Variation
In this topic you will:
- learn what is meant by variation
- think about how genes help to produce variation
- investigate and record some examples of variation.
Key Words
- genetic differences
- variation
Variation within a species
Organisms that belong to different species usually look very different from one another. Horses look different from donkeys. Lions look different from tigers.
But individual organisms that belong to the same species also have differences between them. You and all of the other people in your class all belong to the same species. But you are all very different from one another.
The differences between individuals belonging to the same species are called variation.
Common Mistake
Don't assume that variation only occurs between species. A key part of biological variation also happens within a species — even among individuals in the same family or classroom.
Questions
Show Answer
All the goats have four legs, hooves, two ears, horns, and similar body shape. These shared features show they belong to the same species.
Show Answer
They differ in colour, size, pattern of fur, and horn shape. Some goats are mostly white, others are black or brown. Some have long ears, others short. These are examples of variation within a species.
Using a bar chart to show variation
A species of plant called kidney vetch has flowers that can have any one of five different colours.
Zara studied the kidney vetch plants growing in a small area of a field. She counted the number of plants with each colour of flower.
She recorded her results like this. Each stroke represents one plant. When she gets to five, she puts the stroke for five across the first four.
| Colour | Red | Pink | Orange | Yellow | Cream |
|---|---|---|---|---|---|
| Tally | |||| | || | ||| | |||| |||| | | ||| |
| Number | 5 | 2 | 3 | 9 | 3 |
When she had recorded the flower colour of each plant, she added up all the tally strokes and wrote the number in the last row.
Then she used her results to draw a bar chart, like this.
Example Observation
Bar charts help to clearly show patterns in variation. In this case, the most common flower colour was yellow, with 9 plants, while pink was the least common with just 2.
Questions
Show Answer
She found 22 plants in total (5 red + 2 pink + 3 orange + 9 yellow + 3 cream).
Show Answer
The most common flower colour was yellow, with 9 plants.
Show Answer
The bar chart shows the results more clearly because it lets you quickly compare the number of plants in each group by looking at the height of the bars. It’s easier to spot the most or least common colours visually.
Think Like a Scientist
Some trees have leaves that are divided up into several smaller parts, called leaflets. You are going to investigate the variation in the number of leaflets in a leaf.
You will need:
- one or more trees belonging to the same species, with leaves divided up into leaflets.
Safety
Choose trees where you can collect leaves without having to climb onto anything.
Method
- Collect at least 20 leaves from the same species of tree.
- Count the number of leaflets on each leaf and write them down in a list, like this:
11, 15, 12, 11, 13 … - Draw and complete a tally chart like this. You will need to change the number of columns depending on your results.
| Number of leaflets | Tally | Number of leaves |
|---|---|---|
| [Your data] | [Tally marks] | [Total] |
- Now you are ready to draw a bar chart. Use the bar chart for the flower colour results to help you.
Questions
Show Answer
The median is the middle number when all leaf counts are arranged in order. For example, if your list is: 11, 11, 12, 13, 15 — the median is 12.
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The mode is the number of leaflets that appears most often. For example, if 11 appears the most, then the mode is 11.
Show Answer
Add together all the leaflet numbers, then divide by the total number of leaves. For example: (11 + 12 + 13 + …) ÷ 20.
Show Answer
Yes, differences might exist even on the same tree. These could be due to genetic factors or environmental conditions such as sunlight, damage, or nutrients affecting individual leaves.
DNA and variation
Look again at the photograph of the goats, near the beginning of this topic. Can we tell what has caused the differences between them?
Each goat in the picture began life when a sperm cell fertilised an egg cell. A sperm cell and an egg cell each contain chromosomes, made of many genes. The genes are made of DNA.
All the sperm cells and all the egg cells of goats contain genes that affect the development of horns. There are several versions of these genes, each made of slightly different DNA. One version of the genes might produce long horns, another short horns and another one perhaps no horns at all. There could also be different genes for making curved horns or straight horns.
