Starter

Which of the following is true of polyploid plants? They have:

A – reduced yield and the diploid chromosome numberB - increased yield and the diploid chromosome numberC – reduced yield and sets of chromosomes greater than diploidD – increased yield and sets of chromosomes greater than diploid.

Key Area 1.7

Evolution

Learning Outcomes

Key Area 1.7 Evolution and Inheritance

Learning Outcomes

1. Define ‘evolution’2. Describe the process of vertical inheritance3. Describe the process of vertical inheritance as a result

of sexual reproduction4. Describe the process of vertical inheritance as a result

of asexual reproduction5. Describe the process of gene transfer6. Explain how HGT led to rapid evolutionary change7. State which type of gene transfer is safe and explain

why8. Describe the process of gene transfer from prokaryotes

into eukaryotes9. Describe the process of gene transfer from viruses to

their host cell

Evolution

• Evolution is the change that occurs in organisms over generations as a result of genomic variations.

• These variations take the form of changes in the frequencies of certain genetic sequences.

Introduction to Evolution

Vertical Inheritance

• Genetic sequences are transferred vertically from parent down to offspring as a result of sexual or asexual reproduction.

Vertical InheritanceSexual Reproduction

• In sexual reproduction, the parents are normally genetically different and produce offspring that vary further in genetic makeup.

Vertical InheritanceAsexual Reproduction

• In asexual reproduction, a single parent with a certain genome produces offspring with exactly the same genome and no variation occurs among the successive generations.

• Vertical inheritance occurs among eukaryotes and prokaryotes.

Horizontal Transfer of Genetic Material

• Prokaryotes can transfer genetic material from one cell to another horizontally.

• A much faster method of obtaining a beneficial gene than waiting to evolve by natural means!

• The cells involved may not even belong to the same species.

Rapid Evolutionary Change in Prokaryotes

• During the early stages of prokaryotic evolution the rate of gene loss was high.

• To compensate, there was a high rate of horizontal gene transfer (HGT).

• This led to a rapid spread of new genetic sequences which promoted the build up of larger genomes and allowed rapid evolutionary change to occur among prokaryotes.

Horizontal Gene Transfer (HGT)

• There is no guarantee that a genetic sequence gained horizontally will be an advantage.

• It may useless or even harmful, which makes HGT a risky evolutionary strategy.

• Vertical gene transfer is much safer as the genes have been “tried and tested” by the parents.

Tree of Life

• As organisms reached a certain level of multicellular complexity, the importance of gaining sequences by HGT decreased.

• As the role of vertical inheritance increased in importance, distinct lineages began to emerge with their own sets of specific genes, eventually giving rise to the “tree of life”.

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MRSA

• HGT still takes place in modern prokaryotes.• A current example among certain bacteria is the

exchange of plasmids carrying the genes that are resistant to antibiotics.

• This has resulted in the emergence of strains of bacteria that are resistance to all known antibiotics.

HGT from prokaryotes into eukaryotes

• Some bacteria can transfer genetic material horizontally to the genomes of eukaryotes.

• For example: Agrobacterium tumefacians infects wounded plant cells with a plasmid which integrates a sequence of its DNA into the genome of the host plant cell.

HGT from viruses to host cells

• Some viruses are able to transfer their DNA horizontally into the genome of their host cell.

• For example, the herpes virus reproduces within the host cells and destroys them, however, it is able to integrate its DNA into the genome of other healthy cells and remain dormant as a provirus until the person’s state of health drops.

• HIV behaves in the same way.

You should now be able to . . .

1. Define ‘evolution’2. Describe the process of vertical inheritance3. Describe the process of vertical inheritance as a result of

sexual reproduction4. Describe the process of vertical inheritance as a result of

asexual reproduction5. Describe the process of gene transfer6. Explain how HGT led to rapid evolutionary change7. State which type of gene transfer is safe and explain why8. Describe the process of gene transfer from prokaryotes into

eukaryotes9. Describe the process of gene transfer from viruses to their host

cell

Starter

• Number you and your partner ‘1’ and ‘2’.

• Number 1’s should write a brief description of vertical gene inheritance on their show me board.

• Number 2’s should write a brief description of horizontal gene transfer on their show me boards.

• Swap boards and evaluate each others comments.

Outcomes Covered

Key Area 1.7Selection

Learning Outcomes

1.Describe the process of natural selection

2.Describe the process of sexual selection

Selection

Selection is the process by

which the frequencies of some

DNA sequences increase in a

population because they have

been selected for and some

decrease because they have

been selected against.

Natural Selection• Organisms produce more offspring than the environment can

support.

• Many offspring die before reaching reproductive age for various reasons.

• Members of a species show variation.

• Those offspring better adapted (have a selective advantage) survive, reproduce and pass on favourable characteristics.

• The process is sometimes referred to as ‘survival of the fittest’.

Natural Selection

Natural Selection Example: DDT

• DDT is a poisonous chemical which has been widely used against many insects, including mosquitoes which carry malaria and yellow fever.

• Within just a few years of use, mutant forms of insects resistant to DDT had ‘appeared’.

• The mutants are able to make an enzyme that renders the chemical harmless.

• They had not arisen in response to DDT!

