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Unit 18
Variation and selection
18.1 Variation is the difference between individuals of the same species.
There are two types of variation within a species.
1. Continuous variation (height, skin colour, hair length)
2. Discontinuous variation (albino, blood groups in humans, fingerprints, ability to roll
tongue, ear lobed.
1. Continuous variation
Continuous variation is influenced by genes and the environmental conditions. e.g.
height of humans, skin colour.
Continuous variation
2. Discontinuous variation
Discontinuous variation is caused by genes alone and results in a limited number of
distinct phenotypes e.g. A, B, AB and O blood groups in humans, fingerprints, ability
to roll tongue, ear lobed.
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18.2 Investigations into variation
Investigation into continuous variation: the length of limpets on a rocky shore.
Limpets are aquatic snails with distinctive conical shell and a strong muscular foot.
They are found between the high and low watermarks on rocky shores. They are
truly international, being found on rocky shores of the Caribbean South Africa,
Mediterranean and North Sea.
You cannot measure all the limpets on a particular rocky shore. This would be far
too laborious and time –consuming. The best strategy is to take a representative
sample of the population.
Quadrats are often used when sampling in the rocky shore studies. A frame quadrat
is a metal square and the most common size is 0.25m2. About 10 to 15 quadrats can
be laid down at random sample points on the shore and length of any limpet shell
found within each quadrat can be measured with a caliper.
In our example the data was divided into six equal size groups and the number of
limpets in each group was recorded in a tally chart.
Figure: Tally chart for limpet shell length Figure 18.2.1 A quadrat in use
This was used to draw a histogram-which is used to show frequency distribution, in
this case the number of limpets of different lengths found on the rocky shore. As you
can see, the graph displays a normal distribution reflecting continuous variation.
Length of limpet/mm
Figure 18.2.4 Histogram
of continuous variation Figure 18.2.3 Measuring the length of a
in a limpet shell/length limpet with a caliper
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Investigation into discontinuous variation: the different coloured of sweet pea
plants
With discontinuous variation, the characteristics can be divided into distinct
categories into which individuals can be placed. There is no overlapping between
categories since categories are distinctive, e.g., ABO blood groups.
Gregor Mendel tended to choose characteristics for his experiments that were
controlled by a single gene displaying discontinuous variation, e.g., shape and colour
of pea seeds.
In this investigation, a group of students bought a packet of sweet pea seeds with a
variety of coloured flowers.
The seeds were sown in pots of soil, watered and placed into in sunlight so that they
could germinate.
Eventually they grew into mature plants and produced flowers. The flowers were
placed into four groups depending on their colour. Numbers were recorded in a tally
chart.
This data was used to draw a bar chart since the independent variable (flower
colour) was categoric and the dependent variable (number of plants) was
continuous.
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Mutations
A mutation is a spontaneous change in chromosome or DNA or a gene that may
cause a change in a phenotype characteristic.
Often this change can be harmful, but some mutations are beneficial and some have
no effect at all. There are two types of mutation; they are gene mutation and
chromosome mutation.
(1) Gene mutation
(a) Example of gene mutation
First mutation studied was in the fruit fly. Normal eye colour of fruit flies is red, but a
mutant form with white eyes was discovered. This was due to the change in the DNA
so that the gene no longer coded for the production of the red pigment.
(b) Example of gene mutation
Albino is the example of gene mutation.
A gene controls production of the skin, hair and eye pigment melanin, which
protects the skin and eyes from ultra-violet light. The gene can mutate that does not
produce melanin. It is a recessive allele and homozygous recessive individuals are
albino (e.g. aa, bb, cc)
(c) Gene mutations are the cause of many human genetic diseases.
Sickle cell anemia, color-blindness and cystic fibrosis
Albino is gene mutation, genotype is always homozygous recessive (aa)
Down syndrome is chromosome mutation, one extra chromosome (47)
Turner syndrome is chromosome mutation, one lack of chromosome (45)
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2. Chromosome mutation
Down’s syndrome is caused by a chromosome mutation.
The most common cause of this is a mutation that occurs during meiosis when an
egg cell is formed in the ovary.
The mutation occurs when two of the chromosomes do not separate properly.
Instead of the egg having 23 chromosomes there is an extra chromosome, giving a
Down’ syndrome. If this egg is fertilized the baby will have 47 chromosomes, one
more than normal.
Causes of mutations
- Gene mutations are the only way in which new genetic material is produced.
- Ultra-violet radiation, X-rays and gamma rays are the most damaging.
- Benzpyrene in cigarette smoke is a cause of mutations.
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Homologous chromosome(22) pairs (Autosomal)
One chromosome (1) pair sex chroeosone
Male Down’s syndrome
(46) XX x XY (46)
meiosis
meiosis
X X X Y
22 24 23 23
45 : 47
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Adaptive features
Define the term adaptive feature.
An adaptive feature is an inherited feature that helps an organism to survive and
reproduce in its environment.
The following animals are adapted to extreme conditions:
- The mayfly nymph has a flattened, streamlined body and clings to the underside
or rocks so it is not washed away.
- In winter, Arctic fox grows white fur as a camouflage.
- In desert conditions, a camel has a large hump on the back, each hump stored up
to 36 kilograms of fat which consisting even months without food.
