Variation: Investigating Variation

If one species differs from another it is called Interspecific Variation. But members of the same species also differ from each other. This is called Intraspecific Variation. Every one of the billions of organisms on planet Earth is unique. Even identical twins, who are born with the same DNA, vary as a result of their different experiences. How the do we measure the differences between these characteristics?

Making Measurements

All scientists measure things, but this is a particular problem for biologists. This is because they are usually measuring some aspects of living organisms and all living organisms are different. For this reason, biologists have to take many measurements of the same thing. They cannot reliably determine the height of buttercups or the number of red blood cells in every 1mm3 of human blood by taking a single measurement. Equally, they cannot measure every buttercup or human being in extistence. What they do is take samples.

Sampling

Sampling involves taking measurements of individuals, selected from the population of organisms which is being investigated. In theory, if these individuals are representative of the population as a whole then the measurements can be relied upon. But are the measurements representative? There aree several reasons why they might not be, including:

• Sampling Bias - the selection process may be biased. The investigators may be making unrepresentative choices, either deliberately or unwittingly.Are they as likely to take buttercups from a muddy area as a dry one? Will they aviod areas covered in cow dung or in rich nettles?
• Chance - Even if sampling bias is avoided, the individuals chosen may, by pure chance, not be representative. The 50 buttercups selected might just happen to be the 50 tallest in the population.

The best way to prevent sampling bias is to eliminate, as far as possible, as far as possible, any human involvement in choosing the samples. This can be achieved by carrying out random sampling. One method is to:

• Divide the study area into a grid of numbered lines, e.g. by stretching 2 long tape measures at right angles to each other
• Using random…