Download Slide 1

Quick activity:
How could the equipment below be used to
demonstrated natural selection?
A table tennis table represents the environment and the
variously coloured balls represent genotypes.
The net symbolises the selection pressure, like predators or
disease.
Damaged orange (non-round) balls symbolise organisms with
unfavourable characteristics caused by mutation, with yellow
and blue balls symbolising individuals with more favourable,
but slightly different, characteristics.
Those balls (organisms) which do not make it across the net
have not reached sexual maturity and have not passed on
their genes.
Wednesday, May 24, 2017
Title: Natural selection
Keywords:
• mutation
• gene pool
• differential survival
Learning Objectives:
We are learning….
• How does natural selection work?
• What is mutation?
• How is variation caused and why is it important in evolution?
Starter:
‘The race is survival. The finishing line
is reproduction.’
Discuss this statement with the person
next to you
Natural selection
The theory of evolution states that evolution happens by natural
selection. Here are the key points:
Individuals in a species show a wide range of variation.
This variation is because of differences in genes.
Individuals with characteristics most suited to the environment
are more likely to survive and reproduce.
The genes that allowed the individuals to be successful are passed
to the offspring in the next generation.
Individuals that are poorly adapted to their environment are less
likely to survive and reproduce. This means that their genes are
less likely to be passed to the next generation. Given enough time,
a species will gradually evolve.
• Natural selection
Darwin’s observations
1. All organisms produce more offspring than
survive to adulthood
• Natural selection
Darwin’s observations
2. Populations remain more or less constant
in numbers
What factors keep populations stable?
• Natural selection
Darwin’s observations
3. Members of the same species show
variation in characteristics
What are the causes of variation?
Mutations – Sudden changes to genes and chromosomes which
may have an advantageous effect. Can you think of any examples
from GCSE Biology?
Meiosis – Homologous chromosomes (containing genes coding for
the same characteristics) pair up and then separate during nuclear
division. This forms gametes containing mixed up genetic material
(independent segregation on chromosomes). The gametes are
therefore different.
Draw a Punnet
square showing the
possible genotypes
of the children.
Fusion of gametes – In sexual reproduction the offspring inherit
some characteristics from each parent and are therefore different
from both of them. The fusion of gametes at fertilisation is
completely random.
• Natural selection
Darwin’s observations
4. Some characteristics are inherited and so
are passed on to the next generation
• Natural selection
Deductions
Survival of the fittest
All organisms are involved in a struggle for survival
and only the best adapted survive.
Differential survival and reproduction
Organisms that are better adapted will survive and
are more likely to reproduce, therefore passing on
their useful adaptations to their offspring.
Inheritance, gene pools and frequencies of alleles
The gene pool is the set of all genes, or genetic information, in
any population, usually of a particular species.
An allele is a different form of the same gene (e.g. eye colour).
So why don’t allele frequencies remain stable from generation
to generation?
Why don’t all genotypes (genetic makeups) in the population
have equal reproductive success?
Some organisms with one particular genotype are;
• More likely to die before reproducing
• Unable to grow sufficiently well
to reproduce successfully
• Unable to attract a mate
Natural selection and allele frequencies
When organisms of one particular genotype reproduce less
successfully than others in the population with different genotypes
we say there is differential reproductive success between the
genotypes in the population.
Differential reproductive success is common in populations.
In beech woodlands, yellow-banded snails are
very conspicuous against the pink leaf litter
lying on the ground.
The snails with pink shells are better camouflaged and so are more
difficult to find.
Song thrushes eat more of the yellow snails than the pink ones,
resulting in fewer yellow-shelled snails surviving to reproduce. This
means that fewer Cy alleles are passed on to the next generation.
This is an example of natural selection.
Mutation and natural selection
New forms of genes result from changes
in existing genes.
These changes are known as mutations.
They are tiny changes in the long strands of DNA.
Mutations occur naturally through mistakes when
copying DNA during cell division.
Mutations introduce more variety into the genes of a
species – they are very important for survival because if
they are beneficial and produce a useful adaptation, the
organism is more likely to survive and breed.
Mutant genes become more common in the population
as a result and cause species to evolve.
B1b 7.2 Theories of evolution
Darwin’s finches
Finches can be seen in your garden.
They are small, seed-eating birds.
AQA Science © Nelson Thornes Ltd 2006
13
B1b 7.2 Theories of evolution
The Galapagos Islands
Darwin visited these islands in 1835.
AQA Science © Nelson Thornes Ltd 2006
14
B1b 7.2 Theories of evolution
Darwin’s theory of evolution
That’s 24
years before
writing his
famous book:
Corel 78(NT)
The Origin of Species
AQA Science © Nelson Thornes Ltd 2006
15
B1b 7.2 Theories of evolution
Darwin’s finches
Darwin found similar seed-eating finches to
those he had found in South America nearly
1000 km away.
AQA Science © Nelson Thornes Ltd 2006
16
B1b 7.2 Theories of evolution
Seed-eating finch
Darwin knew that many
more birds were born
than survived.
He thought the ones
that did survive must
be best adapted to
eating seeds.
AQA Science © Nelson Thornes Ltd 2006
17
B1b 7.2 Theories of evolution
Insect-eating finch
Darwin also found
a finch with a
thinner beak for
catching insects
from small
spaces.
AQA Science © Nelson Thornes Ltd 2006
18
B1b 7.2 Theories of evolution
Insect-eating finch
Darwin knew that many
more birds were born
than survived.
He thought the ones
that did survive must
be best adapted to
eating insects.
AQA Science © Nelson Thornes Ltd 2006
19
B1b 7.2 Theories of evolution
More finches!
Some even had
beaks that would
hold a cactus
spine to get a
grub out of a tiny
hole
In fact Darwin found many finches.
AQA Science © Nelson Thornes Ltd 2006
20
B1b 7.2 Theories of evolution
Natural selection
This is just one of the examples that
got Darwin thinking about
natural selection.
AQA Science © Nelson Thornes Ltd 2006
21
Microorganisms such as bacteria and viruses reproduce
very rapidly and can evolve in a relatively short time.
One example is the bacterium E. coli.
Its DNA can be damaged or changed during
replication, and most of the time this causes
the death of the cell.
But occasionally, the mutation is beneficial - for the
bacteria. For example, it may allow resistance to an
antibiotic.
When that antibiotic is present, the resistant bacteria have
an advantage over the bacteria that are not resistant.
Antibiotic-resistant strains of bacteria are an increasing
problem in hospitals.
Plenary: Summary questions
Copy and complete using the words below:
Adaptation breed
generation
Natural selection
environment
mutation
organism
survive
When a …………..has a good effect it produces an …………..
which makes an………………….better suited to it’s
………………. This makes it more likely
to………………….and……………………The mutation then
gets passed on to the next…………………..
This is ……………
………….
How successful were we this lesson?
Learning Objective
We were learning…..
• How does natural selection work?
• What is mutation?
Keywords:
• mutation