CHAPTER 7: GENETIC CHANGES IN POPULATIONS

Question 1

what is a population

Answer 1

members of one species living in one region at a particular time

Question 2

what is a gene pool

Answer 2

• all of the alleles present in a population
  
  • allele - version of a gene
  • changes to the gene pool result in evolution
• each population has a gene pool that consists of all the alleles present for each gene within the population
• the greater the variation and number of alleles within a genetic pool, the greater the genetic diversity

Question 3

mutations as a source of variation

Answer 3

• generally, the genetic material of an organism is stable in its base sequence and chromosomal location and is passed unchanged from generation to generation
• mutations are changes in the DNA sequence
  
  • can be small → changes to nucleotides (point)
  • can be larger → changes to sections of chromosomes (block)

Question 4

base substitution

point mutation

Answer 4

• changes one nucleotide (substituted out)
  
  • silent → doesn’t change the amino acid
  • nonsense → codes for STOP codon
  • missense → changes one amino acid

Question 5

frameshift

point mutation

Answer 5

• change all amino acids following the mutation
• insertion or deletion of a nucleotide
  
  • alter the base sequence of a gene so that the message it encodes no longer makes sense - referred to as changing the reading frame

Question 6

block mutations

Answer 6

• duplication - a segment is copied
• deletion- a segment is removed
• inversion - segment of a chromosome rotates through 180 degrees (removed and then replaced in reverse orfer
• translocation - section of chromosome breaks off and joins a new chromosome

Question 7

polyploidy

Answer 7

• a change in the number of sets of chromosomes
• most species are diploid - 2 sets of chromosomes
• some species have more than two sets of chromosomes - polyploid (eg. 3 sets of chromosomes (3 of each chromosome)
• can occur naturally through crossbreeding/hybridisation or can be induced using chemicals

Question 8

aneuploidy

Answer 8

• change in chromosome numbers (NOT SETS - WHICH IS POLYPLOID)
• can result when homologous chromosomes fail to segregate in anaphase of meiosis stage I or when sister chromatids fail to segregate in anaphase of mitosis or meiosis stage II
• down syndrome and turner’s syndrome

Question 9

hardy-weinberg equilibrium

Answer 9

• in large, randomly mating populations where there are
  
  • no mutations
  • no migration
  • all phenotypes are equally suited to the environment
• there will be no change in allele frequencies
  
  • → gene pool/allele stability
  • → no evolution (remain the same)

Question 10

factors affecting a populations gene pool

Answer 10

• gene pools tend to remain constant (unchanging)
• unless something acts to change the gene pool
  
  • mutation
  • selection (natural or sexual)
  • random events (genetic drift)
  • gene flow (emigration and immigration)
  • human intervention

Question 11

selection pressures

Answer 11

• an external agent which affects an organism’s ability to survive in a given environment
  
  • physical - climate change, shelter, food availability
  • biological - competition, predators, disease
  • chemical - pollutants, drugs (antibiotics)
• selection pressures can select for or against phenotypes
• selection pressures act on the phenotype and change the genotype (gene pool - allele frequencies)

Question 12

natural selection

Answer 12

• selection pressures lead to natural selection
• natural selection occurs when any selecting agent acts on a population creating a selective advantage
• the differences in survival and reproduction result in changes to allele frequencies
  
  • results in evolution

Question 13

mechanism of natural selection

Answer 13

• natural selection → evolution → change over time
• Darwin formed the theory based on the following observations
  
  • organism produce more offspring than actually survive
  • every organism must struggle to survive (selection pressures)
  • there is variation within a population (alleles)
  • some variations allow members of a species to survive and reproduce better than others (better adapted to survive)
  • organisms that survive and reproduce pass their traits to their offspring and the helpful trait gradually appear in more and more of the population
    
    • pass on alleles and increased allele frequency
  • over many generations, natural selection can cause the advantageous alleles to become common within a population
  • this can often lead to a decrease in genetic diversity as those alleles that lead to a beneficial trait may become fixed while others that confer a selective disadvantage may be lost

