AQA Biology Unit 2: 7 Variation

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7.1 Investigating Variation

Interspecific Variation: One species differs from another

Intraspecific Variation: Members of the same species differ

Why Sampling Might not be Representative:

  • Sampling Bias: Investigators might deliberately or unwittingly choose their area of work
  • Chance: Pure chance can ruin representative results
  • The best way to remove sampling bias is reduce human involvement as much as possible

Random Sampling

1. Divide study area into a grid of numbered lines

2. Use random numbers from a number generator to obtain coordinates

3. Take samples at the intersection of each pair of coordinates

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Investigating Cont.

How to Minimise Chance

  • Large sample size: Reduces the probability of chance influencing results increasing reliability
  • Analyse data: Use statistical tests to see how much chance influenced the data

Genetic Differences Causes

  • Mutations: Cause sudden changes to genes/chromosomes or genes may not have passed on to the next generation
  • Meiosis: Genetic information is passed to the gametes by meiosis which gives varied genetic information
  • Fusion of Gametes: Characteristics inherited from both parents, it is a random process increasing variety
  • Variety in asexual reproducing organisms occurs due to mutation
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Investigating Cont.

Environmental Influences

  • The environment can influence how genes are expressed
  • Genes set limits but the environment decides where in those limits an organism is
  • Temperature, nutrients, chemicals can all affect the environment
  • Variation us usually a combination of genetic differences and environmental influences
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7.2 Types of Variation

Due to Genetic Factors

  • No intermediate values: they are catagoric
  • Characters like blood type are usually only controlled by a single gene 
  • Data can be represented in a bar chart or pie chart
  • Environmental factors have little influence to this type of variation

Due to Environmental Influences

  • Characters are not controlled by single gene but many (polygenes)
  • This data usually forms a normal distribution curve (bell shaped curve)
  • The values can be continuous
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Types Cont.

Mean: measurement at the maximum height of the curve, it provides an average value but doesn't give use information about the range of values

Standard Deviation: measure of the width of the curve, gives indication of the range either side of the mean. 68% lies within +1 standard deviation, 95% is +2 and 99% is +3

Calculate Standard Deviation

1. Calculate mean value

2. Subtract the mean from each measured value

3. Square all numbers

4. Add the squared numbers together

5. Divide this number by the original

6. Square root the results

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