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
- A representative sample of 50 snails from each habitat are photographed
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 | 17 | 34 | 12 | 24 | ||
Yellow plain | 7 | 14 | 17 | 34 | ||
Pink/brown striped | 18 | 36 | 7 | 14 | ||
Yellow striped | 8 | 16 | 14 | 28 |
- 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