Key points
- Variation is the differences between individuals of the same species, caused by genetic and environmental factors.
- Surveys into variation give data that are continuous, which means to come in a range, or discontinuous, which means to come in groups.
Video
Learn about the different types of variation
Every crisp is unique and thanks to variation, every organism is unique too.
Due to genetic and environmental factors, members of the same species will have different characteristics or variation. We measure and visualise variation based on whether it is continuous or discontinuous.
Discontinuous variation refers to things like eye colour or blood group, which have a limited number of possible values. In other words, a person's blood can be A, B, AB or O, but it can't be anything in between. So we can simply count how many in our sample are in each group, which we can represent well with a bar graph.
Continuous variation refers to characteristics like weight or height, which change gradually. A person's height could measure any value in between the shortest and tallest person in the world. Line graphs show continuous variation well and often, but not always turn out to be bell shaped because most specimens have close to average characteristics.
Thanks to variation the world is full of interesting, unique organisms, just like my bowl is full of unique crisps. Well it was anyway.
Can you answer these questions based on the video?
1. What are the two ways in which we categorise variation?
2. Which type of variation changes gradually?
Continuous or discontinuous
Continuous
Variation
Small changes in living organisms. is all the differences that exist in a population of the same species. These differences are caused by:
- Genetic variation - these are differences between individuals that are inherited from parents, such as the colour of your eyes, hair and skin.
- Environmental variation - these are differences between individuals that are not inherited but caused by the environment that the organism lives in, including scars and tattoos.
- Genetic and environmental variation - differences between individuals that are caused by both genetic and environmental factors, such as height and weight.
Data on variation is collected by surveying the population. This data can be described as either continuous variation or discontinuous variation. It is important to remember that these are not the causes of variation, which are described above, but how we analyse the results of the surveys.
Continuous variation
Surveys of Variation that shows a wide range of values between two extremes. give us results that come in a range. Human height is an example of continuous variation. It ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values, so this is continuous variation. For example, you can be 150 cm tall, 151 cm tall, or any height in between this - if you had a ruler that could measure small enough values.
So, a characteristic that changes gradually over a range of values shows continuous variation. Examples of such characteristics are:
- height
- arm span
- weight
Results from surveys of continuous variation are presented in line graphs or bar charts with a A line drawn on a graph which shows the trend without necessarily passing through all points. Can be curved or straight. drawn through them. If you record the heights of a group of people and draw a graph of your results, it usually looks something like this:
Normal distribution
Surveys of continuous variation often give us results in a characteristic shape seen in the green line on the graph above. If there are fewer readings at either ends of the scale and far more in the middle, we see a A graph shaped like a bell of normal distribution often seen in surveys of continuous variation. of The distribution of values from a survey of continuous variation which gives a characteristic bell-shaped graph with decreasing values from the mean.. The more people you measure and the smaller the categories you use, the closer the results will be to this shape.
Discontinuous variation
Surveys of Differences between individuals in a characteristic that can only be put into different groups. Sometimes called discrete. give us values that come in groups rather than a range. Human blood groups are an example of discontinuous variation. In the ABO blood group system, only four blood groups are possible - A, B, AB or O. You cannot have a blood group in between these four groups, so this is discontinuous variation.
Here are some examples:
- blood group
- eye colour
Results from surveys of discontinuous variation are presented in charts. These is no line of best fit drawn because the values on the x-axis - blood groups in the graph below - could be placed in any order. If you record the blood groups of a group of people and draw a graph of your results, it usually looks something like this:
More examples
| Continuous variation | Discontinuous variation |
|---|---|
| Height | Blood group |
| Weight | Hand used to write with |
| Arm span | Eye colour |
| Head circumference at birth | Ability to roll tongue |
Test your knowledge
Quiz
Test questions
Write a paragraph for each of the following questions. Tap 'Show answer' to see four points you could have included.
Describe the process involved in a survey of discontinuous variation. Give examples in your answer.
- A survey involves measuring a characteristic in a number of people or organisms.
- If results from a survey come in groups then this is discontinuous variation.
- Examples include blood group and eye colour.
- Results are presented in a bar chart without a line of best fit.
Remember to award yourself the marks if you chose a different but correct example.
Describe the process involved in a survey of continuous variation. Give examples in your answer.
- A survey involves measuring a characteristic in a number of people or organisms.
- If results from a survey come in a range then this is continuous variation.
- Examples include height and weight.
- Results are presented in a line graph or a bar chart with a line of best fit.
Remember to award yourself the marks if you chose a different but correct example.
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