Download Natural Selection

Natural Selection
Learning intentions
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I know that sexual reproduction results in
variation in a population
I can explain natural selection as survival of the
fittest
I know that if variation exists in a population the
population can adapt if the environmental
conditions change
I can describe some examples of natural
selection
Success criteria
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I can state that sexual reproduction results in
variation in a population
I can carry out an experiment that allows
measurement of success of individuals in a
population of brine shrimps
I can state that if variation exists in a
population the population can adapt if the
environmental conditions change
I have completed notes on some examples of
natural selection
Natural Selection
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If mice arrived in a habitat that had no other
mice and ideal conditions existed eventually
the habitat would become over run.
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So under ideal conditions the number of mice
increase rapidly because there is;
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Plenty food to eat so none die of starvation
No predators to eat the mice
Plenty of shelter so do not need to compete for
shelter (will not die from exposure)
However, this does not happen as only a few
mice survive to produce offspring.
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There are a number of ways in
which mice can be killed off and
prevented from breeding;
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Mice with poor camouflage or
slow reaction will be eaten
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Some that are not as good at
competing for food will die of
starvation
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Some weaker mice will die of
disease
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Mice not as good at competing
for shelter or that have thinner
coats will die of exposure or cold
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This means that only the mice with the best characteristics will
survive to breed.
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This for example could be mice with the allele for thick fur or speed
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So only mice with useful combinations of genes survive to pass on
the genes. Mice with less useful combinations of genes die and so
these genes are not passed on!
Your turn………
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See if you can
survive the process
of natural
selection?
Natural Selection over time……..
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It is this process that allows species to adapt to
environments, survive and breed (weeding out)
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This process is called natural selection and it
has operated for millions of years to give rise to
new species
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Evolution is the appearance of new species as
a result of natural selection
A definition
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The environment ‘selects’ the best
individual members of a species to
survive and pass on their genes.
Over a long period of time this leads
to evolutionary changes and results in
specialised organisms able to exploit
their environment effectively.
Factors involved
Reproduction
 Variation
 Selection pressure of environment
 ‘Survival of the fittest’
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1858 Charles Darwin + Alfred Wallace
‘evolution is the result of natural selection’
1859 ‘The origin of species’
Some thoughts about Natural selection
Fact 1.
All species have great potential
fertility. Their populations would
increase exponentially if all their
offspring reproduced successfully.
Fact 2
Apart from annual fluctuations
populations are normally stable
Fact 3
Natural resources are limited. In a
stable environment they remain
relatively constant
Idea 1.
If more individuals are produced than
can be supported by resources but
the population remains stable – there
must be a struggle for existence
among individuals
 Survival of a very small part of each
generation
Fact 4
No two individuals are the same, every
population displays enormous variation
Fact 5
Much of the variation is inherited
Idea 2.
Survival is not random but depends
on inherited characteristics of
individuals ‘survival of the fittest’. This
unequal survival constitutes the
process of natural selection
Idea 3
Over the generations the process of
natural selection will lead to a
continuing gradual changes of
populations and the production of new
species = evolution
Darwin's theory – summary
Overproduction of offspring
 Consistency of numbers
 Struggle for existence (environmental
resources determine survival)
 Variation among offspring
 Survival of the fittest by natural
selection
 Like produces like
 Formation of new species
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Darwins Finches
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From 1 common
mainland ancestor
populations of
finches on the
Galapagos islands
developed into 13
different species by
the process of
natural selection
leading to
evolution.
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The diet of the food they ate fruit,
seeds, insects, fruit and the habitat
they lived in ground, tree cactus
resulted in very different types of
finches with varying beak shapes and
sizes
Peppered Moth
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This nocturnal insect rests by day on
exposed surfaces such as tree trunks.
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Insect eating birds prey on the resting
moths.
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In the early 19th century moths were well
adapted to survive as they had light
coloured speckled wings to camouflage
them against the lichen covered tree
surfaces.
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There is also a genetic variety of this moth
which is melanic (nearly all black coloured).
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These moths showed up easily so the birds
ate them.
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This meant that the melanic moths very
rarely survived to pass on their genes
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Starting in the 1840s significant changes took place
– the Industrial Revolution
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Burning loads of coal produced sulphur dioxide
(lethal to plants) and sooty smoke.
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This meant in industrial areas the speckled moths
were easy to see and the melanic moths were more
camouflaged.
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So in industrial areas there were
more melanic moths and outside these
areas there were more speckled moths
This is an example of how changes in the
Environment leads to changes in the adaptation
Of a species (can still interbreed so not yet a
Different species
Antibiotic resistance
Antibiotics are more effective against
bacteria in their early years.
 In bacteria variation in the population
sometimes leads to resistance
 Antibiotics will not work if a bacteria
becomes resistant to it
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The gene for
resistance
can be
passed from
one
bacterium to
another
Staphylococcus aureus
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This strain of bacterium is responsible for 1
in 5 hospital infections.
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Previously it was controlled by antibiotics.
However one stain of it ‘super Staph’ is
resistant to all but 1 antibiotic.
It is only a matter of time before a strain
appears that is resistant to all know
antibiotics.
This is why its vital to find new antibiotic
sources (fungi)
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