Natural selection

Question 1

Natural selection

Answer 1

Non random process that results in the increase in frequency of the DNA sequences that confer a selective advantage and increase. It also decreases the frequency of DNA sequences that confer a selective disadvantage.

Question 2

Process of natural selection.

Answer 2

1. Organisms produce more offspring than the environment can support
2. Sexual reproduction means that all members of the species show variation form each other.
3. Struggle for survival occurs - and many offspring die before they can reproduce due to selection pressures like disease, lack of food and overcrowding.
4. Only the better adapted organisms to that environment will survive and pass on those adaptations to their offspring.
5. This process is repeated generation after generation.

Question 3

Quantitative traits

Answer 3

Phenotypic traits that can be measured - height, weight etc.

These fall over a range of values and are controlled by many genes (polygenic).

Question 4

Phenotype

Answer 4

Physical or biochemical characteristics of an organism resulting from the expression of its genes.

Question 5

How does normal distribution of a quantitative trait of a large population look -

Answer 5

Like a bell shaped curve. With the average value being the axis of symmetry.

Question 6

Selection altering frequency - Stabilising Selection

Answer 6

Selection against extreme variants and favours average phenotypes - leads to a reduction in genetic diversity.
Redrawn graph will be thinner than previous.
Example - Gall flies producing average sized eggs since they won’t be predated (too large) or be parasites by wasps (too small)

Question 7

Selection altering frequency - Directional selection

Answer 7

Common during periods of environmental change, selection favours one extreme of the phenotype which was initially less common.
Causes a progressive shift of mean value (still looks the same but shifted either left or right)
Example - Peppered moth became darker due to more soot on the trees so better camouflage.

Question 8

Selection altering frequency - disruptive selection

Answer 8

Selection pressures select extreme variants of a phenotype at the expense of intermediate versions.
Graph looks like eildons.
Example - In salmon larger fish are selected because they can better compete for territories where they will breed with female salmon there, smaller rich are also selected because they are better able to sneak into territories and fertilise eggs. Medium sized fish cannot do either well.