Variation (Cambridge O Level Biology)

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Types of Variation

  • Variation is defined as differences between individuals of the same species
  • Phenotypic variation is the difference in features between individuals of the same species
  • Phenotypic variation can be caused in two main ways:
    • It can be genetic - controlled entirely by genes
    • Or it can be environmental - caused entirely by the environment in which the organism lives
  • Examples of genetic variation in humans include:
    • Blood group
    • Eye colour
    • Ability to roll tongue
    • Whether ear lobes are free or fixed

Earlobe variation diagram

Variation in attached and detached earlobesWhether earlobes are attached (lobeless) or free (lobed) is an example of genetic variation

  • Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle
  • In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime
  • Examples include: 
    • An accident may lead to scarring on the body
    • Eating too much and not leading an active lifestyle will cause weight gain
    • Being raised in a certain country will cause you to speak a certain language with a certain accent
    • A plant in the shade of a big tree will grow taller to reach more light
  • Phenotypic variation can be divided into two types depending on how you are able to group the measurements:
    • Continuous variation
    • Discontinuous variation

Continuous Variation

  • Continuous variation is when there are very many small degrees of difference for a particular characteristic between individuals and they are arranged in order and can usually be measured on a scale
  • Examples include height, mass, finger length etc. where there can be many intermediate groups
  • Continuous features often vary because of a combination of genetic and environmental causes, for example:
    • Tall parents will pass genes to their children for height
    • Their children have the genetic potential to also be tall
    • However if their diet is poor then they will not grow very well
    • Therefore their environment also has an impact on their height
  • When graphs of this data is plotted, continuous variation gives smooth bell curves (a result of all the small degrees of difference)

Continuous variation bell-shaped curve

Graph showing continuous variation of height in a populationHeight is an example of continuous variation which gives rise to a smooth bell-shaped curve when plotted as a graph

Discontinuous Variation

  • Discontinuous variation is when there are distinct differences for a characteristic
  • For example, people are either blood group A, B, AB or O; are either male or female; can either roll their tongue or not - there are no intermediary values
  • Seed shape and seed colour in peas are also examples of discontinuous variation
  • Discontinuous variation is usually caused by genetic variation alone
  • When graphs of these data are plotted it is presented on a bar chart

Bar chart of discontinuous variation

Discontinuous variation bar chartBlood group is an example of discontinuous variation which gives rise to a step-shaped graph

Investigating variation

  • There are various ways in which we can investigate examples of continuous and discontinuous variation
  • The example below shows an example of an investigation into discontinuous variation

Polymorphism in snails

  • Two species of snail from the same genus, Capaea, are found commonly in woodlands and grassland habitats in the UK
  • The different variations in shell pattern across the two snail species show polymorphic variation (poly = many, morphism = form)
  • There are four shell patterns:

Snail shell pattern variation diagram

variation-in-snails 

Snails in the genus Capaea show polymorphic variation due to the different shell patterns observed in different habitats

Investigating polymorphic variation in snails

  • We can investigate the frequency of the different patterns in the populations of snails in woodland and grassland areas to show the effect of habitat on variation
  • The method is as follows:
    • A representative sample of 50 snails from each habitat are photographed
      • Quadrat sampling could be used to select snails in a random sample in each habitat
      • Taking a photograph of the snails in their habitat means that no snails would need to be removed
        • This is an ethical consideration
    • For each habitat, the snails are categorised into one of the four different variants: 
      • Pink/brown plain
      • Yellow plain
      • Pink/brown striped
      • Yellow striped
    • Results can be recorded in a table and presented in a bar chart for each habitat

Results

  • An efficient way to record the frequency of snails within each category is to use a tally chart:

Example of a tally chart for recording results

Snail type

Woodland
Grassland
Tally Number % Tally Number %
Pink/brown plain up diagonal strike vertical line vertical line vertical line vertical line end strike space space up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line 17 34 up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line 12 24
Yellow plain up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line 7 14 up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line 17 34
Pink/brown striped up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line vertical line 18 36 up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line 7 14
Yellow striped up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line vertical line 8 16 up diagonal strike vertical line vertical line vertical line vertical line end strike space up diagonal strike vertical line vertical line vertical line vertical line end strike space vertical line vertical line vertical line vertical line 14 28

  • These results can then be presented in a bar chart:

Example of a bar chart to represent tallied resultsvariation-in-snail-shells-bar-chart

Results analysis and conclusions

  • Results analysis is about looking at the data to identify the key trends
  • In the table above, we can see that...
    • all snails were recorded in both habitats
    • more pink/brown striped and pink/brown plain snails recorded in woodland compared to grassland
    • more yellow plain and yellow striped snails recorded in grassland compared to woodland
    • more than double the number of yellow plain snails recorded in grassland compared to woodland
    • more than double the number of pink/brown striped snails recorded in woodland compared to grassland
  • Conclusions are then made to try and explain the patterns seen in the results
  • Questions that might be relevant when writing conclusions for the results above might include:
    • Can the four categories of snail patterns be considered common in both habitats?
    • Are there more snails with a certain shell pattern found in one habitat because they are more adapted to that habitat?
    • Do the results suggest that the habitat has an impact on the variation of snails?

Evaluating your methods

  • In an evaluation, you should consider the strengths and limitations of your investigation
  • The sampling techniques used to collect the results will determine how representative the sample is and therefore how valid the results are
    • How was the area in the habitat selected?
    • Were the snails collected randomly?
    • Was the sample size big enough?
    • How were snails located within the habitat?
    • Were some snails less obvious than others due to camouflage? Did these snails get missed in the sampling techniques?
    • Did methods take into account the ethical considerations?
      • Snails are live animals so methods should be designed to prevent any harm

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Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!