Evolution

Starter: Create a mind map of what you remember about natural selection and evolution

Evolution

Learning intentions

​

To learn about the process of evolution, natural selection and speciation.

Success Criteria

I can describe what evolution is.

I can define natural selection.

I can explain what the effects of stabilising, directional and disruptive selection.

I can explain why natural selection happens more rapidly in prokaryotes.

I can describe the process of speciation.

I can give examples of allopatric and sympatric barriers.

Evolution Introduction

Evolution is the gradual change in the characteristics of a population of organisms over successive generations as a result of variation in the population’s genomes.

http://statedclearly.com/videos/what-is-evolution

Natural Selection

What can you remember?

Natural Selection

Organisms produce more offspring than the environment can support

All members of a species show variation from each other

A struggle for existence occurs and many offspring die before they can reproduce

Only those who are better adapted to the environment (the fittest) will survive and breed and pass those adaptations on to their offspring.

This process is repeated generation after generation causing gradual change in the characteristics of a species.

Natural selection is the non-random increase in frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious sequences.

Natural Selection

Why is it non-random?

Sequence: -

random mutations occur - made more likely by the rapid rate of reproduction (about once every 20 mins)

​

a beneficial dominant allele would confer a selective advantage

​

selection pressure - those without the advantageous form of allele will die when the antibiotic is administered

​

the resistant form rapidly multiply and over successive generations will become more numerous (the “norm”) - natural selection

Essay Question:

‘Describe how mutation and natural selection bring about changes to a species.’

Marker:

• A random and spontaneous mutation occurs altering the protein produced
• Protein produced is altered from the ‘norm’.
• The protein conveys an advantageous characteristic.
• Organisms with this characteristic have are more likely to survive environmental pressures.
• Organisms with mutation survive and are more likely to produce offspring.
• Surviving offspring also have the advantageous allele.
• New allele becomes the norm.
• This happens gradually over many generations

Warfarin resistance

• Warfarin – prevents blood from clotting
• Used as rat poison – rats die of internal bleeding
• When warfarin was introduced in 1950s, some

rats already had a mutation that gave them

resistance to the poison

• The frequency of the resistance allele rapidly

increased as the poison is a selection factor

• Majority of rats became resistant

Evolution

Learning intentions

​

To learn about the process of evolution, natural selection and speciation.

Success Criteria

I can describe what evolution is.

I can define natural selection.

I can explain what the effects of stabilising, directional and disruptive selection.

I can explain why natural selection happens more rapidly in prokaryotes.

I can describe the process of speciation.

I can give examples of allopatric and sympatric barriers.

Effects of Selection

Phenotype Frequency

Height (cm)

In any population there is what is known as a “normal distribution” of genetic traits.

Natural Selection

Natural selection is the non-random increase in the frequency of DNA sequences that increase survival and the non-random reduction in the frequency of deleterious (harmful) sequences

Effects of Selection

Changes in the frequency of phenotypes can result from:

​

• Stabilising Selection
• Directional Selection
• Disruptive Selection

Stabilising Selection

This type of selection exerts pressure on extreme variants and favours the average phenotype. It leads to a reduction in genetic diversity.

Stabilising Selection

Example: Human birth weight – babies who have a low birth weight and babies who have a high birth weight are more likely to experience complications during birth and life. Babies with an average birth weight have a better chance of survival.

Directional Selection

Common during a period of environmental change when a population has to adapt. It favours an extreme phenotype.

​

The advantageous trait that was initially rare has now become common.

Directional Selection

Example: Black bears increased in body size during each ice age because larger body sizes are more favourable in colder climates as they lose less heat than smaller bodies.

Disruptive Selection

This type of selection favours extreme phenotypes like the very tall or very short, and selects against the average i.e. the average sized for examples. The population is split into two, or more, distinct groups with different characteristics

Disruptive Selection

Example: This might happen in shallow water among rocks. Light-colored oysters are less easy for a predator to see because they match the rock color. Dark-colored oysters blend into the shadows cast by the rocks. In this case, intermediate-colored oysters would be most heavily preyed upon by the crabs, and very light and very dark oysters would survive to reproduce.

Task: Peppered Moth Case Study

https://askabiologist.asu.edu/peppered-moths-game/play.html

Task: Peppered Moth Case Study

https://askabiologist.asu.edu/peppered-moths-game/play.html

Example

Birds in an environment had a range of colourings, ranging from light to grey to dark.

​

• The climate changes and becomes dull and overcast. The black and white birds become prey to predators. The numbers of grey birds increased.

​

What type of selection does this show?

Stabilising Selection

Example

Birds in an environment had a range of colourings, ranging from light to grey to dark.

​

• The climate becomes very cold and snow persists. White birds can blend into the background and hide from predators. Black birds can blend into the dark background below the snowline. The grey individuals stand out.

​

What type of selection does this show?

