Question
List two causes of variation within a gene pool.
Describe how variation contributes to evolution by natural selection.
Outline what is required for speciation to occur.
▶️Answer/Explanation
Markscheme
a. sexual reproduction / random fertilization / meiosis
b. mutation
No mark for crossing over unqualified.
Reject natural selection/evolution as causes of variation.
a. (variation is) different phenotypes/differences between individuals in a population/species
b. struggle/competition for survival
c. some individuals have advantageous characteristics/are better adapted/have greater chance of survival/reproduction (than others)
d. favourable alleles/genetic variations passed on/inherited by offspring/next generation
Reject “pass on phenotypes”.
a. divided species/gene pool / part of species/gene pool becomes separated / species splits into separate populations
b. reproductive isolation / lack of interbreeding
Mark point b refers to a lack of interbreeding between separated populations in a species, not the lack of interbreeding after speciation.
c. may be due temporal/behavioural/geographic isolation
d. different natural selection/different selective pressures
Question
Outline the types of evidence that can be used to support the theory of evolution.
Explain two examples of evolution in response to an environmental change.
▶️Answer/Explanation
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
a. fossils (give evidence of evolution);
b. fossils show different species existed in the past/species changed over time;
c. selective breeding of (domesticated) animals/crop plants;
d. selective breeding shows that (artificial) selection can cause rapid change;
e. homologous (anatomical) structures/vestigial organs (give evidence of evolution);
f. homologous structures/pentadactyl limbs/other example show common ancestry;
g. DNA/base/amino acid sequences show (common) ancestry/species diverged;
Do not award marks for examples of evolution in response to environmental change such as melanism as this is tested in part (c) of this question.
Remember, up to TWO “quality of construction” marks per essay.
For each example:
a. a named example of a species that has evolved in this way;
b. description/clear statement of the change that occurred in the environment;
c. description/clear statement of different varieties (that existed at the same time);
d. explanation of/reason for one variant having a selective advantage;
e. the change in the population/species due to natural selection/evolution;
Do not award the last mark if the change is explained using Lamarckism rather than natural selection.
Example:
f. Staphylococcus aureus/MRSA/Clostridium difficile/other named species;
g. introduction/use of an antibiotic/named antibiotic;
h. some bacteria were resistant and others were not;
i. resistant bacteria survived (and multiplied) while non-resistant were killed;
j. percentage of the population showing resistance increased;
[8] can be awarded if the candidate scores [5] for one example and [3] for the other.
Do not accept examples where the evidence of evolution comes from fossils, or where the variation is not heritable.
Question
Outline how antibiotic resistance in bacteria can arise in response to environmental change.
Outline the principle of immunity.
Discuss the benefits and dangers of vaccination.
▶️Answer/Explanation
Markscheme
antibiotic resistance can be inherited;
alleles for resistance can be passed from one cell to another by exchange of plasmids/conjugation;
some varieties are more resistant than others;
bacteria reproduce very rapidly and have high mutation rate;
evolution can occur rapidly;
increased exposure to antibiotics is the environmental change that selects for resistant varieties;
for example, in hospitals / animal feed / inappropriate prescriptions / not finishing prescriptions;
bacteria without resistance die / resistant bacteria survive and pass on genes to next generation;
results in change in genetic makeup of population;
immunity is the ability of an organism to resist infection;
due to presence of (specific) antibodies;
immunity can be active or passive;
passive due to receiving antibodies from external sources/across placenta/from breast milk/injection;
active results from facing an infection directly/through vaccination;
pathogen/foreign cell invades body;
leads to clonal selection/formation of B memory cells;
B-cells produce specific antibodies;
if same pathogen enters body again memory cells activated/stimulated to divide;
antibodies produced faster and in greater amounts;
Benefits: [4 max]
immunity results
can limit pandemics/epidemics/spread of (infectious) diseases;
diseases can be eradicated/smallpox eliminated;
reduces mortality/deaths due to disease;
can protect vulnerable groups/young/old/with other conditions;
decrease crippling effects of diseases (such as polio);
decreased health care costs;
Dangers: [4 max]
may produce (mild) symptoms of the disease;
human error in preparation/storage/administration of vaccine;
individual may react badly to vaccine / defective immune system / hypersensitive/allergic reaction;
immunity may not be life-long/booster required;
possible toxic effects of mercury-based preservatives/thimerosal;
Question
Describe the relationship between the rise in the concentration of atmospheric carbon dioxide and the enhanced greenhouse effect.
Outline the precautionary principle.
Antibiotic resistance in bacteria is an example of evolution in response to environmental change. Using another example, explain how an environmental change can lead to evolution.
▶️Answer/Explanation
Markscheme
CO2 is a greenhouse gas;
increases in CO2 increase/enhance the greenhouse effect;
greenhouse effect is a natural phenomenon but not its increase;
Earth receives short wave radiation from the sun;
reradiated from Earth as longer wave radiation/infra red/heat;
CO2 /greenhouse gases trap/absorb longer wave radiation/infra red/heat;
global warming happened during same time/period as CO2 rise;
CO2 concentration correlated (positively) with global temperature / global temperature increases as CO2 concentration increases;
(causal) link accepted by most scientists;
no proof that man-made increases in CO2 have caused global warming;
those proposing something must prove that it causes no harm;
before they start to do it;
objectors do not have to prove that there will be harm;
activities that risk/threaten/may cause harm are banned;
trials/tests must be done first;
precautionary principle is applied when possible consequences are severe;
precautionary principle should be used in the case of global warming;
action should be taken to reduce CO2 emissions before proved it is the cause;
another example of implementation of the precautionary principle;
natural selection (in correct context);
better-adapted individuals survive/more likely to survive;
more reproduction/genes passed on by better adapted individuals;
name of species; (accept even if remainder of answer is invalid)
description of original/decreasing phenotype;
type of environmental change that led to evolution;
consequence of environmental change
description of new/increasing phenotype;
genetic basis of phenotypes;
reason for new phenotype being better adapted;
detail of reason for adaptedness of new phenotype;
The following has been provided as an example answer.
great tit;
bird that lays its eggs in spring;
global warming/climate change;
more caterpillars (on trees) in early spring;
laying eggs earlier in spring;
time of egg laying is (partly) genetically controlled;
eggs laid early hatch at start of period of greatest food abundance;
more young can be fed/young grow faster/fewer deaths;