3.4.7

• Variation

  Differences (in characteristics) between individuals, within a species (intraspecific variation) or between different species (interspecific variation)

  Variation within a species could be the result of…

  Genetic factors i.e. different alleles

  Environmental factors

  • Or a combination of both
• Continuous variation

  No distinct categories

  Data teds to be quantitative

  Controlled by many genes

  Strongly influenced by the environment

  • Example: height
• Discontinuous variation

  Distinct, discrete categories

  Data tends to be qualitative

  Controlled by a single gene or a few genes

  Unaffected / not strongly influenced by the environment

  • Example; blood groups
• Genetic diversity is the number of different alleles in a population Genetic diversity within, or between species, can be made by comparing…

  The frequency of measurable / observable characteristics

  Indicates genetic diversity because is based on the fact that different alleles determine different characteristics

  So the higher the variety of a characteristic, the higher the variety of alleles of that gene and hence the higher the genetic diversity

  The base sequence of DNA and mRNA

  • The amino acid sequence of the proteins encoded by DNA and mRNA
• How and why has gene technology caused a change in the methods of investigating genetic diversity?

  Early estimates of genetic diversity made by looking at frequency of measurable / observable characteristics in a population

  Limitations:

  Many observable characteristics coded for by more than one gene (polygenic) → vary continuously → difficult to distinguish one from another

  • Characteristics could be modified by the environment so differences may be as a result of different environmental conditions rather than different alleles
• Gene technologies have made it possible to directly obtain DNA sequences. These technologies can be used to give more accurate estimates of genetic diversity within a population / species because:

  Different alleles of the same gene have slightly different base sequences

  Comparing DNA base sequences of same gene in different organisms in a population → find out how many alleles of that gene in a population

  • Different alleles transcribed into slightly different mRNA base sequences and may produce polypeptides with slightly different amino acid sequences which can also be compared
• Quantitative investigations of variation within a species

  Taking a representative sample

  Random sample → eliminates bias

  Example of random sampling in a field:

  Divide the area into a grid of numbered lines

  Using random numbers from a table, obtain a series of coordinates

  • Take samples at the intersection of each pair of coordinates i.e. using quadrats
• Large sample size

  Minimise effects of chance (lower probability that chance will influence the data)

  Anomalies have less influence and can be identified

  Analyse results with a *named depending on the question* statistical test

  • See if variation observed is or isn’t due to chance
• Calculating a mean of the collected data and the standard deviation of that mean

  Mean = (sum of all measurements)/(total number of measurements)

  Standard deviation (you won’t be required to calculate standard deviations in your exam)

  Shows the spread of values around the mean

  68% of all measurements lie within ± 1 standard deviation

  About 95% of all measurements lie within ± 2 standard deviations

  • Note: this is only true if data shows a normal distribution i.e. when plotted as a graph it forms a bell-shaped curve
• Interpreting the mean values and their standard deviations

  Mean → can show if there is variation / differences between samples

  Useful for comparison, but provides no info about the range

  Standard deviation

  The higher the value standard deviation, the higher the variation

  • If standard deviations overlap, causing values of each set of data to be shared, any difference between the two may be due to chance
• Mean and standard deviation can be shown in different ways, e.g.

  9 ± 3 mean = 9 and standard deviation = 3

  Standard deviation can be plotted on graph / chart of mean values using error bars

  • Error bars extend one standard deviation above and one standard deviation below the mean