Variation (Cambridge (CIE) O Level Biology): Revision Note

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

Whether 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

Height 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

Blood 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

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

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

Example of a bar chart to represent tallied results

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