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Chapter 7.1 Investigating Variation
If one species differs from another, it is known as interspecific variation. Members of the
same species also differ from each other. This is known as intraspecific variation.
Taking measurements of individuals, selected from the population of organisms which is
being investigated. If these measurements are representative of the population as a
whole, then they are reliable. The measurements might not be representative for a
number of reasons, including:
Sampling bias investigators may make unrepresentative choices, either
deliberately or unknowingly.
Chance by pure chance, the individuals chosen may not be representative.
The best way to prevent bias is to eliminate, as far as possible, any human involvement
in choosing the samples. This can be achieved by random sampling. One method is to:
1. Divide the study area into a grid of numbered lines by stretching two long tape
measures at right angles to each other.
2. Using random numbers, perhaps computer generated, obtain a series of
3. Take samples at the intersection of each pair of coordinates.
It is impossible to completely remove chance from the sampling process, but its effect
can be minimised by:
Using a large sample size the more individuals that are selected, the smaller is
the probability that chance will influence the result.
Analysis of the data collected data collected can be analysed using statistical
tests to determine the extent to which chance may have influenced the data.
Causes of variation:
Genetic differences are due to the different genes that each individual organism
possesses. These differences arise from generation to generation. Genetic variation
arises as a result of:
Mutations may or may not be passed on to the next generation.
Meiosis form of nuclear division which forms the gametes.
Fusion of gametes offspring inherit some characteristics from each parent.
Which gametes fuses with which at fertilisation is a random process further
adding to the variety of offspring two parents can produce.
Variety in asexually reproducing organisms can only be increased by mutation. Sexually
reproducing organisms increase variation by all three methods. It follows that populations
of sexually reproducing organisms are more varied than asexually reproducing organisms.
The genes of an organism set limits, but it is largely the environment that determines
where, within those limits, an organism lies. Environmental influences include climatic
conditions temperature, rainfall and sunlight soil conditions, pH and food availability.
In most cases variation is due to the combined effects of genetic differences and