Chapter 7: Genetic Changes in Populations Flashcards
Define population and gene pool
- 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
State how to determine whether evolution has occurred
- 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
Define and provide examples of environmental selection pressures
- 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)
State how selection pressures can cause change in the allele frequency of a gene pool
- 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
Define natural selection and its impact on genetic diversity
- 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
Explain gene flow
- 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)
State the difference between immigration and emigration
- Immigration involves the movement of alleles into a population
- Emigration involves the movement of alleles out of a population
Define genetic drift
- 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
Explain and provide an example of the bottleneck effect (genetic drift)
- 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
Explain and provide an example of the founder effect (genetic drift)
- 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
Describe the difference between natural selection and genetic drift
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Natural selection favours organisms that are better adapted to the environment
- Those with advantageous genes are more likely to survive
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Genetic drift does not favour any organism (or allele) over the other
- Each organism is equally subject to being affected (completely random)
Define mutation
- A mutation is a permanent change in the DNA sequence of a gene that leads to the formation of new alleles
State the difference between point and block mutations
- Point mutations involve a change in one single nucleotide base
- Block mutations involve change in a section of a chromosome
Name and describe the three types of point mutations
- Silent mutations do not change the amino acid
- Nonsense mutations lead to the production of a STOP codon
- Missense mutations change the amino acid
Name and describe the types of block mutations
- 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
Explain frameshift mutations
- 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
State the difference between polyploidy and aneuploidy
- Polypoloidy involves a change in the number of sets of chromosomes
- Aneuploidy involves a change in chromosome numbers
TIP: P.S. and N/A.
State how mutations can bring about new alleles
- Mutations create new DNA sequences and thus new alleles
Explain the mechanism of natural selection
- 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
State examples of when increasing genetic diversity can be beneficial or detrimental to a population
- 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
State examples of when decreasing genetic diversity can be beneficial or detrimental to a population
- 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
Define selective breeding
- Involves breeding parents with particular characteristics to produce offspring with desired traits
Give an example of when and why humans have used selective breeding
- 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
State the effect of selective breeding on genetic diversity
- 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
State the difference between natural selection and selective breeding
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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
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Selective breeding
- Occurs artificially as a result of human intervention
- Occurs more rapidly
- Acquired traits do not always enhance the species’ survival
Explain how bacteria can become resistant to antibiotics
- 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
Explain the causes for bacterial resistance
- 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
Explain the consequences for human health of antibiotic-resistant bacteria
- 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
Explain how molecular homology can provide evidence of antimicrobial resistance in bacteria
- 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
List the characteristics of farming that may be contributing to antimicrobial resistance in pathogens
- Selective breeding of animals
- Overuse of medication
- Animals living in close proximity of each other
Describe the difference between antigenic drift to antigenic shift
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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
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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
NOTE: Both involve change in the virus’ genetic makeup.
Describe the conditions that make antigenic shift more likely
- 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
Describe how changes in the antigenic properties of viruses can lead to ineffective vaccines
- 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
Explain why there is lower genetic diversity for populations impacted by the founder effect
- 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
Describe how natural variation exists in a gene pool
- Through mutations that may have created new alleles
- Different allele combinations via sexual reproduction
Provide reasons that may account for a lack of genetic diversity between two separate species/populations
- Interbreeding may still be occuring
- Similar selection pressures in the two regions
