Module 4: Variation
These free OCR A Level Biology Variation revision notes have been written for specification point 4.2.2(f).
Variation
Variation is the differences between individual organisms.
These differences may occur and be measured within a species (intraspecific variation) or between species (interspecific variation).
| Type of variation | Description | Example |
|---|---|---|
| Intraspecific | Variation between individuals of the same species | Eye colour in humans |
| Interspecific | Variation between different species | Cats and dogs have different teeth |
Continuous and Discontinuous Variation
Variation is either continuous or discontinuous, which affects its distribution in a population.
- Continuous variation is usually caused by multiple genes (are polygenic) and may be influenced by the environment. It shows a range of intermediate values.
- Discontinuous variation is usually caused by one gene (monogenic) and is not affected by the environment. It shows distinct categories.
Continuous data should typically be plotted onto a histogram, but line graphs are commonly used.
The graph below gives an example data set of a random sample of 1000 people, presented as a histogram, showing a typical bell-shaped curve.
Discontinuous data is typically plotted onto a bar chart.
The graph below gives an example data set of a random sample of 1000 people’s blood type, presented as a bar chart, showing distinct categories of traits.
Quantitative vs Qualitative
Measuring variation produces data that can be quantitative or qualitative.
Quantitative data involves numerical measurements and is typically associated with continuous variation (e.g. height, mass).
Qualitative data describes categories or characteristics and relates to discontinuous variation (e.g. blood group, flower colour).
Examples of Continuous and Discontinuous Variation
The table below outlines some common examples of continuous traits:
| Trait | Description | Genetic Control |
|---|---|---|
| Height in humans | Varies gradually; influenced by the environment | Polygenic and environmental factors |
| Body mass in animals | Influenced by diet, activity, and genetics | Polygenic and environmental factors |
The table below outlines some common examples of discontinuous traits:
| Trait | Description | Genetic Control |
|---|---|---|
| Human blood group | A, B, AB, or O | Single gene with multiple alleles |
| Flower colour in peas | Distinct categories (e.g. purple or white) | Single gene |