Chapter 18 Variation And Selection

Question 1

Define variation

Answer 1

The differences between individuals of the same species

Question 2

Distinguish between Genetic Variation and Phenotypic Variation

Answer 2

Genetic Variation are variations determined by genes
Phenotypic variations (observable characteristics) may be brought about by genes, but can also be caused by the environment, or a combination of both genes and the environment

Question 3

Explain Continuous Variation

Answer 3

It results in a range of phenotypes between two extremes

Extra: 
controlled by a lot of genes and the environment 
Tends to be quantitative 
No limit
No distinct categories 

Represented by line graph

E.g. height, weight, heart rate

Question 4

Explain Discontinuous Variation

Answer 4

It results in a limited number of phenotypes with no intermediates

Extra: 
it is under the control of a single pair of alleles or a small number of genes 
No in between categories 
Distinct categories 
Tends to be qualitative (non numerical)

Represented as a bar chart

Eg. Blood type, gender, tongue rolling

Question 5

NIS: Genetic variation can be caused by

Answer 5

Mutation
Random mating
Random fertilization
Recombination between homologous chromosomes during meiosis

Question 6

Define mutation

Answer 6

Mutation is spontaneous genetic change. It is the way new alleles are formed.

Question 7

What are the causes of mutation?

Answer 7

Ionizing radiation and some chemicals increase the rate of mutation

• chemical mutagens (e.g tar from cigarette smoke )
• ionizing radiation (e.g gamma rays, X rays and ultraviolet rays)

the greater the dose of radiation a cell gets, the greater the chance of mutation

Question 8

Define gene mutation

Answer 8

A change in the base sequence of DNA

Question 9

Symptoms of sickle-cell anaemia

Answer 9

Weakness, aching joints and poor circulation

Question 10

Describe what happens when one has sickle cell anaemia

Answer 10

RBC contain globular proteins called hemoglobin which are made of two alpha and two beta protein subunits.

Sickle cell anaemia is a blood disorder caused by a single base substation in the gene that codes for the beta subunit of hemoglobin.
When the mutation is transcribed by the mRNA, the resulting codon also contains the substitution and when the mRNA is translated, the mutant polypeptide has valine instead of glutamic acid as the sixth amino acid in the polypeptide chain.
Valine and glutamic acid have different chemical properties, so the polypeptide folds differently, the hemoglobin molecule differs in configuration and the RBC develops into sickle shape.

Sickle shaped RBC deliver oxygen inefficiently and their irregular shapes often block blood vessels. A person with the disease may develop severe symptoms of anaemia. The disease is deadly if left untreated

Question 11

Using genetic diagrams, show how sickle cell anemia is inherited

Answer 11

Refer to slides

Question 12

Who are resistant to malaria

Answer 12

People who are heterozygous (Hb^S Hb^A) for the sickle cell allele have a resistance to malaria

Question 13

Distribution of sickle cell

Answer 13

The selection pressure of malaria favour the Heterozygotes over the homozygotes and the potentially harmful Hb^S allele is kept in the population
Hence, sickle cell disease is more prevalent in places with malaria.

When Africans migrate to countries where malaria does not occur, the selective advantage of the Hb^S allele is lost and the frequency of this allele in the population diminishes.

Question 14

Define Adaptive features

Answer 14

Inherited functional features of an organism that increase its fitness and help it to survive and reproduce in its environment.

Question 15

Define Fitness

Answer 15

The probability of an organism surviving and reproducing in the environment in which it is found

Question 16

Explain the adaptive features of xerophytes

Answer 16

Xerophytes are plants modified to cope with lack of water (e.g: pine tree, cactus, marram grass)

Features:

• Waxy layer covering fleshy leaves to help store water
• storing water in special tubers or bulbs
• Long vertical roots growing deep into soil to reach water table
• Roots growing horizontally in the top layers of the soil to collect surface water
• No leaves to prevent water loss through transpiration
• Prickles and spikes on stem to protect against thirsty animals

Question 17

Explain the adaptive features of hydrophytes

Answer 17

Hydrophytes are plants adapted to living in water (e.g. water lily)

Features

• Leaves contain large air spaces to make them buoyant, so they float on or near the surface, enabling them to gain light for photosynthesis
• The lower epidermis lacks stomata to prevent water entering the air spaces, while stomata are present on the upper epidermis for gas exchange

Question 18

Describe Natural Selection

Answer 18

1. Variation within populations
2. Production of many offspring
3. Competition of resources
4. Struggle for survival
5. Reproduction by individuals that are better adapted to the environment than others
6. Passing on their alleles to the next generation.

The variations have to be heritable for natural selection to be effective. Both genes and the environment can cause variation, but only genetic variation can be passed to the next generation

Question 19

Describe Artificial selection (Selective Breeding)

Answer 19

1. Selection by humans of individuals with desirable features
2. Cross-breeding these individuals to produce the next generation
3. Offspring with the most desirable features are chosen to continue the breeding programme and the process is repeated over a number of generations

Question 20

State the differences between natural selection and artificial selection

Answer 20

Natural Selection

• Occurs in groups of living organism through the passing on of genes to the next generation by the best adapted organisms, without human interference.
• Those with genes that provide an advantage, to cope with changes in environmental conditions for example, are more likely to survive, while others die before they can breed and pass on their genes
• Variation within the population remains

Artificial Selection

• Used by humans to produce varieties of animals and plants that have an increased economic importance.
• It is considered a safe way of developing new strains of organisms, compared with genetic engineering, and is a much faster process than natural selection.
• However, artificial selection removes variation from a population, leaving it susceptible to disease and unable to cope with changes in environmental condition.
• Hence artificial selection may put a species at risk of extinction.

Question 21

Describe Evolution

Answer 21

It is the change in adaptive features of a population over time as a result of natural selection.

Question 22

Define Adaptation

Answer 22

Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.

Question 23

Describe the development of strains of antibiotic resistant bacteria

Answer 23

Over time, bacteria can become resistant due to random mutations in the genes to certain antibiotics (such as penicillin). This is an example of natural selection.
In a large population of bacteria, there may be some that are not affected by an antibiotic. These survive and reproduce – producing more bacteria that are not affected by the antibiotic.
The number of strains of antibiotic-resistant bacteria has increased, partly due to the misuse of antibiotics.

Question 24

How is selective breeding by artificial selection carried out over many generations to improve crop plants and domesticated animals?

Answer 24

For example, the largest fruit on a tomato plant might be picked and its seeds planted next year. In the next generation, once again only seeds from the largest tomatoes are planted. Eventually it is possible to produce a true-breeding variety of tomato plant that forms large fruits.

Similar principles can be applied to farm animals.