4.4.2- Natural selection

Question 1

Explain how natural selection results in

development of new characteristics. (3)

Answer 1

1- Random mutations result in new alleles

2. Some alleles are advantageous which make the individual more likely to survive and reproduce
3. Their offspring receive the new allele and frequency continues to increase over many generations

Question 2

5 principles of natural selection (4)

Answer 2

1. Mutations arise spontaneously
2. Some mutations are beneficial
3. Changes allele frequency
4. Genes are inherited by offspring

Question 3

What is stabilising selection?

Answer 3

Occurs when environmental conditions
stay the same. Individuals closest to the
mean are favoured, and any new
characteristics are selected against.
Results in low diversity.

process where natural selection favours an average phenotype.

Question 4

What is a selection pressure? and give an example

Answer 4

external factor that influences the reproductive success of an individual e.g. Disease for those who are immune

Question 5

How does selection pressure affect stabilising selection?

Answer 5

The selection pressures in stabilising selection select against the extreme phenotypes.
Stabilising selection takes place in environments that do NOT change.

E.g. Human birth weights

Question 6

What type of environment does stabilising selection take place in?

Answer 6

Non changing

Question 7

Describe differences in the effect of increasing the concentration of antibiotic on the
SR strain and the non-SR strain. (2)

Answer 7

1. Non-SR strain falls more / SR strain falls less / up to 10(μg / cm−3);
   Must include 10 but only required once in either MP1 or MP2
   Ignore: units or absence of
   This must be a comparative statement
2. Above 10(μg / cm−3), SR strain levels out / off and non-SR strain
   continues to decrease;
3. Greater difference between strains with increasing concentration of
   antibiotic.

This must be a comparative statement

Question 8

The stringent response involves a number of enzyme-catalysed reactions.
Explain how scientists could use this knowledge to design drugs that make the
treatment of infections caused by the SR strain more successful. (2)

Answer 8

1. Make a competitive / non-competitive inhibitor;
   Mark in pairs
   either MP1 and MP2 OR MP3 and MP4
2. Competitive competes with / blocks active site / non-competitive inhibitor
   affects / changes active site;
   Do not mix and match
   OR
3. (Make a drug) that inhibits / denatures / destroys enzyme / stringent
   response;
   Accept: drug that ‘knocks out’ / destroys enzyme
4. Give at the same time as / before an antibiotic.

Question 9

Use the information provided and Figure 2 to suggest an explanation for the greater
resistance of the SR strain to this antibiotic.

Answer 9

1. Fewer free radicals (than non-SR);
   Note: has to be comparative statement
2. Produces more catalase (than non-SR);
   Accept converse statements for non-SR.
3. Catalase (might be) linked to production of fewer free radicals / breaking
   down / removing free radicals.
   Accept: hydrolysis of radicals by catalase.

Question 10

One farmer stated that the increase in the use of Bt crop plants had caused a
mutation in one of the insect species and that this mutation had spread to other
species of insect. Was he correct? Explain your answer.

(Why do mutations not pass on to others?) (4)

Answer 10

(No – no mark)

1. Mutations are spontaneous / random;
2. Only the rate of mutation is affected by environment;
3. Different species do not interbreed / do not produce fertile offspring;
4. So mutation / gene / allele cannot be passed from one species to
   another.
   Ignore references to correlation does not prove causation

Question 11

There was a time lag between the introduction of Bt crops and the appearance of
the first insect species that was resistant to the Bt toxin.
Explain why there was a time lag. (3)

Answer 11

1. Initially one / few insects with favourable mutation / allele;
2. Individuals with (favourable) mutation / allele will have more offspring;
3. Takes many generations for (favourable) mutation / allele to become the
   most common allele (of this gene).