AQA AS Level Biology Unit 2 - Variation

The 2nd AS Biology unit broken up into its sub-topics - variation

2 forms of variation


  • Interspecific variation - when one species differs from another
  • Intraspecific variation - when members of the same species differ from one another
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Investigating variation

Sampling: involves taking measurements of individuals, selected from the population of organisms. If they're are representative of the population reliability is increased. There are several reasons why they may not be representative:

  • Sampling bias: this selection process may be biased. The investigators may make un representative choices, wither deliberately or unwittingly.
  • Chance: Even if avoided, individuals chosen may not be representative.

The best way to prevent sampling bias is to eliminate any human involvement in choosing the samples, by carrying out random sampling. One method is to:

  • 1. Divide the study area into a grid of numbers e.g. by stretching two long tape measures at right angles to each other
  • 2. Using numbers from a table/generated by a computer to get coordinates
  • 3. Take samples at the intersection of each pair of coordinates

Using a large sample size or by analysis of the data collected(accepting chance may play a part

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Causes of variation

Genetic differences caused by different genes in the individuals. Genetic variation arises as a result of:

  • Mutations - sudden changes to genes and choromasome
  • Meiosis - special form of nuclear division that gorms gametes
  • Fusion of gametes - sexual reproduction cause offspring to inherit parental characteristics

Variety in asexually reproduction organisms can only be increased by mutations

Environmental differences: these influences affect the way the organism's genes are expressed. The genes set limits, but it is largely the environment that determines where, withough  those limits, an organism lies. Environmental influences include, climatic conditions, temperature, rainfall, sunlight, soil conditions, pH and food availability

In most cases of variation is due to the combined affects of genetic differences and environmental influences. It is very hard to distinguish between the effects of many genetic and environmental influences that combine together,

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Types of variation:

Where variation is the result of genetic factors organisms fit into a few distinct forms and there are no intermediate types. A character displaying this type of variation is controlled by a single gene, This variation can be represented on a bar chartor pie chart graph. Environmental factors have little influence on this type of variation.

Environmental factors play a major role in determining where on the continuum an organism actually lies. This type of variation is the product of polygenes and the environment. e.g height - forms a normal distribution curve

Mean - the meaurement at the maximum heigh of the curve. Provides an average value and is useful information when comparing one sample to another. (doesnt give information about the range of data)

Standard Deviation - the measure of the width of the curve. It gives an indication of the range of values either side of the mean to the point where the curve changes from being convex to concave (point of inflexion) 68% = all meaurements lie within +/- 1.0 standard deviation. 95% = all meaurements lie within +/- 2.0 standard deviation

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Standard deviation

1. calculate the mean value. 2. subtract mean value from all measured values. 3. square all the numbers to make them positive. 4. Add all the squared numbers together. 5. Divide this number by the original number of meaurements. 6. square root the number.


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Smart Girl


This is extremely helpful, thank you for uploading this! :)

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