiotic molecules. This is similar to the insects
mentioned earlier in this Focus. When a person takes
the antibiotic, it kills all the bacterial cells that are not
resistant. Any resistant ones may survive. The resistant
cells reproduce. The offspring will also be resistant.
FOCUS
4.12
[ Questions ]
Use your book
The process of natural selection
1 What is natural selection?
2 Explain what is meant by ‘selective agent’. Give an
example.
7HEN PLANTS ARE SPRAYED
AGAIN MORE RESISTANT
INSECTS SURVIVE
Insects become resistant to insecticides
by natural selection.
Fig 4.12.5
Bacteria reproduce by dividing in half. Those then
each divide again, often about 20 minutes later. Every
20 minutes or so the number of bacteria inside you
may double. Eventually there will be enough bacteria
to make you ill. These will be resistant bacteria
though. So if you return to the doctor for more of the
antibiotic, it will not work. You will have to have a
different antibiotic, one to which the bacteria will not
already be resistant.
Single cell
begins to divide
Two ‘daughter’
cells formed.
Each divides again
Four daughter
cells formed
Fig 4.12.6
Bacteria reproduce by dividing in two.
One very dangerous type of bacteria is ‘golden
staph’. This is resistant to many antibiotics, and is
very difficult to kill. It became resistant because of
the widespread use of antibiotics in hospitals. This
created an environment in hospitals where natural
selection was continually happening to this type
of bacteria. The resistant bacteria were selected to
survive, and eventually the species became resistant.
3 Explain what is meant by ‘variation’ and give some
examples.
Evidence for natural selection
4 Explain how the light-coloured peppered moths
gradually died out in the cities where pollution had
changed the environment.
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Natural selection in insects and bacteria
5 What do we mean by resistant insects or bacteria?
6 What are antibiotics and from where did they come?
7 In the examples of natural selection in insects and
bacteria, what were the selective agents in each case?
8 What was the inherited variation in the examples of
natural selection in insects and bacteria?
Use your head
9 Why must there be variation in a species before natural
selection can occur?
10 Choose one of the following adaptations and explain
how it may have developed through natural selection:
warning colouration in poisonous insects, venom in
tiger snakes, the hind legs of a grasshopper, the front
legs of a mole cricket, the frill of a frill-necked lizard,
the echolocation of a dolphin, the long tongue of a
honeyeater.
11 Consider the ancient bird-like Archaeopteryx from the
previous Focus. Explain how they could have evolved
into modern birds.
FOCUS
4.12
12 Can a single organism become adapted during its
lifetime? Explain your answer.
FOCUS
4 .12
13 Natural selection is happening to all organisms each day
of their lives. Choose an animal species and give a few
examples of this during a typical day for the animal.
14 Are humans being subjected to natural selection?
Explain your answer, giving some examples.
15 Several early science fiction movies showed humans
several million years into the future. They showed us
with a huge brain and head about twice as big as now.
Why do you think the film-makers showed us like that?
Investigating questions
16 When taking antibiotics, why must you finish taking
them all even if you feel better after only taking some of
the pills?
17 Find out about the famous Australian scientist Howard
Florey and explain his contribution to the fight against
infectious diseases.
18 Find out about Charles Darwin and write a paragraph
on his contribution to the understanding of evolution by
natural selection.
[ Practical activity ]
Does camouflage work?
Purpose
Prac 1
Focus 4.12
To use a model to discover if camouflage in
organisms improves their chances of survival.
Requirements
Two A3 sheets of paper each a different colour,
20 toothpicks of each paper colour, stopwatch,
1 pair of forceps (tweezers).
picked up. Write your results in the table. Calculate the
percentage of the total toothpicks picked up that were
the same colour as the paper.
6 Repeat the experiment, but use the other coloured sheet
of paper.
Number of toothpicks picked up in the camouflage experiment
Toothpick
colour A
Procedure
1 Work with a partner. One person is the experimenter and
the other is the subject.
Paper colour A
2 The subject must copy the table opposite.
Paper colour B
3 The experimenter must mix the coloured toothpicks up
and spread them evenly over one of the sheets while the
subject is copying the table.
Toothpick
colour B
Percentage different
colour from paper
Questions
4 The subject has 20 seconds to pick up as many of the
toothpicks as they can with the forceps, one at a time.
They must pick them off the paper and put them in a
pile in front of them. The experimenter will say when to
start and when to stop.
2 Discuss any problems you had with this experiment.
5 When the time is up the experimenter must count the
number of toothpicks of each colour that the subject
3 Explain how this activity is similar to the peppered moth
research on natural selection.
1 Find out the results of the other students in your class.
What was picked up more often: the same colour as the
background paper, or a different colour from the paper?
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