Question 14

mutations of viruses

Answer 14

• viruses mutate at a high rate during replication and antigens can change
  
  • especially for viruses that have RNA as their nucleic material - no proofreading mechanisms like enzymes that check for mutations in RNA as compared to DNA
  • eg, influenza, stars cov 2, HIV, ebola
• the degree to which the antigens are altered will result in either antigenic drift or antigenic shift

Question 15

darwin vs lamarcks theory

Answer 15

• Darwin: Giraffes with short neck die, giraffes with long neck pass on the trait
• Lamarck: physical changes in organisms during their lifetime - such as greater development through increased USE, could be transmitted to their offspring.

Question 16

definition of selection pressures

Answer 16

selection pressure is a factor in an organism’s environment that removes unsuited individuals.

Question 17

what is genetic flow/drift

Answer 17

chance events that cause changes to gene pools

Question 18

genetic drift

Answer 18

• change in allele frequency from one generation to the next due to chance events
• does not favour one allele over another → both are equally subject to being affected by genetic drift (completely random)
• more likely to decrease genetic diversity and occur in smaller populations
• the smaller the population the greater the potential impact of genetic drift
• can lead to the decrease, and eventual loss of favourable alleles from the gene pool

Question 19

genetic drift - bottleneck effect

Answer 19

• population size reduced drastically by a chance event for at least one generation
• survivors/new populations that reproduce to give the next generation may be an unrepresentative sample of the gene pool (original population)
• Increasing population number after the bottle neck does not increase genetic diversity of the gene pool.

Question 20

examples of chance events

Answer 20

• destruction of habitat
• hunting and poaching
• drought
• bushfire
• introduced diseases

Question 21

genetic drift - founder effect

Answer 21

• a new colony is started by a few members from a larger population
  
  • founder population eg. small mating pair
  • new gene pool highly likely to
    
    • have reduced genetic variation
    • not contain alleles from the original population/ be an unrepresentative sample of the original population - as the founder population would not have all the alleles of the original population

Question 22

gene flow

Answer 22

• movement of alleles between interbreeding population
• transferring alleles from one gene pool to another
• movement of alleles can increase the genetic diversity of a population when a new allele is introduced
• no gene flow between populations → become isolated and any new alleles that arise will remain in the one population
• immigration → increase/introduce
• emigration → decrease/lose

Question 23

change in population

Answer 23

change in population = births - deaths + immigration - emigration

Question 24

impact of events on genetic diversity

Answer 24

• natural selection → decrease in genetic diversity
  
  • during natural selection, a particular phenotype has a selective advantage so particular alleles become more common, while others are removed
• gene flow → increase in genetic diversity
  
  • usually, the movement of alleles results in new alleles coming into a population
• genetic drift → decrease in genetic diversity
  
  • random chance events may lead to a loss of alleles in a population
• bottleneck → decrease in genetic diversity
  
  • event leading to the death of many members of a population may lead to a loss of alleles in a population
• founder effect → decrease in genetic diversity
  
  • the population is descended from individuals with limited diversity in their genetic material
• mutations → increase in genetic diversity
  
  • mutations are the source of new alleles

Question 25

high genetic diversity

Answer 25

• gene pool with a large number of alleles for each gene
• these populations:
  
  • are resilient to environmental changes
  • tend to be large in size
  • have gene flow with other populations
• more variation → reduced chance of extinction

Question 26

low genetic diversity

Answer 26

• gene pool with a small number of alleles for each gene
• these populations:
  
  • are small
  • do not have gene flow
  • high rates of inbreeding
  • limited ability to adapt to environmental changes
• less variation → higher chance of extinction

Question 27

why can bacteria become resistant to antibiotics

Answer 27

• have a short generation - shorter time between successive generations (one organism to offspring - for bacteria around 20 min)
• inherited genes will affect the gene pool faster
• increase rate of evolution and how pathogens are treated
• some bacteria become resistant to antibiotics
  
