In this topic of IBDP Biology, we will learn about what variation is and causes of variation.

• Variation means the differences in characteristics (phenotype) between organisms.
• There is variation within each species – intraspecific variation.
• Also, there are differences between different species – interspecific variation.

• Take a representative sample:
  • Measure the characteristic in a large number of different individuals
  • Essential that sampling is random – this eliminates bias.
    • Use random number tables or the button on a calculator
• The larger the sample size, the more representative the data.
• The smaller the sample, the higher the likelihood that chance can influence the data.
• Then plot a frequency histogram.
• This graph has the values of the characteristic on the X axis and the number of individuals showing that characteristic on the Y axis (frequency).
• The larger the number of samples

• Commonly referred to as the average.
• Add up all measurements and divide by total number of measurements:
• Mean = Sum of all measurements/ Total number of measurements

Standard Deviation

• There are two kinds of numerical data acquired by biologists:

1. Counting; e.g. the number of females in a population The true value is obvious for this.

2. Measuring a continuous variable such as length or weight

• The measured values always reflect a range.
• The size of this is determined by such factors as:
  • Precision of the measuring instrument
  • Individual variability among the objects being measured.
• Biologists like to be confident that the data they achieve is within acceptable limits of variance from the mean.
• Standard deviation can be used to quantify the spread of the data around the mean.
• The data must show a normal distribution around the mean.
• I.E. when plotted as a graph it forms a bell shaped curve.
• Standard deviation actually measures the distance from the mean to the point where the graph goes from being convex to concave.
• This includes 68% of the measurements.
• Two times the standard deviation includes 95%.
• The larger the standard deviation, the greater the spread of data around the mean.
• This number can then be compared with the standard deviation of another set of data.
• So, the larger the difference between the two standard deviations, the more certain we are that the two seta of data are different.

When it comes to IBDP Biology, there are many causes of variation:

This is the end of part 1. You can go to part 2 now!

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