7B - Variation and selection

Question 1

What can variation be caused by?

Answer 1

Genes, the environment or both.

Question 2

What is variation?

Answer 2

The differences that exist between individuals.

Question 3

What does variation within species mean that individuals in a population can show?

Answer 3

Means that individuals in a population can show a wide range of different phenotypes.

Question 4

What is variation within a species called?

Answer 4

Intraspecific variation.

Question 5

What causes genetic variation within a species?

Answer 5

Although individuals of the same species have the same genes, they have different alleles (versions of genes).

Question 6

What does the fact that, although individuals of the same species have the same genes, they have different alleles (versions of genes) cause?

Answer 6

Genetic variation within a species.

Question 7

What is the main source of the genetic variation within a species?

Answer 7

Mutation - changes in the DNA base sequence lead to the production of new alleles.

Meiosis - through the crossing over of chromatids and the independent segregation of chromosomes).

Random fertilisation of gametes during sexual reproduction.

Question 8

Other than mutations, meiosis and random fertilisation (genetic factors), what else can cause variation within a species?

Answer 8

Differences in the environment, e.g. climate, food, lifestyle.

Question 9

What is most variation within a species caused by?

Answer 9

A combination of genetic and environmental factors.

Question 10

What results in evolution?

Answer 10

Only genetic variation.

Question 11

Does genetic variation result in evolution?

Answer 11

Yes.

Question 12

Does environmental variation result in evolution?

Answer 12

No.

Question 13

What is evolution?

Answer 13

A change in allele frequencies over time.

Question 14

Why does evolution occur?

Answer 14

Natural selection, genetic variation and genetic drift.

Question 15

What does natural selection cause to occur?

Answer 15

Evolution.

Question 16

How does natural selection work?

Answer 16

1) Individuals of the same species vary because they have different alleles.
2) Predation, disease and competition (selection pressures) create a struggle for survival.
3) Because individuals vary, some are better adapted to the selection pressures than others.
4) This means there are differential levels of survival and reproductive success in a population. Individuals with a phenotype that increases their chance of survival are more likely to survive, reproduce and pass on their genes (including the beneficial alleles that determine their phenotype), than individuals with a different phenotype.
5) This means that a greater proportion of the next generation inherit the beneficial alleles.
6) They, in turn, are more likely to survive, reproduce and pass on their genes.
7) So the frequency of the beneficial alleles in the gene pool increases from generation to generation.

Question 17

What are selection pressures?

Answer 17

A selection pressure is anything that affects an organism’s chance of survival and reproduction.

Question 18

What do different types of natural selection lead to?

Answer 18

Different frequency patterns.

Question 19

What are the types of natural selection that affect allele frequency in different ways?

Answer 19

Stabilising selection, directional selection and disruptive selection.

Question 20

What are stabilising selection, directional selection and disruptive selection types of?

Answer 20

Natural selection that affect allele frequency in different ways.

Question 21

What are the 3 types of natural selection?

Answer 21

Stabilising selection, directional selection and disruptive selection.

Question 22

What is stabilising selection?

Answer 22

Where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.

Question 23

When does stabilising selection occur?

Answer 23

When the environment isn’t changing, and it reduces the range of possible phenotypes.

Question 24

What is directional selection?

Answer 24

Where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce.

Question 25

What could directional selection be a response to?

Answer 25

An environmental change.

Question 26

What is disruptive selection?

Answer 26

Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce.

Question 27

Which types of selection are opposites?

Answer 27

Disruptive selection is the opposite of stabilising selection because characteristics towards the middle of the range are lost in disruptive selection whereas they survive in stabilising selection.

Question 28

When does disruptive selection occur?

Answer 28

When the environment favours more than one phenotype.

Question 29

What is assortative (non-random) mating?

Answer 29

Where an animal is more likely to mate with other animals in that species with the same phenotypes as them - this causes two or more breeding populations.

Question 30

Which type of selection shows 2 breeding populations?

Answer 30

Disruptive selection.

Question 31

Discontinuous variation

Answer 31

• Distinct categories with no overlap.
• Controlled by a small number of genes.
• largely unaffected by the environment.

Question 32

Environmental variation

Answer 32

• Genes set limits.
• Environment determines where, within the limits, an organism lies.
• Usually due to polygenes.

E.g. height and weight in humans.

Question 33

Continuous variation

Answer 33

• No categories to place individuals.
• Controlled by a large number of genes.
• Significantly affected by environment.
• Normal distribution curve.

Question 34

What is there none of if variation is mostly due to genetic factors?

Answer 34

No intermediate types, e.g. blood group system.

Question 35

What are the 3 types of variation?

Answer 35

Discontinuous, environmental and continuous.