Adaptations of plants to different environments
Desert adaptations - All plants have to balance water uptake with water loss. It is
important that they maintain the turgor in their cells, or they will wilt. Very high
rates of transpiration can kill a plant if it cannot absorb enough water to prevent
long-term wilting.
How xerophytes absorb sufficient water in the conditions in which they live.
Cacti reduce water loss and conserve water in the following ways:
• A thick, waxy cuticle covers the plant’s surfaces and reduces transpiration.
• Their leaves are reduced to spines. This reduces the surface area of the leaf over
which water can be lost.
They have swollen stems containing water-storage tissue.
• They have a shallow, spreading root system to quickly absorb any water from rain
and overnight condensation.
• Many cacti have a round, compact shape which reduces water can be lost.
• They have shiny surfaces which reflect heat and light.
• Their stomata are closed during the day to reduce water loss.
They open their stomata at night to absorb the carbon dioxide which they store for
use in photosynthesis during the day. Photosynthesis occurs in the outer layers of
cells in their stems.
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Pond plants (Hydrophytes)
- Hydrophytes are plants that grow submerged or partially submerged on water.
- Roots, if present, are for anchorage and there is no need for the roots to absorb
water or mineral ions, there are no root hairs.
- The leaves and stems have little or no cuticle; there is no need to conserve water.
- Hydrophytes is needed carbon dioxide for photosynthesis, diffuses through water
much more slowly than it does through air.
- Therefore, many hydrophytes have an extensive system of air spaces in their stems
and leaves through which gases diffuse quickly.
- The air spaces provide buoyancy to keep the plants close to the light and are a
reservoir of Oxygen and carbon dioxide.
Water Hyacinth Water Lily
Unit(Hydrophytes)
Pond plants 18.5
Selection
Natural selection
Natural selection is the process by which organisms that are well adapted to their
environment a greater chance to breed and pass on their genes to the next
generation than those that are less well adapted.
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The features of natural selection:
1. Struggle for existences (Competition for resources)
2. Survival of the fittest (Production of many offspring)
3. Variation within population
Describe natural selection with reference to:
(a) genetic variation within populations
-Variation is the differences that exit within a species.
-It is also produced by sexual reproduction between two individuals.
-Gene mutation in which completely new genetic material is produced.
-Some mutations may give an advantage to the individual.
e.g. If individual has good camouflage to avoid being seen by a predator.
(b) production of many offspring
Over-population
- The sockeye salmon females produce millions of eggs, many of which are fertilized.
- In spite of huge over- production of the eggs, sockeye salmon species remain fairly
stable.
- That’s because, most of the salmon’s eggs are eaten by the predators and many
young salmon die from disease and starvation.
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(c) Struggle for survival including competition for resources
Among the organisms that survive the early stages of life there is competition for
resources.
- Plants compete for space, light, water and nutrients.
- Animals compete for food, water, space (territories) and mates.
- This competition is referred to by biologists as the “struggle for existence”.
- There is a “struggle for existence” due to competition, predation and disease.
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(d) A greater chance of reproduction by individuals that are better adapted to the
environment than others
(e) these individuals pass on their alleles to the next generation.
Adaptation
The organisms that survive their struggles are the best adapted individuals to their
particular environment.
Antibiotic resistance
- Antibiotics are chemicals that kill bacteria or inhibit their growth.
- The introduction of antibiotics in the 1940s. Some bacteria developed a resistance
to their effects.
Explain how bacteria become resistant to antibiotics. (Feb/Mar 2017)
When bacteria are exposed to an antibiotic such as penicillin, most are killed. However,
some bacteria have a mutation giving them resistance to the antibiotic. The bacteria may be
able to produce an enzyme that breaks down the antibiotic. These resistance bacteria can
survive and reproduce and pass on the resistance gene to their offspring due to the natural
selection but non-resistance bacteria have died.
Selective breeding
Define the term artificial selection.
The selection of plants and animals for breeding because of their useful
characteristics, e.g. high crop yield. Also called selective breeding.
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The selection of plants and animals for breeding because of their useful
characteristics, e.g. high crop yield. Also called selective breeding.
Selective breeding (artificial selection) involves humans finding organisms with
desirable features, crossing them and selecting the best from the next generation.
Selective breeding has produced new varieties of animals and plants with increased
economic importance, e.g. high-yielding crops, cattle that produce more milk or
better meat and sheep that produce more wool.
- Human choose a desirable feature of an animal or plant to improve, e.g. fast
growth, short stems, docile (not fierce) nature, high yield, disease resistance,
resistance to drought.
Differences between artificial selection and natural selection
There are two main methods of carrying out selective breeding:
1. Outbreeding and
2. Inbreeding
1. Outbreeding involves the breeding of unrelated animals or plants. This may be
used to combine the good characteristics of separate individuals, for example
crossing a crop plant with high yield, and another crop plant that is resistance to
disease. Outbreeding often results in tougher individuals with a better chance of
survival. This is called hybrid vigour.
2. Inbreeding involves breeding close relatives in an attempt to retain desirable
characteristics. However, there can be harmful effects as a result of inbreeding.
These can include a loss of vigour, with the population weakened by a lack of gene
diversity and reduced fertility.
