1 Genetic variation within a population is the raw material on which natural selection can act.
2 Meiosis, random mating and the random fusion of gametes produce genetic variation within populations of sexually reproducing organisms. Variation is also caused by the interaction of the environment with genetic factors, but such environmentally induced variation is not passed on to an organism’s off spring. The only source of new alleles is mutation.
3 All species of organisms have the reproductive potential to increase the sizes of their populations, but, in the long term, this rarely happens. This is because environmental factors come into play to limit population growth. Such factors decrease the rate of reproduction or increase the rate of mortality so that many individuals die before reaching reproductive age.
4 Within a population, certain alleles may increase the chance that an individual will survive long enough to be able to reproduce successfully. These alleles are therefore more likely to be passed on to the next generation than others. This is known as natural selection.
5 Normally, natural selection keeps allele frequencies as they are; this is stabilising selection. However, if environmental factors that exert selection pressures change, or if new alleles appear in a population, then natural selection may cause a change in the frequencies of alleles; this is directional selection.
6 Over many generations, directional selection may produce large changes in allele frequencies. This is how evolution occurs.
7 The evolution of antibiotic resistance in bacteria and the spread of industrial melanism in moths are examples of changes in allele frequencies. The role of malaria in the global distribution of sickle cell anaemia is an example of how two strong opposing selection pressures can counterbalance each other in maintaining two alleles within certain populations.
8 A species can be defi ned as a group of organisms with similar morphology, behaviour, physiology and biochemistry that are capable of interbreeding to produce fertile off spring. In practice, however, it is not always possible to determine whether or not organisms can interbreed.
9 New species arise by a process called speciation. In allopatric speciation, two populations become isolated from one another, perhaps by some geographical feature, and then evolve along different lines until they become so diff erent that they can no longer interbreed. In sympatric speciation, new species may arise through polyploidy.
10 Artificial selection involves the choice by humans of which organisms to allow to breed together, in order to bring about a desirable change in characteristics. Thus artificial selection, like natural selection, can aff ect allele frequencies in a population.
1. End-of-chapter questions
1 Which of the following gives rise to genetic variation in a population?
1 crossing over and independent assortment in meiosis
2 different environmental conditions
3 random mating and fertilisation
4 mutation
A 1,2,3 and 4 B 1, 2 and 3 only C 1, 3 and 4 only D 2, 3 and 4 only
2 A species of finch living on an isolated island shows variation in beak size. Birds with larger beaks can eat larger seeds.
After a period of drought on the island, large seeds were more plentiful than small seeds and the average size of the finches' beaks increased.
What explains this increase in size of beak?
A artificial selection acting against finches with small beaks
B directional selection acting against finches with small beaks
C increased rate of mutation resulting in finches with larger beaks
D stabilising selection acting against finches with the smallest and largest beaks
3 Which effect of natural selection is likely to lead to speciation?
A Differences between populations are increased.
B The range of genetic variation is reduced.
C The range of phenotypic variation is reduced.
D Favourable alleles are maintained in the population.
4 There are three genotypes of the gene for the β-globin polypeptide: HbAHbA, HbAHbs and HbsHbs.
Copy and complete the table to show which genotypes have a selective advantage or disadvantage in different regions of the world.
5 The wings of butterflies are covered with microscopic scales that give them their colour and also provide waterproofing.
The wings of some species have large transparent areas through which the colour of the vegetation on which the butterfly has settled can be seen. Because they lack scales, these areas have poor waterproofing. The butterflies are eaten by birds.
a Describe two selection pressures that are likely to control the size of the transparent areas of the wings of these butterflies.
b In what circumstances might there be selection for larger transparent areas in the wings?
6 Rearrange the order of the following statements to give a flow diagram showing the evolution of resistance to the antibiotic streptomycin by the bacterium Escherichia coli.
1. Most of the population of E. coli is resistant to streptomycin.
2. A mutation in a DNA triplet of a plasmid, changing TTT to TTG, gives an E. coli bacterium resistance to streptomycin.
3. The resistant bacterium divides and passes copies of the R plasmid to its offspring.
4. Sensitive bacteria die in the presence of streptomycin as a selective agent.
5. The frequency of the mutated gene in the population increases.
6. The resistant bacterium has a selective advantage and survives.
A Differences between populations are increased.
B The range of genetic variation is reduced.
C The range of phenotypic variation is reduced.
D Favourable alleles are maintained in the population.
4 There are three genotypes of the gene for the β-globin polypeptide: HbAHbA, HbAHbs and HbsHbs.
Copy and complete the table to show which genotypes have a selective advantage or disadvantage in different regions of the world.
5 The wings of butterflies are covered with microscopic scales that give them their colour and also provide waterproofing.
