17.13 Past Paper Practise
Question 1:
(a) Explain the meaning of each of the following terms.
(i) Species. [2]
a population of similar organisms that are capable of interbreeding to form fertile offspring;
they are reproductively isolated from other such populations/cannot interbreed with other species to form fertile
offspring;
(ii) Isolating mechanism. [2]
an obstacle to interbreeding;
thus limiting gene flow between parts of the gene pool;
thus enabling divergence;
(b)Distinguish between each of the following pairs.
(i) Allopatric speciation and sympatric speciation. [4]
Allopatric: speciation due to populations occupying different geographical areas;
thus there is no gene flow between the populations;
Sympatric: speciation where the populations occupy the same geographical locality;
but gene flow is restricted between the populations/demes;
(ii) Prezygotic reproductive isolation and postzygotic reproductive isolation. [4]
Prezygotic: prevents fertilisation and the formation of zygotes;
Any two examples:
geographical isolation/
seasonal/mature at different times/
ecological/live in different habitats but in same region/
behavioural/incompatible mating rituals/incompatibility/cannot fertilise due to physiological incompatibility;;
Postzygotic: fertilisation can occur but hybrids are either not formed or are sterile;
Question 2:
Myxomatosis is a virus disease in rabbits which is transmitted from rabbit to rabbit by rabbit fleas. The virus
causes blindness and then death. 12 rabbits were released in Australia in 1859. They reproduced rapidly and
started to spread quickly, so that by 1886 they were advancing at about 66 miles per year. By 1905 they had
spanned the continent. Rabbit proof fencing was inadequate to halt their progress.
In an attempt to control the huge rabbit population myxomatosis was introduced in 1950. It spread through the
rabbit population resulting in a 99.9% mortality rate. Over the next few years, however, the rabbit population
recovered although not to the pre-myxomatosis numbers. The mortality rate to myxomatosis in rabbits at present
is around 40%.
(a) Suggest two reasons for the rapid spread of rabbits across Australia between 1859 and 1905. [2]
prolific breeding/rapid reproduction rate/quick maturing/huge reproductive capacity;
abundant food supply enabling rapid growth/little or no competition for food;
no/few predators to reduce numbers;
could burrow under fences;
(b) Suggest two reasons for the very high mortality rate in the rabbit population when myxomatosis was introduced
in 1950. [2]
no natural resistance/immunity to the virus;
rabbit population was dense/animals lived close together in burrows;
thus supported a huge flea population/fleas could easily jump from rabbit to rabbit;
thus the virus was transmitted very easily from rabbit to rabbit;
(c) With reference to the genetic mechanisms involved explain the recovery of the rabbit population from the
devastating effects of myxomatosis after its introduction in 1950. [4]
a few rabbits developed resistance/immunity to the virus;
possibly as a result of gene mutation;
these rabbits survived and bred;
passing on the resistance/mutant gene to their offspring;
these also developed immunity to the virus (before it caused symptoms/killed them);
thus resistant rabbits were selected and non-resistant rabbits died;
(d) Suggest why there is still a 40% mortality rate to myxomatosis in present day rabbit populations. [2]
not all rabbits inherit the resistance gene and so some sucumb to the virus;
the virus may have mutated changing its infectivity/pathogenicity;
Question 3:
The graphs below show three types of natural selection. The shaded areas marked with arrows show the individuals in the population which are being selected against. The dotted vertical lines show the statistical means.
(a) What names are given to the types of selection shown in graphs A, B and C. [3]
A = stabilising; B = directional; C = disruptive;
(b) Draw graphs on the three sets of axes below to show the distribution of phenotypes in A, B and C after the selection has operated for several generations. Indicate the new means with dotted lines.
3 correct graphs;;; 3 correct means;;;
(c) Describe one specific example of the type of selection shown in graph B, naming the organism and describing
the character selected in your answer. [3]
artificial selection practised by humans;
cattle/sheep/pigs or other example;
improved milk/beef yield/improved wool yield/improved hardiness/improved bacon yield/ any other example;
Question 4:
Diagrams A to E below show the heads of five species of finch inhabiting a volcanic oceanic island in the Galapagos group. The Galapagos islands lie about 600 miles from the South American mainland.