Different sperm cells and different egg cells probably contain different versions of these genes. They will also contain different versions of other genes – for example, for height, or coat colour or tail shape. Each gene is different because its DNA is different.
When a sperm cell and egg cell fuse together at fertilisation, the zygote that is produced has a new combination of DNA. The combination is not exactly the same as in its parents, or in its brothers and sisters.
Differences in the DNA of organisms within a species are called genetic differences. Genetic means ‘to do with genes’.
The people in this photograph belong to three generations of the same family. Some of the DNA from the grandparents has been passed on to their children and grandchildren. You can probably see some similarities between them, because some of their DNA is the same. But everyone in the family has a slightly different combination of DNA from everyone else. There are genetic differences between them.
Quick Fact
DNA carries the instructions that determine your characteristics, like eye color, height, and whether you can roll your tongue. Everyone's DNA is slightly different—this is why individuals within a species are not identical.
Important Concept
Genetic variation arises because each new organism inherits a unique combination of DNA from its parents. Even within a family or species, no two individuals (except identical twins) have exactly the same DNA, which explains why we all look slightly different.
Environment and variation
DNA is not the only cause of variation between individuals. An organism’s environment also affects it.
For example, think again about the variation in the goats. Let us for the moment just think about the two adult goats. They have different coat colours. This is determined by their DNA. Variation in coat colour is caused by genetic differences.
The size of the goat could also be affected by its DNA, but its environment will also affect size. For example, one goat might eat more than another, so it would grow fatter and have a greater body mass. This is not caused by its genes. Not all variation within a species is caused by differences in DNA.
Common Mistake
It’s a common mistake to think that all variation between individuals is caused by differences in their DNA. In reality, environmental factors—like food availability or temperature—can also lead to visible differences that have nothing to do with genes.
Questions
- genes only
- environment only
- genes and environment
Show Answer
Example answers:
- Eye colour: genes only
- Language spoken: environment only
- Height: genes and environment
- Body weight: genes and environment
- Skin tone (with sun exposure): genes and environment
The classification depends on the specific feature and whether it is influenced by inherited DNA, external conditions, or both.
Think Like a Scientist
Measuring variation in humans
You are going to investigate variation in people’s wrist circumference. The data you collect are a little bit more difficult to deal with than the leaflet data. This is because they do not fit into definite categories, so you will have to determine the categories yourself.
Method
Step 1: Measure the circumference of the wrist of every person around you. If needed, include people beyond your class to reach at least 20 participants. Write down your measurements in a list with units.
Step 2: Sort your measurements into suitable categories. Aim for categories with equal width (e.g. 1 cm intervals). You may adjust the range depending on your results.
Step 3: For each measurement, place a tally in the correct category.
Step 4: Count up the tallies and record the total number of people in each group.
Step 5: Draw a frequency diagram. Place wrist circumference on the x-axis and number of people on the y-axis. Make sure the bars touch each other.
| Wrist circumference in cm | Tally | Number of people |
|---|---|---|
| 8.0 – 8.9 | ||
| 9.0 – 9.9 | ||
| 10.0 – 10.9 | ||
| 11.0 – 11.9 | ||
| 12.0 – 12.9 | ||
| 13.0 – 13.9 | ||
| 14.0 – 14.9 |
Questions
Show Answer
The smallest wrist circumference recorded was around 8.0 cm and the largest was close to 14.9 cm, depending on your class measurements.
Show Answer
The most common category will be the one with the highest tally. Typically, this may fall between 10.0 – 11.9 cm, but it depends on your group's actual data.
Peer Assessment
How well do you think you have completed this task?
Did you:
- Measure wrist circumference in exactly the same way for each person?
- Group the measurements into suitable categories?
- Label the frequency diagram axes correctly?
- Choose appropriate scales for both axes?
- Draw the bars carefully and accurately?
Write down one thing you did well.
Write down one thing you could improve next time.