Quantitative investigations of variation

• Interspecific variation - If one species differs from another
• Intraspecific variation - If members of the same species differ from each other
• Sampling - Taking measurements from a limited number of individual organisms from a larger population
• Why is sampling used:

  • Sampling is used to estimate the number of organisms or particular characteristics of those organisms without counting all of them
• Sampling bias - The selection process may be biased

  • The investigators may be making unrepresentative choices, either deliberately or unwillingly
• Random sampling: (reduces bias)

  1. Divide study area into a grid of numbered lines
  2. Use random number generator to select coordinates
  3. Take samples at the intersection of each pair of coordinates
• Large sample size:

  • Ensures sample is representative
  • Reduces probability that any patterns are due to 'chance'
  • Less influence anomalies will have
• Normal distribution curve:

  • Populations and samples usually follow a normal distribution
  • If you plotted the information taken from each member of the population, they would create a bell-shaped curve as the distribution of data is symmetrical about the mean
  • Most values are close to the mean
• Standard deviation - A measure of the spread of data around the mean
• LOW standard deviation = LITTLE variation in the sample
• HIGH standard deviation = LOTS OF variation in the sample
• Standard deviation is a measure of the width of the curve

  • It gives an indication of the range of values either side of the mean