Chapter 7: Genetic Changes in Populations

Question 1

Define population and gene pool

Answer 1

• A population refers to members of one species living in one region at a particular time
• A gene pool is all of the alleles present in a population

Question 2

State how to determine whether evolution has occurred

Answer 2

• Changes in allele frequencies over generations indicate evolution
• E.g. Initially, the dominant and recessive allele both have a frequency of 0.5 but overtime, the dominant allele becomes more prevalent

Question 3

Define and provide examples of environmental selection pressures

Answer 3

• Environmental selection pressures are external agents that influence the ability of an organism to survive in their environment
• Physical → climate change, food shortages, shelter availability
• Biological → infectious disease, predation, competition and sexual selection
• Chemical → soil/water pollutants and drugs (antibiotics)

Question 4

State how selection pressures can cause change in the allele frequency of a gene pool

Answer 4

• Due to selection pressures, the number of favourable alleles within a gene pool increase while the number of unfavourable alleles decrease
• Survival of the fittest

Question 5

Define natural selection and its impact on genetic diversity

Answer 5

• Natural selection is the process in which organisms better adapted for an environment are more likely to survive and pass their genes on to the next generation
• Over many generations, better adapted alleles become more common in a population
• This decreases genetic diversity

Question 6

Explain gene flow

Answer 6

• Gene flow involves the movement of organisms and their genetic material between populations (transferring alleles from one gene pool to another)
• A lack of gene flow prevents the introduction of new alleles into a population which decreases genetic diversity (only current members of the population contribute to the genome of future populations)

Question 7

State the difference between immigration and emigration

Answer 7

• Immigration involves the movement of alleles into a population
• Emigration involves the movement of alleles out of a population

Question 8

Define genetic drift

Answer 8

• Genetic drift involves random and unpredictable changes in allele frequencies from one generation to the next
• Due to chance events such as floods and bushfires

Question 9

Explain and provide an example of the bottleneck effect (genetic drift)

Answer 9

• The bottleneck effect is a drastic reduction in population size due to a chance event
• E.g. Due to bushfire, a population of koalas has drastically dropped

Question 10

Explain and provide an example of the founder effect (genetic drift)

Answer 10

• The founder effect occurs when a new colony is started by a few members of a larger population
• E.g. Due to a flash flood, fish are seperated from the main population and subsequently establish a new population in a nearby lake

Question 11

Describe the difference between natural selection and genetic drift

Answer 11

• Natural selection favours organisms that are better adapted to the environment
  
  • Those with advantageous genes are more likely to survive
• Genetic drift does not favour any organism (or allele) over the other
  
  • Each organism is equally subject to being affected (completely random)

Question 12

Define mutation

Answer 12

• A mutation is a permanent change in the DNA sequence of a gene that leads to the formation of new alleles

Question 13

State the difference between point and block mutations

Answer 13

• Point mutations involve a change in one single nucleotide base
• Block mutations involve change in a section of a chromosome

Question 14

Name and describe the three types of point mutations

Answer 14

• Silent mutations do not change the amino acid
• Nonsense mutations lead to the production of a STOP codon
• Missense mutations change the amino acid

Question 15

Name and describe the types of block mutations

Answer 15

• Duplication → a section of a chromosome is duplicated and is reconnected to the same chromosome
• Deletion → a section of a chromosome is removed
• Insertion → a section of a chromosome is added
• Inversion → a section of a chromosome is flipped but is still connected to the same site
• Translocation → a section of a chromosome is removed and is relocated to a new position on the chromosome

Question 16

Explain frameshift mutations

Answer 16

• Frameshift mutations are a type of point mutation in which the insertion or deletion of a single base causes change in all amino acids
• Insertion mutations occur when a base is added
• Deletion mutations occur when a base is removed
• Results in alteration of the ‘reading frame’ as the number of nucleotides is no longer divisible by three

Question 17

State the difference between polyploidy and aneuploidy

Answer 17

• Polypoloidy involves a change in the number of sets of chromosomes
• Aneuploidy involves a change in chromosome numbers

TIP: P.S. and N/A.

Question 18

State how mutations can bring about new alleles

Answer 18

• Mutations create new DNA sequences and thus new alleles

Question 19

Explain the mechanism of natural selection

Answer 19

• Variation exists in the population’s gene pool (created by new alleles via mututations)
• Environmental selection pressures act upon the population and there is a struggle for all organisms to survive
• Better adapted organisms are more likely to survive/reproduce and, thus, pass their alleles on to the next generation
• Alleles that allow for survival will be inherited by subsequent generations and increase in frequency in the gene pool over time

Question 20

State examples of when increasing genetic diversity can be beneficial or detrimental to a population