The wings of some species have large transparent areas through which the colour of the vegetation on which the butterfly has settled can be seen. Because they lack scales, these areas have poor waterproofing. The butterflies are eaten by birds.
a Describe two selection pressures that are likely to control the size of the transparent areas of the wings of these butterflies.
b In what circumstances might there be selection for larger transparent areas in the wings?
6 Rearrange the order of the following statements to give a flow diagram showing the evolution of resistance to the antibiotic streptomycin by the bacterium Escherichia coli.
1. Most of the population of E. coli is resistant to streptomycin.
2. A mutation in a DNA triplet of a plasmid, changing TTT to TTG, gives an E. coli bacterium resistance to streptomycin.
3. The resistant bacterium divides and passes copies of the R plasmid to its offspring.
4. Sensitive bacteria die in the presence of streptomycin as a selective agent.
5. The frequency of the mutated gene in the population increases.
6. The resistant bacterium has a selective advantage and survives.
7 Copy and complete the table to compare artificial selection with natural selection.
a 40 dark moths were placed on dark bark. Calculate the number of moths taken by birds. Show your working. [2]b Suggest an explanation for the differences in the numbers of moths taken by birds. [4]
[Total: 6]
9 The snail Cepaea nemoralis may have a yellow, pink or brown shell. Each colour shell may have up to five dark bands, or have no bands. Both shell colour and number of bands are genetically controlled. The snails are eaten by birds such as thrushes, which hunt by sight.
The following observations were made:
• Most snails living on a uniform background, such as short grass, have no bands.
• Most snails living on a green background, such as grass, are yellow.
• Most snails living on a non-uniform background, such as rough vegetation, have bands.
a Suggest an explanation for these observations. [4]
b Predict the phenorype of snails living on a dark background of dead leaves.[2]
c Suggest what will happen, during the course of a year, to the frequencies of the different alleles controlling shell colour and banding in a snail population living in deciduous woodland. (Deciduous trees shed their leaves in autumn. The background for the snails will be made up of dead leaves in the autumn and winter, and green vegetation in the spring and summer.)[4]
[Total: 10]
10 The heliconid butterflies of South America have brightly coloured patterns on their wings. A hybrid between two species, Heliconius cydno and H melpomene, has wing patterns that are different from both parental species.
An investigation was carried out to see whether the hybrid was a new species.
Separate groups of four butterflies, each consisting of a male and female of one of the parental species and a male and female of the hybrid, were placed together and their choices of mates recorded. The results are shown in the table.
a With reference to the information in the table, explain whether or not the results of the investigation suggest that the hybrid butterfly is a separate species. [4]
b Suggest how the ybrid could be reproductively isolated from the two parent species of butterfly. [2]
c Briefly describe how allopatric speciation can occur.[4]
[Total: 10]
2. End-of-chapter answers
1 C
2 B
3 A
5 a predation by birds, tending to increase the size of the transparent areas of the wings as they increase camoufl age;
rainfall, because smaller transparent areas give an advantage;
b increased predation/drier conditions;
6 2, 4, 6, 3, 5, 1
1 mark for every 2 correct answers
Exam-style questions
2 B
3 A
5 a predation by birds, tending to increase the size of the transparent areas of the wings as they increase camoufl age;
rainfall, because smaller transparent areas give an advantage;
b increased predation/drier conditions;
6 2, 4, 6, 3, 5, 1
1 mark for every 2 correct answers
Exam-style questions
8 a 40 × 40 ÷ 100 = 16; [2]
b pale moths are camouflaged on pale bark, and dark moths on dark bark;
predators/birds, hunt by sight;
fewer moths taken that match bark;
refer to figures: 20% v. 44% of pale moths/15% v. 40% of dark moths; [4]
[Total: 6]
9 a camouflage from bird predators hunting by sight;
yellow blends into grass but pink or brown are easily seen;
bands break up outline against rough vegetation;
yellow or pink without bands are easily seen; [4]
b brown/five bands; [2]
c selection favours alleles for brown shell and for bands in autumn and winter;
selection favours alleles for yellow shell and few or no bands in spring and summer;
gradual change in selection pressures as seasons change;
keeps all alleles in the population; [4]
[Total: 10]
10 a behaves as good species with no intermating in relation to H. melpomene;
15 matings between H. melpomene males and females and between hybrid males and females; behaves as less good species in relation to H. cydno;
no matings between H. cydno males and hybrid females;
but three matings between H. cydno females and hybrid males; [max. 4]
b select mates on basis of wing colours and patterns;
hybrid wing pattern sufficiently different from parent species to give good isolation from H. melpomene; [2]
c needs geographical separation;
selection pressure diff erent in the separated populations;
different alleles selected for;
in time the diff erences between the two populations are so great that they do not interbreed should they happen to meet; [4]
[Total: 10]