​

​

​

​

Disruptive

Selection

Example

Birds in an environment had a range of colourings, ranging from light to grey to dark.

​

• The snow has now gone and left a dark earth-scape. Both white and grey varieties stand out and are predated on. They dark phenotype is selected for.

​

What type of selection does this show?

​

​

​

​

Directional Selection

Extended Response

Write notes on evolution under the following headings:

natural selection; 5

​

Mark Scheme

• Organisms show variation
• Those that are best adapted survive to reproduce.
• Favourable/beneficial genes/DNA sequences are passed onto offspring/next generation
• There is a non-random reduction of deleterious alleles/gene sequences.
• 2 marks for naming and describing two of the types of selection Stabilising/Directional/Disruptive
• 1 mark for naming all three but no explanation
• Over many generations/a long time
• There is an increase in certain genetic sequences/genes/alleles.

Evolution

Learning intentions

​

To learn about the process of evolution, natural selection and speciation.

Success Criteria

I can describe what evolution is.

I can define natural selection.

I can explain what the effects of stabilising, directional and disruptive selection.

I can explain why natural selection happens more rapidly in prokaryotes.

I can describe the process of speciation.

I can give examples of allopatric and sympatric barriers.

Title: Natural Selection in Prokaryotes

LO: How does natural selection occur in Prokaryotes?

Starter: Name the type of selection in graphs a, b and c.

a)

b)

c)

Key Words:

Horizontal

Vertical

Resistance

There are 2 types of gene transfer

Natural Selection in Prokaryotes

Natural selection is more rapid in prokaryotes.

​

In prokaryotes (who reproduce using asexual reproduction) – genetic material can be exchanged between organisms through horizontal gene transfer.

​

Natural Selection in Prokaryotes

Horizontal gene transfer is where genes are passed between individuals in the same generation.

​

Methods of Horizontal Gene Transfer

Horizontal Gene Transfer

Allows for the rapid spread of new genetic sequences and allows for rapid evolutionary change (the acquisition of a beneficial gene from a neighbouring cell is faster than waiting for natural mutations!).

​

BUT

​

No guarantee the genetic sequences will be advantageous so therefore is a risky evolutionary strategy.

Antibiotic Resistance

A large amount of horizontal gene transfer occurs in modern prokaryotes which makes fighting them very difficult! This is the case in circumstances where certain prokaryotes have resistance to antibiotics and pass this resistance onto other prokaryotes.

​

Vertical Gene Transfer

• Genes are transferred vertically from parent down to offspring.

​

• This vertical inheritance may be the result of sexual or asexual reproduction.

Vertical Gene Transfer

Sexual Reproduction

This involves two parents who differ from one another genetically.

​

Offspring inherit different combinations of genes from each parent.

Bingo

• Splice site mutation
• Gene Expression
• Lagging strand
• Antiparallel
• Hydrogen bonds
• Translation
• Meristem
• Prokaryotes
• Eukaryotes
• Vertical inheritance
• Deleterious

​

• Horizontal transfer
• Chromosomal mutation
• Single gene mutation
• Disruptive selection
• Stabilising selection
• Transcription
• PCR
• Pluripotent

Evolution

Learning intentions

​

To learn about the process of evolution, natural selection and speciation.

Success Criteria

I can describe what evolution is.

I can define natural selection.

I can explain what the effects of stabilising, directional and disruptive selection.

I can explain why natural selection happens more rapidly in prokaryotes.

I can describe the process of speciation.

I can give examples of allopatric and sympatric barriers.

Title: Speciation

LO: How are new species formed?

Starter:

Compare and contrast vertical and horizontal gene transfer.

Marker:

• Horizontal gene transfer occurs between two prokaryotes and is a rapid process.
• Vertical gene transfer is a slower process involving reproduction.
• This can be sexual or asexual reproduction.

Definition of a Species

A group of organisms capable of interbreeding to produce fertile offspring.

Speciation

Organisms move around and breed with different populations of the same species and so genes are continually moving between populations. When this stops, and populations become isolated, different species may emerge.

Speciation

The formation of a new species by the process of evolution as a result of isolation, mutation, natural selection and speciation.

There are two types of speciation: allopatric and sympatric.

Speciation

Isolation barriers prevent gene flow between sub-populations

SPECIATION

1. Large freely interbreeding population of one species.

2. Isolation by barrier (geographical, ecological or behavioural.)

Geographical

Ecological

Behavioural

Population A

​

Population B

3. Mutations arise in both populations (some of these give “selective advantage”).

In a dry warm environment large ears will help the fox to cool and a sandy coloured coat will camouflage it. A mutation that caused a thinner coat and smaller body size might also be favoured.

In a cold snowy environment a coat that is thicker and paler in the winter will aid survival. A mutation causing smaller ears that radiate less heat would also give advantage.