(a) What major difference between the five species is shown in the diagram? [1]
shape of beak;
(b) How might this difference be related to the habits of the finches? [3]
different diets;
some eat insects some eat seeds/nuts;
beaks become adapted over many generations by variation (mechanisms) and most efficient forms are selected;
(c) Darwin suggested that these finches probably descended from a common ancestral stock of finches. Suggest how these ancestral finches may have reached the Galapagos islands. [2]
ancestors from South American landmass/Ecuador blown by storms/winds;
or carried on driftwood;
(d) Suggest how the modern diversity of finches arose from the ancestral stock. [4]
new variations would arise due to meiosis/fertilisation;
(and) due to continued mutation;
most successful variations would survive better (than less suitable adaptations);
ref to many different niches on the island to which the birds might become adapted;
(e) The different finch species on the island do not interbreed. What does this suggest? [2]
that they are now separate species;
due to reproductive isolation;
(f) The plumage of the different finch species present is very similar. Suggest two ways in which males and females of the same finch species may recognise each other. [2]
bill shape/bird song/mating dances or ritual movements;; (any two)
Question 5:
Explain why each of the following statements is incorrect.
(a) Cross pollination enlarges the gene pool of a species. [2]
cross pollination can only occur within the gene pool of a species;
cannot bring in new genes because of reproductive isolation between different species;
(b) Different phenotypes are always the result of different genotypes. [2]
some alleles express multiple effects in the phenotype;
for example ‘mottled colour’ in mice and a ‘defect in copper absorption’ are controlled by the same allele/any other
valid example;
effect of a particular genotype may be modified by different environmental influences;
(c) Offspring from self fertilisation are genetically identical. [2]
even in self fertilisation gametes vary because of meiosis;
variation introduced due to random assortment/chiasmata;
mutation may produce variation;
(d) DNA replication must be 100% accurate. [3]
DNA replication must be accurate enough to give genetic stability;
but a low level of inaccuracy allows mutation;
and so gives variation allowing evolutionary potential/development;
Question 6:
In the box below is a list of evolutionary terms, (i) to (x), and a jumbled list of definitions, A to J is provided underneath. Select the correct definition for each term by placing the appropriate letter in the appropriate box.
| Evolutionary term: | Letter: |
| (i) Isolation | |
| (ii) Polymorphism | |
| (iii) Deme | |
| (iv) Adaptation | |
| (v) Sympatric speciation | |
| (vi) Analogy | |
| (vii) Natural selection | |
| (viii) Allopatric speciation | |
| (ix) Industrial melanism | |
| (x) Homology |
A. A local unit of population of a species in which there is random mating.
B. The restriction of interbreeding thus preventing gene flow between demes or species.
C. The production of species by isolation mechanisms operating on a gene pool in the same geographical region.
D. The presence of two or more distinct forms of a species found in the same locality at the same time.
E. The production of species by physical separation of the gene pool into different geographical regions thus restricting gene flow in the gene pool.
F. Structural similarity which results from similar specialisation in unrelated organisms.
G. Features evolve in an organism which enable it to cope better with its environment.
H. Structural similarity due to common ancestry.
I. Differential reproduction rates in nature leading to the increase in frequency of some genes and a decrease in frequency of other genes.
J. The selection of darkened phenotypes of a species due to industrial pollution. [10]
(i) B; (ii) D; (iii) A; (iv) G; (v) C; (vi) F; (vii) I; (viii) E; (ix) J; (x) H;
Question 7:
(a) (i) What is meant by the term ‘allele frequency’? [2]
the relative proportions of the alleles of a gene present in the population;
can be measured by geneticists and monitored for changes;
(ii) Suggest three causes of allele frequency change in a population. [3]
mutation;
migration;
natural selection;
(b) The algebraic statement of the Hardy-Weinberg principle is: p2 + 2pq + q2 = 1
where p and q are the frequencies of the two alleles and p + q = 1.
State the Hardy-Weinberg principle in words. [3]
in a large randomly-mating population there is a fixed relationship between gene and genotype frequencies;
in the absence of mutation, migration and natural selection;
these frequencies remain constant from generation to generation;
(c) A gene in humans controls the ability to taste the chemical phenylthiocarbamide (PTC). The ability to taste is
endowed by the presence of the dominant allele T, so that people with genotypes TT and Tt are tasters.
Individuals with genotype tt are non tasters. In a group of people, 195 individuals were able to taste PTC and
105 could not taste it.
