Within a species there is usually a great deal of variation between individuals

These snails are all the same species
( Cepaea nemoralis) but they vary considerably
What variations can you see ?
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The snails’ shells vary in
• their colour
•the presence or absence of bands
•the number of bands
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These tiger moths are from the same family
What variations can you see?
© Biophoto Associates
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The uppermost moth is larger than the other two
The dark bands on the abdomen differ in number and thickness
The patterns of the forewings are quite different from each other
The number, shape and distribution of the black spots on the hind wings differ
The amount of orange colour in the hind wings varies
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What variations can you see ?
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There are variations in skin colour, hair colour, hair curliness, eye colour and sex

Variations may be inherited or acquired
Inherited variations result from the activity of genes
They are genetically controlled
Genetically controlled variations cannot be altered
For example, hair colour, skin colour*, blood group, finger prints and sex cannot be changed naturally
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Acquired characteristics result from an individual’s activities or nutrition or from environmental conditions during a lifetime
Examples of acquired conditions in humans are* language obesity athletic skills mental skills body building sun tan
Acquired characteristics cannot be inherited
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The next side shows apples collected from different positions on the same tree
The differences between the apples are acquired during the growing season
The differences will not be inherited
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North side, upper branches South side upper branches
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North side, lower branches South side, lower branches
© Horticultural Research International
Apples

What environmental conditions in the different parts of the tree might have caused the differences in size of the apples ?
The upper branches will receive more sunlight than the lower branches, but on the North side the sunlight would have been less intense
The lower branches will receive less sunlight because of the shade cast by the upper branches. This effect will be more pronounced on the North side
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Environmental conditions

A great many variations are influenced by both genetic and environmental effects.
For example your height will depend on what genes you inherit and the amount of food you get during your growing period
You may inherit a good physique but unless you take exercise you will not develop to your full potential
You may inherit the genes to make the pigment, melanin, in your skin but you will need to expose your skin to sunlight to acquire a tan
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Discontinuous variations are entirely genetically controlled
They cannot be altered by external conditions.
You are either male or female, there are no intermediates
Your ABO blood group is either A, B, AB or O
Genetic defects such as colour blindness, albinism, achondroplastic dwarfism, sickle cell anaemia are all
Genetically controlled and expressed in a discontinuous way
( See ‘Heredity’ slide 19 )
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You either have these conditions or you do not. There are no intermediate states

percentage in population of Britain
Discontinuous variation in blood group . The figures cannot be made to fit a smooth curve because there are no intermediates
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Continuous variation describes the situation in which there are a great many intermediates between the extremes
For example, there is every shade of hair colour between black and blond. People do not belong to one or other of a small number of distinct categories
Variations such as these are under genetic control but there are several pairs of genes involved
The genome AA BB CC DD might give black hair while the genome aa bb cc dd might give blond hair.*
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Genomes AaBbCcDd or AABbCCdd or aaBBccDd and all the other possible combinations would give intermediate colours

There are as many categories of hair colour as there are children
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Continuous variation also occurs when the characteristics are controlled by the genes and the environment
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Your height will depend on the genes you inherit and on the amount of food you eat during your growing period
See slide 13 for other examples
The next slide shows the range of heights of army recruits
Continuous variation

thousands of men height in inches
There seem to be distinct categories of height but this is because measurements are made to the nearest inch
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If measurements could be made to the nearest millimetre there would be a smooth transition in the heights
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This is how continuous variation would appear in a graph
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Which of these statements are correct?
(a) Wild plants of the same species do not vary
(b) There is always variation between individuals of the same species
(c) Individuals in the same species may show variations
(d) Daschunds and greyhounds are varieties of the same species
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Variations in an individual’s weight…
(a) are entirely genetically controlled
(b) result exclusively from the amount of food taken in
(c) are the result of food intake and genes
(d) are the result of chance
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Which of the following can be changed by conditions other than genetic?
(a) Blood group
(b) Weight
(c) Physique
(d) Finger prints
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Which of the following are acquired characteristics ?
(a) Lung cancer
(b) Athleticism
(c) Curly hair
(d) Big feet
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Discontinuous variations are …
(a) inherited
(b) not inherited
(c) a combination of inherited and acquired factors
(d) acquired during a lifetime
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Which of the following are examples of continuous variation ?
(a) Height
(b) Weight
(c) Sex
(d) Intelligence
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