18.4 Types of selection

Question 1

What is stabilising selection

Answer 1

It preserves the average phenotype (so phenotypes around the mean) of a population by favouring average individuals

So selection against the extreme phenotypes

Question 2

What is directional selection

Answer 2

It changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population

So selection for the extreme phenotypes

Question 3

What is disruptive selection

Answer 3

It favours individuals with both extreme phenotypes rather than the average ones around the mean of the population

Question 4

Describe the process of stabilising selection

What is the main example for this

Answer 4

It eliminates the extreme phenotypes of a population, so eliminates the capacity for evolutionary change

. The environmental conditions are constant over long periods of time

. However the extremes will still survive

Babies birth masses: as those born too heavy or too light are not very likely to survive and will likely die

Question 5

Give an example of stabilising selection, showing how the extremes could still barely survive

Of mammals with fur

Answer 5

. In years where the temperature is hotter, the individuals with short fur will have the phenotype advantage as they can lose body heat more easily
. And in colder years, the opposite is true, and those with longer length fur will survive better as they are better insulated

In average temperatures, the extreme individuals will never be selected for, so the mean will remain the same, but there will be fewer individuals at either mean

Question 6

Describe process of directional selection

Answer 6

The curve has a mean that represents the optimum value for the phenotypic character under existing conditions

So for example, with fur length, if the temperature is 10 degrees, and fur length is 1.5cm, this represents mean fur length for the whole population.

. There are a few individuals in the population who have thicker fur, eg 2.5cm

. If the temperature drops from 10 to 5 degrees, individuals with thicker fur would be selected for, as they will be warmer so will survive more and breed,

. So this causes a shift in the mean fur length, so it is thicker in the population over many generations.
. The graph will shift to the right, in the direction of thicker fur.

Question 7

Describe the process of disruptive selection

How does it bring about evolutionary change

Answer 7

It is the opposite of stabilising selection
It favours both extreme phenotypes at the expense of the average ones

So occurs when an environmental factor eg temperature takes two distinct forms.
So for example in summer if it is around 15 degrees, and in winter if it is around 5 degrees
Individuals with both extremes of fur would be selected for, as the 1cm type is active in summer, and 2cm type is dominant in winter

. It can eventually lead to two separate species of mammal: one with long fur and active in winter, and one with short fur and active in summer

. So those with an average will be selected against so the normal distribution curve will have a big dip in the middle, until there are no common individuals in the middle so there are two
species

Question 8

What is polymorphism

How is this found in the peppered moth

What type of selection does it illustrate

Answer 8

Where individuals in an interbreeding species are genetically distinct

It used to always be light, until a black variety arose as a result of mutation

They stood out on the white trees however, so were subject to more predation from birds, and the white ones could survive

However, in certain cities due to soot from factories that covered trees etc, the peppered moths could camouflage in black and weren’t killed, so survived more.

But the white ones were still more common in rural areas.

It illustrates directional selection