Assuming that the Hardy-Weinberg principle applies in this case, calculate the frequency of individuals
with genotype Tt. Show your working. [4]
proportion of non tasters (tt) = 105/300 = 0.35;
since p + q = l, then (tt) which is q2 = 0.35, thus q = 0.59;
thus p = 1 – 0.59 = 0.41;
thus 2pq (Tt) = 2 x 0.41 x 0.59 = 0.48 or 48%;
Question 8:
Read the passage below and then answer the questions that follow.
(a) What is meant by the following terms?
(i) Natural selection. (line 1) [2]
differential rates of reproduction in nature;
leading to an increase in frequency of some genes/genotypes and a decrease in others;
(ii) Reproductive capacity. (line 2) [2]
the ability of a species to reproduce new offspring;
far more offspring are generally produced than can survive (due to limitations in environmental provisions or to
predation);
(iii) Heritable variation. (line 3) [2]
variations which are inherited from generation to generation;
which if they give advantage will also give survival value;
(iv) Species. (line 4) [2]
organisms which are capable of interbreeding to form fertile offspring;
not separated from other members of the species by reproductive isolation/breeding barriers;
(b) What is the importance of having ‘an enormous reproductive capacity operating in a limited environment’?
(lines 1 & 2) [2]
far more offspring are produced than can be supported by the environment;
thus there will be a struggle for survival and the best adapted will survive to reproduce more of the same/
the least adapted will die out;
(c) Explain how the ‘isolation mechanisms’ (line 7) might operate. [4]
gene flow might be restricted by organisms being sexually mature at different times;
or by failing to mate by having different mating rituals;
or by a geographical barrier;
or by physiological incompatibility/or by living in different ecological niches/any other valid example;
Question 9:
Three distinct populations of birds, X, Y and Z live on live separately on three isolated oceanic islands. The birds are all omnivorous, eating insects, tiny seeds and nectar, but have slightly different beaks and feather colourings.
The islands are frequently swept by strong south-westerly gales. Populations X and Y can interbreed and form fertile offspring. Population Y can interbreed with population Z but the offspring are sterile. Population X will not
even mate with population Z and artificial insemination of Z birds with X bird sperm is unsuccessful.
(a) Suggest an explanation why populations X and Y can still interbreed and form fertile offspring. [4]
populations/islands X and Y are relatively close/not geographically isolated;
thus birds can still come into contact and breed together;
no chance for any mutations to become genetically isolated/no chance for demes to become established;
so little divergence occurs between X and Y/still reproductively compatible;
population Y probably arose from population X because of prevailing winds;
chromosomes of hybrids will still pair in meiosis (so gametes can form);
(b) Suggest an explanation why mating between Y and Z produces infertile offspring. [5]
populations/islands Y and Z are geographically isolated;
thus will not normally interbreed;
thus mutations/genetic variation in the two populations will occur independently;
thus become isolated by post-zygotic isolation/chromosomes of Y differ from those
of Z/will not pair in meiosis (to form gametes);
Z probably arose from Y as blown by winds rather than originating from X;
not diverged sufficiently to have different courtship rituals/behavioural patterns;
some Y may still be blown to Z allowing occasional interbreeding (although this has now become ineffective);
(c) Suggest an explanation for the inability of populations X and Z to mate. [5]
population/island X is geographically isolated from population Z;
by ocean and island Y;
thus mutations/genetic variation in the two populations has continued independently;
they are now reproductively isolated because their courting/mating behaviours differ;
incompatible mating rituals/courtship dances/plumage colours/breeding times;
this is pre-zygotic isolation;
ref to chromosomes of X will no longer match with those of Z even if they could mate;
Question 10:
The diagram below shows plumage variation in four subspecies of the flycatcher, Monarcha castaneoventris of the Solomon Islands in the South Pacific Ocean, and their distribution throughout the islands. These subspecies of flycatcher can still successfully interbreed.
