9.4 Types of selection

Question 1

What is selection

Answer 1

Process where organisms that are better adapted to their environment tend to survive and breed, whilst those who are less well adapted will not.

Question 2

What are the types of selection

Answer 2

Directional selection:
Selection may favour individuals that vary in one direction from the mean of the population.
It changes the characteristics of the population

Stabilising selection:
They favour average individuals, so preserves the characteristics of the population

Question 3

What is a normal distribution curve made by

Answer 3

Most characteristics are influenced by multiple genes (polygenes)

So these characteristics are more influenced by the environment than characteristics influenced by only one gene.
Because diet etc could directly impact one of those genes, changing the whole characteristic

The effect of the environment on polygenes produces individuals in a population that vary about the mean

So plotting this on a graph produces a normal distribution curve, which is shaped quite like a bell

Question 4

What is directional selection

Answer 4

If environmental conditions change, the individuals in the population that contain phenotypes best suited to that environment will survive.

So individuals that fall to the left or right of mean will possess these alleles to be more suited to the conditions.

So they are more likely to survive and breed to make new offspring, so the new generation have higher allele frequency of it.

Over time, the mean moves in the direction of these individuals, so graph moves left or right

So it favours one extreme at the expense of the other

Question 5

What is an example of directional selection

Answer 5

Antibiotic resistance

. When antibiotics started being used, the effectiveness of some at killing bacteria was reduced . So some populations had become resistant to them eg penicillin

. Spontaneous mutation occurs in allele of a gene in bacteria:
. Change in base sequence of gene codes for a different protein , which was an enzyme that could break down penicillin antibiotic. This enzyme was called penicillinase

. The bacteria happened to be in a selection pressure where it was then being treated with penicillin in an individual, so the bacteria with a mutation had an advantage so it could break down antibiotic and survive whilst the others were all killed.

. The small amount bacteria that survived divided by binary fission to make a population of resistant bacteria

. So members of this population were able to survive and reproduce, so the allele frequency increased

. So population of resistant bacteria increased at the expense of non resistant bacteria, so there was a new mean

So normal distribution curve shifted to direction towards high antibiotic resistance

Question 6

What is important to remember about bacteria with mutations

Answer 6

Bacteria don’t only mutate when in contact with the antibiotic, there must already have been mutations in it, that occurred randomly.

Most mutations will be no advantage to bacteria and most will be harmful which kills the bacteria
However sometimes it will be advantageous

As even the mutation making bacteria produce penicillinase wouldn’t be useful unless in the presence of penicillin

Question 7

What is stabilising selection

How does the graph shape change

Answer 7

If environmental conditions remain stable, it is the individuals who have phenotypes closest to the mean that are favoured

So these are more likely to pass their alleles to the next generation than individuals at either extreme

So extreme phenotypes on either side of the mean will be selected against in stabilising selection.

The populations characteristics are preserved rather than changed

The shape of the graph will be more narrow and the peak will be higher

Question 8

What is the example of stabilising selection

How does this change the shape of the graph

Answer 8

Human birth weights:

The mean human birth weight will have the highest number of births, because it is optimum.
Eg underweight babies will probably be premature and not developed so may die, whilst overweight babies will cause birth complications and may die.

So extreme phenotypes on either side of the mean will be selected against.
Whilst the mean will be favoured.

The populations characteristics are preserved rather than changed

The shape of the graph will be more narrow and the peak will be higher

Question 9

What are the 3 types of adaptation and describe them

Answer 9

Anatomical: Eg shorter ears, thicker fur in cold habitats

Physiological: Eg oxidising of fat in kangaroos for water in desert

Behavioural: Eg autumn migration of swallows from UK to africa