10.6 - Representing variation graphically

Question 1

Discontinuous variation

Answer 1

• A characteristic that can only result in certain values
• There are no in-between values
• Variation determined purely by genetic factors falls into this category. An animal’s sex is an example of discontinuous variation as there are only two possible functional values - male or female
• An example is the shape of bacteria (spherical, rods, spiral, comma, corkscrew shaped)
• Represented by a bar or pie chart

Question 2

Continuous variation

Answer 2

• A characteristic that can take any value within a range
• There is a graduation in values from one extreme to the other of a characteristic
• Characteristics that show continuous variation are not controlled by a single gene but a number of genes (polygenes). They are also often influenced by environmental factors
• Data is collected in a frequency table and plotted onto a histogram.

Question 3

Normal distribution curves

Answer 3

• The mean, mode and median are the same
• The distribution has a characteristic ‘bell shape’, which is symmetrical about the mean
• 50% of the values are less than the mean and 50% are greater than the mean
• Most values lie close to the mean value - the number of individuals at the extremes are low

Question 4

Standard deviation

Answer 4

• The standard deviation is a measure of how spread out the data is.
• The greater the standard deviation is, the greater the spread of the data.
• In terms of variation, a characteristic which has a high standard deviation has a large amount of variation
• When you calculate the standard deviation of data that display normal distributions you will find that:
  1) 68% of the values are within 1 standard deviation of the mean
  2) 95% of values are within 2 standard deviations of the mean
  3) 99.7% of values are within 3 standard deviations of the mean

Question 5

Other statistical tests

Answer 5

• Several statistical tests can be used by scientists to determine the significance of data collected.
• These tests can be used in a number of situations, for example when comparing variation within populations, or comparing the effects of abiotic and biotic factors on organisms. These include:
  1) Student’s t test - this is used to compare the means of data values of two populations
  2) Spearman’s rank correlation coefficient - this is used to consider the relationship between two sets of data