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Biology Unit 2 Chapter 7
If one species differs from another it is known as interspecific variation and if members of same
species differ from one another it is known as intraspecific variation.
Samples are taken to make measurements; it involves measurements of individuals, selected from
the population being investigated. If the individuals are representative of the population as whole
then the measurements can be relied upon.
There are two reasons why the individuals might not be representative:
1. Sampling bias: investigators may be making unrepresentative choices, deliberately or
unwittingly. For example they may only take samples of buttercups from the dry area and not
from the muddy area.
This can be avoided using random sampling. It is done by dividing the study area into grid of number
lines (by putting two measuring tapes at right angles to each other). Then use random numbers
(generated from computer or from a table) and take the samples at intersection of each pair of
2. Chance: individuals chosen, by chance, may not be representative. For example 50 selected
buttercups may be the 50 tallest in the population.
We cannot completely avoid chance however we can minimise the effect of it. This can be done in the
a. Using large sample size: the larger the number of individuals used the smaller the
probability that chance will influence the result.
b. Analysis of collected data: data can be analysed using statistical tests to determine
the extent to which chance may have influenced the data.
Causes of variation:
There are two main factors:
1. Genetic differences: this can change from one generation to another. It arises as a result of:
a. Mutation: changes to chromosomes and genes which may or may not be passed on.
b. Meiosis: it is a form of nuclear division which forms gametes. This mixes up the
genetic material before it is passed into the gametes.
c. Fusion of gametes: which gamete fuses with which at fertilisation is a random
In asexual reproduction the variation is due only to the mutation.
2. Environmental influences: this affects the way the organisms' gene is expressed. For
example in buttercups one plant may be determined by its genes to grow taller than the
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Variation is due to mixture of genetic differences and environmental influences combined. Its hard to
distinguish the effect of many genetic differences and environmental influences that combine to
produce differences between organisms. In summary its very hard to draw conclusion about causes
of variation in any particular case.
Types of variation
Variation due to genetic factors:
Where variation is due to genetic factors, organisms hit into few distinct forms and there are no
intermediate forms (e.g. blood groups).…read more