  • occurs through the process of natural selection where the presence of antibiotics acts as the environment selection pressure

Question 28

process of natural selection for antibiotic-resistant bacteria

Answer 28

• during division one of the bacteria undergoes a mutation in its DNA → resulting in antibiotic resistance
• when the antibiotic is added all sensitive bacteria are killed
• antibiotic-resistant bacterium survives and divides
• forms a population of antibiotic-resistant bacteria

Question 29

factors that increase bacterial resistance

Answer 29

• doctors overprescribing antibiotics
  
  • leads to more opportunities for bacteria to mutate and consequently evolve to become resistant
• patients not finishing a course of antibiotics
  
  • the full course will kill infectious bacteria, but if not taken, some bacteria may survive and be more likely to reproduce and evolve
• increased use of antibiotics in livestock farming
  
  • farmers often give livestock antibiotics to grow faster and reduce the infection rate
  • enables resistant bacteria to develop in livestock and can be ingested and transferred to humans
• poor hygiene and sanitation
  
  • increase spread and transmission of bacteria
• lack of infection control in medical centres
  
  • increases the likelihood of resistant bacteria being transferred between patients.

Question 30

consequences of bacterial resistance

Answer 30

• more challenging to treat common infectious diseases
• normally treatable diseases can now be life-threatening if they have an antibiotic-resistant strain of bacteria
• solutions
  
  • new antibiotics - which bacteria aren’t resistant to
  • new treatment options
  • time length of treatments being altered

Question 31

antigenic drift

Answer 31

when a point mutation alters a virus’s nucleic materials resulting in small changes to its antigens

Question 32

antigenic shift

Answer 32

occurs when two or more strains of a virus combine to form a new strain of the virus with antigens from each of the original strains

Question 33

structure of influenza virus

Answer 33

• two main antigens of an influenza virus are hemagglutinin (HA) and neuraminidase (NA)
• three main types of influenza - A, B, C
  
  • influenza B and C are only found in humans
  • influenza A can also infect animals
• influenza A virus has 17 diff HA antigens and 9 NA antigens

Question 34

selective advantage

Answer 34

• the phenotype that makes the greater contribution to the gene pool in the next generation (higher reproduction rate) has a higher fitness value and is said to have a selective advantage.

Question 35

influenza antigenic drift

Answer 35

• RNA undergoes frequent mutations as it replicates
• overtime, accumulation of point mutations means antigenic properties of the mutated virus have changed, causing a new subtype
• no longer recognised by the immune system memory cells → treated as a new pathogen
• all influenza types A, B, and C are subject to the gradual change of antigenic drift

Question 36

influenza antigenic shift

Answer 36

• one host is infected with two different kinds of influenza A virus
• a new combination of genetic material can be produced by re-assortment
• may produce a novel influenza subtype (yet to be seen)
  
  • not likely to have immunity to this subtype and will develop influenza
  • one of the main causes of pandemics and epidemics
  • a livestock and poultry market provides an opportunity for cross-infection of a host by influenza viruses from different species
  • genetic reassortment produces an antigenic shift that creates a new subtype of influenza A virus that can infect people

Question 37

consequences of antigenic drift and antigenic shift

Answer 37

• no mutation of virus (same strain)
  
  • full immune response immediately
  • B memory cells produce antibodies quickly
  • no illness
• antigenic drift of influenza
  
  • partial immune response
  • B memory cells can produce antibodies and can form SOME antibody-antigen complexes
  • no illness
• antigenic shift of influenza
  
  • immune system has no memory of pathogen
  • time required for adaptive immune response
  • illness
• constantly mutating therefore a single vaccination cannot be made and administered to provide long-term protection
• Australian government make a vaccine covering the most common seasonal strains of influenza for each year

Question 38

what is artificial selection

Answer 38

the process by which humans breed animals or plants to increase the proportion of chosen phenotypic traits (desired by humans - not necessarily beneficial to the organism in the environment)

