Investigating and Measuring Variation

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Making Measurements

Interspecific Varaition - when one species differs from another species

Intraspecific Variation - when members of the same species differ to each other

Due to variation, conclusions can't be made about a species based on one set of data. But they also can't collect data from every single living species. Need to take samples

Sampling

  • Involves taking measurements from a few members of the population you're investigating
  • Results can be unrepresentative of the population due to:
    • sampling bias - selection process may be accidentally or purposefully biased
    • chance - data may be unrepresentative simply due to chance. All have certain characterisitcs
  • To avoid sampling bias, carry out random sampling. Done by dividing area using grid and take samples from randomly generated intersection coordinates
  • To avoid chance can use a large sample or by analysing the data
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Causes of Variation

Variation due to either genetic or environmental factors or a combination of both

This makes it hard to know cause of variation so conclusions tentative

Genetic Differences - due to individual's genes, inherited

Members of a species have same genes but different versions of them, alleles. Alleles you have make up genotype. Different genotypes produce different phenotypes (visible characteristics)

  • Mutations - sudden changes to genes and chromosomes. May or may not be passed on
  • Meiosis - nuclear division which makes gamates. Each one different
  • Fusion of gamates - sexual reproduction means that offspring differs from both parents

Variety in asexual reproduction can only happen via mutation

Environmental Influences

  • Phenotype also influenced by environment
  • Include climatic conditions (temp, rainfall, sunlight), soil conditions, pH, food availability, disease
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Presenting Variation

Variation due to genetic differences of one gene can be shown as distinct groupings on a bar graph or pie chart. Eg. The % of the UK population with blood group A, B, AB or O

Sometimes, a characteristic predetermined by several genes (height) can alter due to environmental factors (diet). This variation can be shown on a contiuous graph (line graph) and often forms a normal distribution curve.

Normal Distribution Curve - bell shaped

  • Mean is point at max height of curve (most frequent). Can be used to show if there's variation between samles but does not give range of values
  • Standard deviation indicates range of values either side of mean in a single sample by measuring width of curve
  • A large standard deviation means a lot of variety. Small SD means little variety
  • A graph with little variation, small standard deviation, is steep. One with little variation, large SD, is fat and not as steep
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