4. Natural selection

Population A

​

Population B

Population suited to environment A

​

Population suited to environment B

……….and after a very, very long time

Population A suited to environment A

​

Population B suited to environment B

5. Barrier removed

6. No interbreeding to produce viable offspring occurs between members of 2 populations- 2 different species have arisen i.e. speciation has occurred.

1. Large freely interbreeding population of one species.

2. Isolation by barrier (geographical, ecological or reproductive.)

3. Mutations arise in both populations (some of these confer “selective advantage”).

4. Natural selection over a very long period of time.

5. Barrier removed

6. No interbreeding to produce viable offspring occurs between members of 2 populations- 2 different species have arisen i.e. speciation has occurred.

Barrier

Subpopulation B

​

Subpopulation A

Population suited to environment B

Population suited to environment A

​

Barrier

New Species A

​

New Species

B

There are two types of speciation

Sympatric – populations are

geographically still close

Sympatric - occurs in Same place

Allopatric Speciation

Occurs when groups are physically separated from the main population by a geographical barrier (therefore the gene pool has been separated).

Allopatric Speciation

Large interbreeding population

Allopatric Speciation

Isolation of populations

Population A

Population B

Allopatric Speciation

Large mutant

Small mutant

There will be different selection pressures on each side of the barrier and different mutations will develop.

Allopatric Speciation

Natural selection favours mutants

e.g. Large mutant may favour dry conditions

e.g. Small mutant may favour wet conditions

Allopatric Speciation

Species A

Species B

Over a long period of time natural selection increases frequency of new adventageous alleles

Sympatric Speciation

Occurs when living in same environment but become genetically isolated from each other.

​

Occurs due to:

• Ecological barriers – e.g. changes in pH, temperature
• Behavioural Barriers – e.g. lack of attraction between males and females, unattractive courtship behaviour, failures in pollination.

Sympatric Speciation

Large interbreeding population sharing the same ecological niche.

(e.g. Fruit flies living on hawthorn bushes)

Sympatric Speciation

Alternative ecological niche appears.

(e.g. new species of apple tree introduced by humans)

Some members of the population start to exploit the new niche

Sympatric Speciation

The two populations now exploit different resources (e.g. food source) and no longer interbreed.

Behaviour has become an isolating barrier.

Sympatric Speciation

Mutants better adapted to exploit the new resources appear and successfully breed.

(e.g. better camouflaged on apples)

Sympatric Speciation

Natural selection favours the new mutants and eventually over a period of time two genetically distinct species are formed which can no longer interbreed.

Summary

Evolution

Learning intentions

​

To learn about the process of evolution, natural selection and speciation.

Success Criteria

I can describe what evolution is.

I can define natural selection.

I can explain what the effects of stabilising, directional and disruptive selection.

I can explain why natural selection happens more rapidly in prokaryotes.

I can describe the process of speciation.

I can give examples of allopatric and sympatric barriers.

1 When a horse is crossed with a donkey, the result is a sterile animal called a mule.

• Do a mule’s parents belong to the same species? (1)
• Explain your answer (1)

2 a) Define the term speciation. (1)

b) Arrange the following stages into the correct order in which they would occur during allopatric speciation (1)

A isolation of gene pool

B formation of new species

C mutation

D occupation of territory by one species with one gene poll

E natural selection

3 a) i) Identify the type of speciation that involves geographical barriers.(1)

ii) Give TWO examples of geographical barriers (2)

b) Identify the type of speciation that involves behavioural or ecological barriers. (1)

1 a) No

b) They do not produce fertile offspring.

2 a) the formation of a new species by evolution as a result of isolation, mutation and natural selection.

b) D, A, C, E, B

3 a)i) Allopatric

ii) River, mountain, sea etc.

b) Sympatric

Past Paper Questions

Use the hyperlinked past paper questions on the Biology Site

​

2016 - Section 2 5a

2019 - Section 1 - 5,6,7,8

Section 2 5 a and b

2022 - Section 2, 3 and 5

You should now know…

• Define ‘evolution’
• Define ‘natural selection’
• Explain what happens as a result of ‘stabilising selection’
• Explain what happens as a result of ‘directional selection’
• Explain what happens as a result of ‘disruptive selection’
• Describe how ‘vertical gene transfer’ can take place
• Explain why vertical gene transfer can be referred to as inheritance

Glossary: stabilising, directional, disruptive, horizontal gene transfer, vertical gene transfer, allopatric, sympatric.

• Describe how ‘horizontal gene transfer’ can take place
• Explain the implication for evolution of prokaryotes carrying out horizontal gene transfer
• Define ‘species’
• Define ‘speciation’
• Explain the role of isolation in speciation
• Name 3 types of isolation barriers
• Describe the difference between allopatric and sympatric speciation
• Give an example of allopatric evolution of a species
• Give an example of sympatric evolution of a species
• Explain the role of mutation in speciation
• Explain the role of selection in speciation