(a) (i) Suggest and explain a mechanism by which the different subspecies may have arisen. [5]
(from the distribution map it is clear that) the populations are isolated on different islands/groups of islands;
even though they are not separated by huge distances/may not like flying over water/psychological barrier to crossing water;
gene mutations/genetic variation may cause different plumage patterns/colours;
which are selected for/of survival/camouflage value on different islands;
since interbreeding is restricted these variations can become established;
but if they do interbreed chromosomes can still pair in meiosis so gametes can be made (by offspring)/not reproductively isolated;
also courtship rituals are still compatible/not behaviourally isolated;
(ii) What further events must occur before the subspecies become full independent species? Explain your answer. [4]
continued restriction on interbreeding/isolation;
continued mutation/genetic variation;
this must be selected for and become established;
until behavioural/mating rituals become incompatible/behavioural isolation;
and chromosomes become so different that meiosis cannot occur in any hybrids;
since pairing/synapsis of chromosomes cannot occur;
(b) Distinguish between prezygotic and postzygotic isolation. [4]
prezygotic isolation operates before fertilisation occurs/prevents fertilisation;
for example, geographic/behavioural/ecological/seasonal/incompatible gametes;
postzygotic isolation operates after fertilisation;
for example, hybrid sterility/hybrid inviability or premature death;
Question 11:
Read through the following passage about speciation and isolation mechanisms and then complete it by filling in
the spaces with the most appropriate word or words.
Isolation mechanisms (barriers) restrict……………………. flow between populations thus allowing the separated
populations to diverge genetically from one another leading to the formation of new separate species.
Speciation occurring due to an isolation mechanism operating within a gene pool, in a single geographical region, is
known as …………………………. speciation.
Speciation due to isolation mechanisms operating on gene pools in different geographical regions is known as
…………………………. speciation. The type of isolation mechanism which results in this type of speciation is
……………………………….. isolation.
Reproductive isolation is classed into two main types. The first type is …………………………… isolation in which
populations may be prevented from completing fertilisation and zygote formation. This type of isolation mechanism
includes ………………………………… isolation when populations live in the same locality but mature at different
times of the year and ………………………… isolation when populations live in the same area but in different habitats.
The second type of reproductive isolation is …………………………. isolation in which fertilisation occurs but the
hybrids formed are either …………………………… or ……………………………… . [10]
gene;
sympatric;
allopatric;
geographical;
pre-zygotic;
seasonal;
ecological;
post-zygotic;
inviable; sterile; (these two points can be given either way round)
Question 12:
Suggest explanations for the following evolutionary events:
(a) the rapid development of warfarin resistance in rodents in Great Britain in the 1960s. [4]
ref to use of warfarin as rat poison/its action as an anticoagulant resulting in bleeding;
interferes with action of vitamin K;
mutation of normal gene to mutant gene which gave warfarin resistance;
the mutant resistant gene acted as a dominant;
thus both homozygotes and heterozygotes could survive exposure to warfarin;
allowed rapid spread of the resistant strain of rats/alleles (throughout Britain);
(b) the development of industrial melanism in peppered moths (Biston betularia) in Great Britain in the mid 1800s. [4]
original population of moths were white with black specks;
camouflaged against predation (by birds) on bark of silver birch trees;
silver birch bark became darker during the industrial revolution due to pollutants/soot;
occasional black mutants appeared in the population/ref gene mutation to give melanic form;
these were better camouflaged (on the polluted bark) than the normal/white forms;
thus the white forms were predated on and the black forms survived to breed;
thus the black/melanic forms were selected for (and the population of moths became melanic);
(c) the continual failure to develop a vaccine to give long term protection against influenza virus. [4]
influenza virus has a very high mutation rate;
due to base changes in its RNA/in one or more of its 8 genes/pieces of RNA;
these change the nature of the surface antigens/neuramidase/haemagglutinin;
(every few months) small genetic changes occur causing small antigenic changes/antigenic drift/producing new strains of influenza;
(every few years) much larger genetic changes occur causing major antigenic change/antigenic shift/producing new species/
subtypes of influenza;
ref antigenic drift produces epidemics/antigenic shift produces pandemics;
antibodies against one strain of influenza are unlikely to protect against new strains/species of influenza;
(d) the incompleteness of the fossil record. [4]
only hard parts/bones/exoskeletons/plant cell walls stand a chance of fossilisation;
most dead organisms just rot away/decay/may be eaten;
conditions for fossilisation are relatively rare;
body needs to be buried in anaerobic conditions to prevent decay;
suitable example/peat/ river mud/silt/sand;
and where petrification/impregnation with inorganic salts can occur;
most fossils are still buried/hidden in rocks/sections of the fossil record may be destroyed by earthquakes/weathering;