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Learn the skills you need to do research on Evolution.
Planning an experiment
There are a number of investigations that could be carried out that look at the variation in populations and natural selection. Classic examples might include the differences in predation between dark and light coloured moths of the same species colour and banding in the European snail or variation in leaf size and shape in the common daisy. When planning this type of investigation there are some of a number of important points to consider.
1. Carry out some background research on the topic and the organism being considered. This should include books, websites, course notes and textbooks.
2. Produce one clear testable hypothesis. A hypothesis is an ‘educated guess’ of the likely results of the experiment. Many statistical tests need you to state a ‘null hypothesis’. A null hypothesis would state there is no difference between, for example, mean trunk diameter in trees in area A or from those in area B.
3. A statistical test will need to be included and this should be considered at the planning stage.
4. One set of measurements would not be considered reliable. For example the width of one daisy leaf would give no idea of the range of widths that might be found in an area. Measuring the width of many samples and calculating the mean value would be more reliable.
5. In order to make the results more reliable it is usually necessary to carry out the experiment several times. Time may be a consideration but it is common during experiments on selection to carry out two or three replicates.
6. If you carry out an experiment that involves living organism then it is important that both they, and the environment they live in are unharmed.
Analysing and presenting results
The first manipulation of any data collected during a variation or selection experiment is to calculate the mean and standard deviation of the repeated measurements that have been made. Imagine you had measured the shell width of a number of yellow-banded snails in three different locations. It is difficult to present all the individual data so the mean and standard deviation of each site could be calculated, which summarizes the information for easy comparison. The means and standard deviations can be presented in a graph, histogram or tabular format. It is important to be able to attach some level of confidence to your interpretation of the data. For this reason an appropriate statistical test is usually applied to the data sets.
Graph plotting for variation in populations
It is vital to select the most appropriate form of presentation for the data collected or provided, e.g. bar chart, histogram and line graph and be able to select which variable(s) to plot and plot appropriately on clearly labelled x - and y –axes.
Bar charts are used when one of the variables is not numerical – a discrete variable. For examples the number of lambs born to each ewe in a population in one particular year, the concentration of Vitamin C found in different fruits or the number of people of each different eye colour found in a class. It is important that the bars are of the same width and do not touch.
A histogram is used to plot continuous data. It could be used to plot the shell width in mussel shells or the number of eggs laid by a group of hens during each month of the year. The bars should be drawn in either ascending or descending order and they should touch.
A line graph shows the relationship between two variables.
A scattergram can be used to show variation between two variables e.g. the relationship between height in humans and the length of the foot. The points on the graph are not joined but left as single crosses. The way in which the points fall can show trends.