VARIATION

Question 1

Variation

Answer 1

The differences between individuals of the same species

Question 2

Mutation

Answer 2

Genetic change

Question 3

Adaptive feature

Answer 3

An inherited feature that helps an organism to survive and reproduce in its environment

Question 4

Gene mutation

Answer 4

A change in the base sequence of DNA

Question 5

Adaptive feature

Answer 5

The inherited functional features of an organism that increase its fitness

Question 6

Fitness

Answer 6

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

Question 7

Process of adaptation

Answer 7

The process, resulting from natural selection, by which populations become more suited to their environment over many generations

Question 8

Continuous variation

Answer 8

Continuous variation shows a complete range of the characteristic within a population between two extremes. It is caused both by genes (often a number of different genes) and by the environment. Environmental influences for plants may be the availability of, or competition for, nutrients, light and water and exposure to disease. For animals, environmental influences can include the availability of food or balanced diet, exposure to disease (or the availability of health services for humans), etc. Examples of continuous variation include height, body mass and intelligence. When the frequency is plotted on a graph as in Figure 18.1, a smooth curve is produced, with the majority of the population sample grouped together and only small numbers at the extremes of the graph.

Question 9

Discontinuous variation

Answer 9

Discontinuous variation is seen where there is a limited number of obvious, distinct categories for a feature. There are no intermediates between categories, and the feature cannot usually change during life. It is caused by a single gene or a small number of genes, with no environmental influence (as in Figure 18.2). Examples include blood group, ability to tongue-roll and earlobe shape. When the frequencies are plotted on a graph, bars are produced that cannot be linked with a smooth curve.

Question 10

Mutation

Answer 10

Mutation is a source of variation, caused by an unpredictable change in the genes or chromosome numbers. As a result, new alleles are formed. Mutations are normally very rare. However, exposure to radiation and some chemicals, such as tar in tobacco smoke, increases the rate of mutation. Exposure can cause uncontrolled cell division, leading to the formation of tumours (cancer). The exposure of gonads (testes and ovaries) to radiation can lead to sterility or damage to genes in sex cells that can be passed on to children.

Question 11

Phenotypic variation is caused by both genetic and

Answer 11

environmental factors. For example, a fair-skinned person may be able to change the colour of his or her skin by exposing it to the Sun, so getting a tan. The tan is an acquired characteristic. You cannot inherit a suntan. Black skin, on the other hand, is an inherited characteristic. Many features in plants and animals are a mixture of acquired and inherited characteristics. For example, some fair-skinned people never go brown in the Sun, they only become sunburnt. They have not inherited the genes for producing the extra brown pigment in their skin. A fair-skinned person with the genes for producing pigment will only go brown if he or she exposes themselves to sunlight. So, the tan is a result of both inherited and acquired characteristics.

Question 12

Discontinuous variation is mostly caused by

Answer 12

genes alone. An example is blood groups in humans (Figure 18.2). Environmental factors will not cause a change in a person’s blood group. A gene mutation (see definition at the start of this chapter) can result in a genetic change. The sequence of bases in DNA becomes altered, resulting in a change in coding for one or more amino acids (see Chapter 4). A section of DNA may now start making a different protein that could affect the organism.

Question 13

Sickle cell anaemia

Answer 13

Sickle cell anaemia is caused by a mutation in the blood pigment haemoglobin. The defective haemoglobin molecule differs from normal haemoglobin by only one amino acid (represented by a sequence of three bases). When the faulty haemoglobin is present in a red blood cell, it causes the cell to deform and become sickle shaped, especially when oxygen levels in the blood become low.

In this state, the sickled red blood cells are less efficient at transporting oxygen and are more likely to become stuck in a capillary, preventing blood flow. The distortion and destruction of the red cells, which occurs in low oxygen concentrations, leads to periods of severe anaemia. The faulty allele (HbS) is dominated by the allele for normal haemoglobin, but still has some effect in a heterozygous genotype.

The possible genotypes are:

• HbAHbA – normal haemoglobin, no anaemia;
• HbAHbS – some abnormal haemoglobin, sickle cell trait (not life-threatening);
• HbSHbS – abnormal haemoglobin, sickle cell anaemia (life-threatening).

