CIE Categories Archives: 18. Variation and Selection

18.3) Selection

18.3) Selection


Natural selection:

  • Variation within populations.
  • Production of many offspring.
  • Competition for resources.
  • Struggle for survival.
  • Reproduction by individuals that are better adapted to the environment than others.
  • 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 on to the next generation.


Selective breeding:

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


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

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


Antibiotic-resistant bacteria:

  • 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.


Comparing natural and artificial selection:

  • 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 for example, are more likely to survive, while others die before they can breed and pass on their genes.
  • However, variation within the population remains.
  • Artificial selection 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, compared with genetic engineering, and is 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.

Potentially, therefore, artificial selection puts a species at risk of extinction.

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18.2) Adaptive features

18.2) Adaptive features


Adaptive feature: is an inherited feature that helps an organism to survive and reproduce in it environment.

Adaptive features: is the inherited functional features of an organism that increase its fitness.

Fitness: is the probability of that organism surviving and reproducing in the environment in which it is found.


Adaptations to arid conditions:

  • In both hot and cold climates plants may suffer from water shortage.
  • High temperatures accelerate evaporation from leaves.
  • At very low temperatures the soil water becomes frozen and therefore unavailable to the roots of plants.
  • Plants modified to cope with lack of water are called xerophytes.
  • Loss of leaves removes virtually all evaporating surfaces at a time when water may become unavailable.
  • Pine tree, Cacti and Marram grass.


Adaptations to living in water:

  • Plants adapted to living in water are called hydrophytes.
  • Water lily.
  • The leaves contain large air spaces to make them buoyant, so they float on or near the surface.
  • This enables 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.
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18.1) Variation

18.1) Variation


Variation: is the differences between individuals of the same species.


Genetic variation are variations that are determined by genes.

Phenotypic variations may be brought about by genes, but can also be caused by the environment, or a combination of both genes and the environment.

Acquired characteristic – eg. A tan, you cannot inherit a suntan.

Inherited characteristic – eg. Black skin.


Continuous variation results in a range of phenotypes between two extremes, eg. height in humans.

Discontinuous variation results in a limited number of phenotypes with no intermediates, eg. tongue rolling.


Discontinuous variation is under the control of a single pair of alleles or a small number of genes.

An example is human blood groups. A person is one of four blood groups: A, B, AB or O.

There are no groups in between.


When recording and presenting results of investigations, use line/histogram for continuous and bar chart for discontinuous variation.



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

Gene mutation: is a change in the base sequence in DNA.


Causes of mutation:

Ionising radiation and some chemicals increase the rate of mutation.

  • chemical mutagens – such as tar from cigarette smoke.
  • ionising radiation – gamma rays, X-rays and ultraviolet rays.
  • The greater the dose of radiation a cell gets, the greater the chance of a mutation.


Sickle-cell anaemia:

  • With sickle-cell anaemia, the haemoglobin molecule differs from normal haemoglobin by only one amino acid (represented by a sequence of three base).
  • valine replaces glutamic acid. This could be the result of faculty replication at meiosis.
  • When the relevant parental chromosome replicated at gamete formation, the DNA could have produced the triplet -CAT- (valine) instead of -CTT- (glutamic acid).
  • In this case, a change of just one base (from A to T) makes a significant difference to the characteristics of the protein (haemoglobin).
  • A person with sickle-cell disease has inherited both recessive alleles (Hb^SHb^S) for defective haemoglobin.
  • The heterozygous (Hb^AHb^S) have no symptoms of anaemia but are more resistant to malaria than the homozygotes Hb^AHb^A. It appears that the malaria parasite is unable to invade and reproduce in the sickle cells.
  • The selection pressure of malaria, therefore, favours the heterozygotes over the homozygotes and the potentially harmful Hb^S allele is kept in the population.
  • 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.
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