Chapter 18: Variation and Selection

Question 1

Define variation.

Answer 1

Variation is the differences between individuals of the same species.

Question 2

What can variation be caused by?

Answer 2

genes (genetic variation), the environment or a combination of the two.

Question 3

What are the two types of variation?

Answer 3

Discontinuous
Continuous

Question 4

What is continuous variation?

Answer 4

When the individuals in a population have a range of phenotypes which vary between two extremes.

There are no distinct categories.

Question 5

Give some examples of continuous variation.

Answer 5

Body mass, the number of leaves on a tree

Question 6

What is continuous variation caused by?

Answer 6

both genes and the environment.

Question 7

What is discontinuous variation?

Answer 7

Discontinuous variation is when there are two or more distinct categories.

Each individual falls into only one of these categories so there is a limited number of phenotypes with no intermediates.

Question 8

Give an example of discontinuous variation.

Answer 8

Blood groups

Question 9

What is discontinuous variation caused by?

Answer 9

Genes only.

Question 10

Define mutation.

Answer 10

Mutation is genetic change.

Question 11

When does genetic variation arise?

Answer 11

Genetic Variation can arise due to mutations.

When mutations occur within a gene, they result in a different version of the gene called an allele.

Having different alleles increases genetic variation.

Question 12

When do mutations occur spontaneously?

Answer 12

The rate of mutation is increased by exposing yourself to ionising ration (X-rays, gamma rays) or to chemicals called mutagens.

Question 13

Why are new alleles produced during mutation?

Answer 13

Because mutations change the sequence of DNA bases in a gene.

As the sequence of DNA bases code for the sequence of amino acids, that make up a protein, mutation to a gene sometimes lead to changes in the protein that it codes for.

Question 14

Define gene mutation.

Answer 14

A random change in the base sequence of DNA

Question 15

List additional sources of genetic variation.

Answer 15

Random mating
Meiosis
Random fertilisation.

Question 16

Define adaptive features.

Answer 16

Adaptive features are characteristics that an organism inherits which help it to survive and reproduce in its environment.

Question 17

How are arctic animals adapted?

Answer 17

They have white fur for camouflage.
This helps the to avoid predators and to catch prey.

They also have a thick layer of blubber and a small surface area for their size to help retain heat.

Question 18

How are desert animals adapted?

Answer 18

They have a thin layer of fat to help them lose heat.

Question 19

How are xerophytes adapted?

Answer 19

They have sunken stomata that are surrounded by hairs. This slows transpiration (because they both trap a layer of humid air close to the leaf’s surface)

They have rolled leaves to trap humid air

They have a thick, waxy layer on the epidermis to reduce water loss.

Question 20

How are hydrophytes adapted?

Answer 20

Their leaves contain air spaces so they can float on the surface of the water and be exposed to the most light.

Stomata are usually only present in the upper surface of floating leaves to maximise gas exchange.

The stems are flexible to help them prevent damage from currents.

Question 21

How does natural selection occur?

Answer 21

Individuals in a population show genetic variation because of differences in their alleles.

Some organisms produce many offspring. This means that there aren’t enough resources to support all of the offspring and so there is competition.

Competition for resources and things like predation and disease affect an organism’s chance of surviving and reproducing. There is a ‘struggle for survival’.

Those individuals with characteristics that make them better adapted to their environment have a better chance of survival and so are more likely to reproduce.

This means that the alleles that are responsible for beneficial characteristics are more likely to be passed on to the next generation and become more common in the population.