Natural Selection Example

• A tiny number of resistant mutants just happened to be present within the natural insect population.

• When the DDT spray was applied the non-resistant insects died.

• The resistant mutants had a selective advantage and multiplied.

• Natural selection took place.

Selection Against Deleterious Sequences

• A deleterious genetic sequence can code for an inferior version of a characteristic.

• The individual will be poorly adapted to the environment.

• As a result, it will leave fewer offspring.• In turn, fewer copies of the gene will be

passed on and a non-random reduction in its frequency will occur.

• A frequency that is lethal will disappear much more quickly.

Sexual Selection

• Females tend to be selective and choose one high quality male to breed with.

• Male animals must compete for a sexual partner.

• Sexual selection is a non-random process that results in the increase in frequency of DNA sequences that increase the reproductive success.

Male-to-Male Competition

• Males compete aggressively with one another for territories and access to females.

• They may also use “weapons” such as antlers.

Female Choice

• The female may selects her mate based on the traits he displays.

• Female Choice

Investigating Sexual Selection in Brine Shrimps

• Practical Technique

Starter

Learning Outcomes

1. Describe and explain the outcome of stabilising selection.

2. Describe and explain the outcomes of directional selection.

3. Describe and explain the outcomes of disruptive selection.

Types of Selection for a Quantitative Trait

• A polygenic trait, such as seed mass, is quantitative.

• Natural selection can affect the frequency of a quantitative trait within a large population in one of three ways:

1. Stabilising selection2. Directional selection3. Disruptive selection

Stabilising Selection

• Leads to a reduction in genetic diversity without a change in the mean value.

• Maintains the status quo for the best adapted genotypes.

Directional Selection

• Favours a version of the characteristic that was initially a less common form.

• Results in a progressive shift in the population’s mean value for the trait.

Disruptive Selection

• Extreme versions of a trait are favoured at the expense of the intermediates.

• Results in the population becoming split into two distinct groups each with its own mean value.

You should now be able to . . .

1.Describe and explain the outcome of stabilising selection.

2.Describe and explain the outcomes of directional selection.

3.Describe and explain the outcomes of disruptive selection.

Starter

Learning Outcomes

1.Define the term ‘gene pool’.2.Define the term ‘genetic drift’.3.Describe the ‘founder’ effect.4.Explain the term ‘neutral mutation’.5.Describe the effect of genetic drift on neutral

mutations.

Genetic Drift

• A gene pool is the total of all the different genetic sequences present in a population.

• The random increase of decrease in frequency of genetic sequences (due to sampling error) is called the genetic drift.

• Random genetic drift creates the potential for evolutionary change.

Founder Effect

• A small group of organisms (splinter group) is isolated from the rest of the population and “finds” a new population.

Founder EffectThe original population has 4 alleles of the gene. The splinter group only have 3 and they occur in different frequencies.After several generations, the genetic makeup of the new population is very different to the original and its members become distinctive.

Neutral Mutations

• A neutral mutation is a change to the sequence that has no effect on the amino acid produced.

• E.g. GCC changed to GCA – both still code for alanine.

• Neutral mutations do not affect how well adapted the individual becomes to the environment so are not involved in natural selection.

Neutral Mutations

• Neutral mutations are affected by genetic drift.

• Most of the changes that occur in the frequencies of these neutral genetic sequences are the result of random genetic drift acting as a mechanism of evolution.

You should now be able to. . .

1.Define the term ‘gene pool’.2.Define the term ‘genetic drift’.3.Describe the ‘founder’ effect.4.Explain the term ‘neutral mutation’.5.Describe the effect of genetic drift on

neutral mutations.

Starter

Learning Outcomes

1. Define the term ‘species’.2. Define ‘speciation’.3. Describe the sequence of events in

allopatric speciation and the type of barrier involved.

4. Describe the sequence of events in sympatric speciation and the barriers involved.

5. Explain the formation of hybrid zones and the consequences of a barrier being introduced.

Species

• A species is a group of organisms that are able to interbreed with one another to produce fertile offspring.

Speciation

• Speciation if the formation of new biological species brought about by evolution as a results of isolation, mutation and selection.

Allopatric Speciation

• Gene flow is interrupted by a geographical barrier.

• Examples of these are a river, mountain range, desert or sea.

Allopatric Speciation

Sympatric Speciation

• When two (or more) populations live in close proximity to one another in the same environment, but become genetically isolated.

• Isolation is by a behavioural or ecological barrier (or polyploidy in plants).

• Examples of ecological barriers are temperature or pH.

• Sympatric speciation is promoted by disruptive selection.

Sympatric Speciation

Hybrid Zones

• A region can be occupied by several populations of an organism which vary in their ability to interbreed form hybrid zones.

• These five closely related species belong to the same “ring” species.

THINK. . .PAIR. . .SHARE

• What would happen if a barrier was introduced that separated hybrid zone ‘B’ and ‘C’?

You should now be able to . . .

1.Define the term ‘species’.2.Define ‘speciation’.3.Describe the sequence of events in allopatric

speciation and the type of barrier involved.4.Describe the sequence of events in sympatric

speciation and the barriers involved.5.Explain the formation of hybrid zones and the

consequences of a barrier being introduced.