Answer 20

• Should environmental conditions change, high genetic diversity within a gene pool increases the species’ chance of survival
• High genetic diversity can promote the rise of redundant alleles; if the environment changes and advantageous alleles are less prevalent, population size can decrease and make the species more susceptible to extinction

Question 21

State examples of when decreasing genetic diversity can be beneficial or detrimental to a population

Answer 21

• Decreased genetic diversity (where advantageous alleles are dominant) is beneficial if conditions are stable and unchanging
• If a novel disease is introduced to a population with low genetic diversity, the opportunity for survival is low and therefore makes the species more susceptible to extinction

Question 22

Define selective breeding

Answer 22

• Involves breeding parents with particular characteristics to produce offspring with desired traits

Question 23

Give an example of when and why humans have used selective breeding

Answer 23

• Humans use selective breeding to breed for animals with more desirable characterisics
• E.g. Through selecting breeding, beagles have been bred to have a strong sense of smell which allows them to be great hunters

Question 24

State the effect of selective breeding on genetic diversity

Answer 24

• Selectively bred plants and animals tend to share very similar alleles and thus results in a decrease in genetic diversity
• As a result, populations are at risk should conditions change

Question 25

State the difference between natural selection and selective breeding

Answer 25

- **Natural selection** - Occurs naturally as a result of an organism’s ability to survive - Occurs slowly over numerous generations - Acquired traits increase the species’ chance for survival - **Selective breeding** - Occurs artificially as a result of human intervention - Occurs more rapidly - Acquired traits do not always enhance the species’ survival

Question 26

Explain how bacteria can become resistant to antibiotics

Answer 26

- During cell division, a **bacterium undergoes mutation that results in antibiotic resistance** - There is now variation in the bacterial population; some are susceptible and some are resistant to the antibiotic - **When the antibiotic is present**, all sensitive bacteria are killed while **the antibiotic resistant bacterium is unaffected** - The **resistant bacterium survives and multiplies** establishing a population of antibiotic resistant bacteria

Question 27

Explain the causes for bacterial resistance

Answer 27

- **Doctors over prescribing antibiotics** → provides more opportunities for bacteria to mutate and eventually become resistant - **Patients not finishing a course of antibiotics** → remaining infectious bacteria can survive and thus be more likely to reproduce and evolve - **Lack of infection control in medical centres** → increases the likelihood of resistant bacteria being transferred between patients

Question 28

Explain the consequences for human health of antibiotic-resistant bacteria

Answer 28

- Due to antibiotic resistance, diseases such as pneumonia and tuberculosis are becoming increasingly difficult to treat as the **bacteria no longer responds to antibiotics and thus continues to reproduce in the body** - This can be life threatening for affected patients due to limited treatment options - This can also jeopardise the health of other individuals due to increased chances of infection

Question 29

Explain how molecular homology can provide evidence of antimicrobial resistance in bacteria

Answer 29

- The DNA of resistant and sensitive bacteria can be sequenced and compared - Identifying the molecular changes that provided resistance can then be used to demonstrate how the bacteria has evolved to become resistant overtime

Question 30

List the characteristics of farming that may be contributing to antimicrobial resistance in pathogens

Answer 30

- Selective breeding of animals - Overuse of medication - Animals living in close proximity of each other

Question 31

Describe the difference between antigenic drift to antigenic shift

Answer 31

- **Antigenic drift** - Occurs when a virus’ nucleic material is altered, resulting in small changes to the virus (modified antigens) - Affects influenza A, B and C - Gradual change of the virus - **Antigenic shift** - Occurs when two or more strains of a virus combine to form a new strain (new antigens) - Only affects influenza A - Sudden onset of a new virus ## Footnote NOTE: Both involve change in the virus’ genetic makeup.

Question 32

Describe the conditions that make antigenic shift more likely

Answer 32

- When **different species are in close proximity** of each other (viruses from the different species can combine) - When a **species is infected with more than one strain of a virus**

Question 33

Describe how changes in the antigenic properties of viruses can lead to ineffective vaccines

Answer 33

- Vaccines elicit a particular immune response specific to the virus that it targets - Viral RNA undergoes frequent mutations as it replicates resulting in **new strains or subtypes** of the virus - The **virus will no longer be recognised by the immune system and will be treated as a new pathogen** - As a result, the previous vaccine becomes ineffective

Question 34

Explain why there is lower genetic diversity for populations impacted by the founder effect

Answer 34

- Founder populations carry only a subset of the genetic diversity found in the original population - This makes the population less adaptable to changing environments due to the unlikelihood of favourable characteristics existing in the gene pool

Question 35

Describe how natural variation exists in a gene pool

Answer 35

- Through **mutations** that may have created new alleles - Different allele combinations via **sexual reproduction**

Question 36

Provide reasons that may account for a lack of genetic diversity between two separate species/populations

Answer 36

- **Interbreeding** may still be occuring - **Similar selection pressures** in the two regions