Question 39

mechanism of artificial selection

Answer 39

• there is variation within the population’s gene pool
• humans select individuals with a desirable trait
• these individuals breed (reproduce) and pass their alleles onto the next generation
• the alleles that lead to the desired phenotype will be inherited by subsequent generations and they can increase in frequency in the gene pool over time

Question 40

effect of selective breeding on gene pool

Answer 40

• breeders manipulate the gene pool of their breeding stock
• can favour features that are disadvantageous for survival and reproduction and would be selected against in the wild
  
  • the alleles for these features would likely be lost through natural selection
  • but is retained through selective breeding

Question 41

State 3 differences between natural selection and selective breeding

Answer 41

natural selection
* environmental factor selects for phenotypes
* occurs in the natural environment
* selects for traits that increase the survival of species

selective breeding
* humans select for desired phenotypes
* selected phenotypes may not be advantagous for the organism
* occur in domesticated animals

Question 42

Compare natural selection to genetic drift

Answer 42

• natural selection selects for favourable traits while genetic drift occurs randomly due to chance events
• natural selection occurs over a long period of time, and genetic drift can occur as quick as over one generation

Question 43

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

Answer 43

• vaccine doesn’t target the specific strain of virus (the antibodies produced from the stimulation of the vaccine aren’t specific to that strain)
• Vaccines contain new antigens resulting in the production of new antibodies OR memory cells specific to the antigen
• Memory cells allow for a faster OR larger immune response
• Any existing memory cells specific to the previous antigens would no longer be effective.

Question 44

difference between antigenic drift and antigenic shift

Answer 44

• drift:point mutation, shift: two strains being combined
• both changes antigens on virus
• antigenic drift is more common occurs over time and antigenic shift can occur quickly

Question 45

germ line mutations vs somatic mutations

Answer 45

• Germ-line mutations are the source of mutational variation within a species as the trait needs to be passed onto the next generation.
• Somatic mutations end when the individual dies.
• Germ line mutations can be spontaneous or may be the result of exposure to mutagenic agents such as radiation, or several types of chemical.

Question 46

variation due to recombination

Answer 46

• the crossing over between homologous chromosomes, meiosis and the segregation of alleles produces new recombination’s of genes
• the main cause of variation within a population of sexually-reproducing species.
• the more chromosomes that are involved the greater the chances of recombination.

Question 47

how does a small population likely result in extinction

Answer 47

• a small population size would mean a limited gene pool and
• reduced variation would reduce the chances of survival
• if there were an environmental change the chance of a favourable characteristic existing in the
  population is unlikely as individuals would all be genetically similar
• there is insufficient variation in the population to survive within their current environment
• inbreeding could result in an increased chance of genetic diseases
• there is a change in allele frequency due to genetic drift.

Question 48

how can natural variation exist between individuals in a population

Answer 48

• Natural variation exists in a population through mutations that may have created new alleles
• different allele combinations in sexual reproduction.
• Changes in chromosome number may change phenotype

Question 49

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

Answer 49

• the lower genetic diversity was due to the founder effect.
• The founder population will not have all the variations of alleles as seen in the original population
• the descendants of these two individuals will not be able to inherit other alleles and there will be less variation in phenotypes of the lizards.

Question 50

extinction

Answer 50

when no members of a particular species exist

Question 51

why is it an advantage to have a diverse gene pool

Answer 51

• diversity provides a variety of phenotypes
• there is an increased chance of survival of the population if the environment changes

Question 52

factors that increase chance of antigenic shift

Answer 52

• increased population coming into contact with wild animal habitat
• live stock/poultry markets
• urbanisation of habitats in close proximity with wild animals
• transportation of wild animals

Question 53

how does mass extinctions act as an opportunity for rapid evolution of other species

Answer 53

• Reduces competition – the extinction reduces populations and species. With less competitors, can lead to opportunities for evolution of other species
• Environmental Changes – mass extinctions can cause changes in the environment, these new conditions may allow species to have an advantage and evolve
• Surviving species may have the opportunity to develop new traits, behaviour and adaptations for rapid speciation