Summary of Selection and Evolution

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.

7   Copy and complete  the table to compare artificial selection with natural  selection.

8   Pale and dark peppered  moths were collected and placed on pale and dark areas of bark on trees in a park in Liverpool, England.  Some of the moths were predated  by birds. The results of the investigation  are shown in the table.

 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

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]