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IGCSE BIOLOGY TOPIC 18 VARIATION

Variation SUMMARY NOTES


Variation refers to the differences between each organism in a species. Variation is beneficial to a
species as it allows natural selection to occur and reduces the risk of extinction from disease. There are
two types of variation: genetic variation and phenotypic variation.
● Genetic variation – each organism in a species has a different set of DNA, which is due to genetic
variation. Genetic variation is increased during meiosis, which produces gametes. Each gamete
has a different set of alleles, which means that when the two gametes fuse an entirely new set
of genes are produced.
● Phenotypic variation – The phenotype of an organism refers to its observable characteristics,
such as height or hair colour. Phenotypical variation can be caused by both genetic and
environmental factors. For example, the potential height of an organism is decided in genes
which come from the parents, although some organisms will never reach this height as they do
not receive enough nutrients from their environment.
Variation can be continuous and discontinuous. Continuous variation results in a range of phenotypes
between two extremes, for example height or weight. Discontinuous variation, however, is limited to a
discrete number of categories, such as blood group, which is limited to A, B, AB, or O in humans.
Discontinuous variation is mainly caused by genes alone.
Mutations:
Mutations are genetic changes which result in a change in the sequence of DNA bases. These changes
can occur due to a variety of factors, including exposure to some chemicals and ionising radiation. If the
mutation occurs at a particular allele, this allele may be altered, changing how it functions. This is how
new alleles are formed.
An example of this is sickle-cell anemia: sickle-cell anemia is a condition where red blood cells
become sickle shaped. Sickle cells carry less oxygen and can block blood vessels. This condition
is caused by a mutation in the beta-haemoglobin gene, which alters the allele which produces
haemoglobin (protein). This allele is recessive; thus, it is only present in the phenotype if two
copies of this allele are present (homozygous). The mutation can also have positive effects;
people who are homozygous or heterozygous, i.e. have one sickle-cell allele and one unmutated allele, are immune to malaria, as the malaria parasite cannot infect the sickle-shaped
cells. Sickle-cell anemia is therefore commonly found in areas where malaria is common. This
shows that natural selection for this gene is occurring in these areas, as those with the gene do
not catch the disease and are more likely to survive. This allows this allele to be preserved.

Adaptations
Adaptive features are inherited functional features that help the organism by increasing its
fitness, which is the ability of the organism to survive and reproduce in its environment.
Xerophytes are plants that are adapted to live in very dry climates, such as cacti. They have a
number of adaptive features that help to increase survival by reducing water loss:
● Fewer stomata – water vapour diffuses out of the plant via the stomata, thus less water
is lost if there are fewer stomata. Stomata are also sunken in pits in the leaf, which
allows bubbles of moist air to be trapped around them. This lowers the water potential
gradient, so less water is lost from the leaf.
● Small, rolled leaves or spines – this reduces the surface area of the leaf and traps
moisture to lower the water potential gradient, reducing water loss.
● Deep roots – this allows plants to absorb water from the soil. Roots are also adapted to
absorb lots of water when it rains for storage, e.g. in monsoon seasons.
● Thick waxy cuticle – this provides a waterproof barrier around the leaf to prevent water
loss.
Hydrophytes, in contrast, are plants which are adapted to live in very wet conditions and
includes species such as the water lily and the lotus. These plants are adapted differently to
xerophytes as they do not need to minimize water loss:
● Leaf shape – leaves are usually large and flat to have a large surface area which
promotes water loss.
● Stomata – positioned on the top of the leaf where the sun hits. There is also a large
number of stomata, which are usually open to allow water vapour to diffuse out of the
leaf.
● Thin/no waxy cuticle – water loss does not need to be restricted by this layer in
hydrophytes.
● Small root system – as there is a large amount of water reliably available, root systems
can be shallow, and water can diffuse directly into the stem.

Selection
Natural selection is where organisms with favourable alleles and advantageous characteristics
have a higher probability of surviving and reproducing. This is due to competition within a
population for resources and mates. As there is variation in the alleles of each species, each
organism within a species has different traits, some positive and some negative. Those with
more positive traits can adapt to the environment more effectively and are thus is more likely
to survive and produce many offspring, which inherit these alleles. Over time, negative
characteristics are lost from the species as organisms with those characteristics are not able
reproduce to pass on their alleles. This is known as evolution. Evolution allows a population to
become more adapted to its environment over time, as a result of natural selection.
Antibiotic resistance:
Some bacterial strains become resistant to antibiotics as a result of natural selection:

  1. A mutation occurs in a bacterial cell allele which makes it resistant to an antibiotic.
  2. When that antibiotic is administered, this cell is not killed, whereas cells which have not
    become resistant are killed.
  3. The resistant cell can therefore survive and reproduce, passing on the resistant allele to
    produce more resistant bacteria.
    Selective breeding:
    Selective breeding is where humans select animals or plants with desirable features and breed
    these together to make more offspring with these desirable features. This process is repeated
    over many generations. As this breeding is controlled by humans, it is known as artificial
    selection.
    An example of selective breeding of animals is the German Shepherd. These dogs were
    originally bred as working dogs to herd sheep as they are known for their intelligence and
    agility. Humans selectively breed these dogs to exaggerate desirable qualities, such as their
    sloping backs and large ears. This involves crossing dogs which show these traits so that the
    alleles are passed on to their offspring. Farmers also selectively breed crops. For example,
    bananas are selectively bred for their size, shape and easiness to peel. This means that plants
    which express these characteristics are bred to produce more offspring with desirable
    characteristics.

MAIN NOTES

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.

Mutations:

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

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.

 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.