Question 14

Malaria

Answer 14

is a life-threatening disease caused by a parasite that invades red blood cells. The parasite is carried by some species of mosquito. However, a person who is heterozygous (HbAHbS) for sickle cell anaemia has protection from malaria, because the malaria parasite is unable to invade and reproduce in the sickle cells. A person who is homozygous for sickle cell anaemia (HbSHbS) also has protection, but is at a high risk of dying from sickle cell anaemia. A person with normal haemoglobin (HbAHbA) in a malarial country is at a high risk of contracting malaria. When the distributions of malaria and sickle cell anaemia are shown on a map of the world, it is found that the two coincide in tropical areas because of the selective advantage of the HbS allele in providing protection against malaria.

Question 15

adaptative features of rabbit

Answer 15

Question 16

xerophytes

Answer 16

Some plants are adapted to cope with a lack of water (e.g. in very dry or ‘arid’ environments). These are called xerophytes.

Question 17

hydrophytes.

Answer 17

Some plants are adapted to cope with living in water (e.g. pond plants and seaweeds). These are called hydrophytes.

Question 18

natural selection

Answer 18

Variation describes differences in a population. Some variation is inherited (passed on from parents) and some is acquired (developed during life). Animals and plants produced by sexual reproduction will show variation from their parents, for example in the size of the muscles in the legs of lions. When organisms reproduce, many offspring are often produced. However, not all of them are likely to survive because of competition for resources such as food, water and shelter. The same is true for plants (they compete for resources such as nutrients, light, water and space). There is a struggle for survival. The individuals with the most favourable characteristics are most likely to survive because they have an advantage over others in the population. For example, a lion cub with bigger muscles in its legs would be able to run more quickly and get food more successfully than its siblings. In an environment where there is a food shortage, the individual with the best adaptations to the environment is most likely to survive to adulthood. The weaker individuals die before having the chance to breed, but the surviving adults breed and pass on the advantageous alleles to their offspring. More of the next generation carry the advantageous genes, resulting in a stronger population, better adapted to a changing environment.

Question 19

selective breeding

Answer 19

Selective breeding is used by humans to produce varieties of animals and plants that have an increased economic importance: • Humans first select individuals with desirable features. • These individuals are cross-bred to produce the next generation. • From that generation, the offspring with desirable features are selected for further breeding.

Question 20

Evolution

Answer 20

Slow changes in the environment result in adaptations in a population to cope with the change. Failure to adapt could result in the species becoming extinct. This gradual change in the species through natural selection over time, in response to changes in the environment, is a possible mechanism for evolution.

Question 21

The development of strains of antibiotic-resistant bacteria

Answer 21

This is an example of evolution by natural selection. Bacteria reproduce rapidly – a new generation can be produced every 20 minutes by binary fission (see Chapter 16). Antibiotics are used to treat bacterial infections. An antibiotic is a chemical that kills bacteria by preventing bacterial cell wall formation. Mutations occur during reproduction, which produce some variation in the population of bacteria. Individual bacteria with the most favourable features are most likely to survive and reproduce. A mutation may occur that enables a bacterium toresist being killed by antibiotic treatment, while the rest of the population is killed when treated. This bacterium would survive the treatment and breed, passing on the antibiotic resistance gene to its offspring. Future treatment of this population of bacteria using the antibiotic would be ineffective.

Question 22

Differences between natural and artificial selection

Answer 22

Natural selection occurs in groups of living organisms 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 are more likely to survive, while others die before they can breed and pass on their genes. However, variation within the population remains. Selective breeding is 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 and is a much faster process than natural selection. However, selective breeding removes variation from a population, leaving it susceptible to disease and unable to cope with changes in environmental conditions. Potentially, therefore, selective breeding puts a species at risk of extinction.

Question 23

Examples of improving crop plants and domesticated animals by selective breeding

Answer 23

Wild varieties of plants sometimes have increased resistance to fungal diseases, but have poor fruit yield. Cross-breeding wheat plants over a number of generations and selecting the organisms with the best features at each stage can result in the formation of varieties that have both high resistance to disease and high seed yield. A variety of cattle may have a higher than average milk yield. Another variety may have a very high meat yield. If the two varieties are cross-bred, the individuals in the next generation with the best features are selected to continue breeding until a new breed has been artificially produced with the benefits of both parental varieties (high milk production in females; high meat yield in males).