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B. Life Systems · Biodiversity

Why Variety Within a Species Helps It Survive

B2.3 A1.2 A3.2About 50 minutes
Learning goals
  • I can describe variation among members of the same species.
  • I can explain why diversity within a species helps it survive changes, such as a new disease or a change in climate.
  • I can measure variation in a sample and show it on a line plot.

Words to know

variation
Differences among members of the same species.
trait
A characteristic of an organism, such as height, colour or resistance to a disease.
inherited trait
A trait passed from parents to their young.
genetic diversity
The variety of inherited traits within a species or population.
resistant
Able to fight off or survive something, such as a disease.
variety (of a crop)
A type of a crop plant with its own set of traits, such as a kind of apple or potato.

No two alike

Look at the people in your family, or the kids in your class. Same species, Homo sapiens, yet everyone is different. Now look at a bag of apples, a handful of beans or a flock of Canada geese. Look closely and you will find differences there too.

Those differences are not just interesting. They can be the difference between a species surviving and disappearing.

A wooden bushel basket of apples in an orchard, with apples of many sizes and shades of red, yellow and green.
Apples come in many shapes, sizes and colours. That variety within one species is genetic diversity, and it can help a species survive. AI-generated image

Variation within a species

Members of one species share the same basic body plan, but they vary in many traits:

  • Visible traits: size, colour, the shape of leaves or beaks, thickness of fur.
  • Hidden traits: how well an organism fights a certain disease, how much cold or heat it can handle, how much water it needs.

Many traits are inherited: parents pass them to their young. The variety of inherited traits in a species is called its genetic diversity. This is the first level of biodiversity: diversity within a species.

Why variety helps a species survive

Conditions on Earth keep changing. A new disease can arrive. Summers can become hotter or drier. A new predator can appear.

  • If all members of a species are almost the same, a change that harms one can harm them all.
  • If members are varied, some may have traits that let them survive the change. The survivors reproduce and pass on those helpful traits, so the species carries on.

Think of a sports team. If every player could only play goalie, the team would be in trouble in most games. A team with many different skills can handle more situations.

Worked example

Let's compare two populations facing a new disease

A new plant disease reaches two fields. Field A has 100 potato plants, all grown as copies of one parent plant (so they are almost identical), and none can resist the disease. Field B has 100 potato plants of many different varieties; 15 of them are resistant to the disease. What happens in each field?

  1. Field A: all plants share the same weakness. The disease can kill all 100 plants, and none are left to grow next year's crop.
  2. Field B: the 85 plants without resistance may die, but the 15 resistant plants survive.
  3. The survivors in Field B produce new potatoes for next year, and many of those carry the resistance trait.
  4. Over time, Field B's population can recover, and more of its plants are resistant.

Variety within a species is like insurance. Field A has no back-up; Field B has survivors because its plants are different.

Field A: 100 identical plantsnone can resist the diseaseField B: 100 varied plants15 are resistant0 survive15 survive and can regrow
The two fields from the example. Each circle is a surviving plant and each X is a plant killed by the disease. Variety in Field B means 15 plants survive.

Real examples

  • Potatoes in Ireland: in the 1840s, many farmers in Ireland relied on just a few potato varieties. When a disease called potato blight arrived, crops failed across the country. Many people starved or had to leave Ireland.
  • Bananas: most bananas sold in Canadian stores are one variety, and the plants are grown as copies of each other. An earlier favourite variety was badly hit by a disease in the mid-1900s, and scientists now worry about a newer form of that disease affecting today's variety.
  • Cheetahs: scientists have found that cheetahs have very little genetic variety, likely because their numbers became very small long ago. This may make them more likely to be harmed by diseases.

When a population gets very small, it loses variety, because fewer individuals carry fewer different traits. That is one reason conservation programs try to keep populations large and connected, and why zoo breeding programs carefully choose which animals to pair.

Math connection

Scientists describe variation with numbers. The range (largest minus smallest) shows how spread out a trait is. A line plot shows how often each value appears.

Just for fun

What if a chickadee had to take attendance in your class? "Hmm... no feathers, no beaks... they all look the same to me!" Looking closely is how scientists spot the variety within a species.

Bean12345678910
Length (mm)14151516161617171819
Lengths of 10 dried beans from one bag. All the same species and variety, yet they vary. Range: 19 − 14 = 5 mm. Most common length (mode): 16 mm.
1314151617181920bean length (mm)range: 19 − 14 = 5 mmmode: 16 mm
The 10 bean lengths from the table as a line plot. Each X is one bean. The lengths spread from 14 mm to 19 mm (range 5 mm), and 16 mm is the most common (the mode).
Hands-on activity

Measure the variation

You need: 20 dried beans of one kind (kidney, lima or white beans), a ruler with millimetres, paper and a pencil.

  1. Predict: will all the beans be the same length? How different will they be?
  2. Measure each bean's length to the nearest millimetre. Record the results in a table.
  3. Find the shortest, the longest and the range (longest − shortest).
  4. Make a line plot: draw a number line covering your lengths and put an X above the number for each bean.
  5. Find the mode (the most common length).
  6. Explain: if a bird could only swallow beans shorter than a certain length, how would variety help the bean plants survive?
  7. Bonus: measure the hand span of everyone in your family (thumb tip to pinky tip) and compare the range.

Safety: dried beans are a choking hazard for little brothers and sisters. Keep them away from young children and don't eat raw dried beans.

What to look for: careful measurements to the nearest mm, a correct range and mode, a line plot with one X per bean, and the idea that variation means some individuals survive when conditions change.

Your turn

1

What is variation?

2

Give two visible traits and one hidden trait that can vary in a species.

3

Why is diversity within a species important when a new disease appears?

4

What happens to genetic diversity when a population becomes very small?

5

Beans measured 12, 13, 13, 14 and 17 mm. What is the range?

6

What happened to potatoes in Ireland in the 1840s?

7

What does "inherited" mean?

More practice: warm-up

1

True or false: all members of a species are exactly the same.

2

Name a trait that varies among people.

3

What is the most common value in a set of data called?

4

Which population is more likely to survive a change: a varied one or a uniform one?

5

Beans measured 15, 16, 16, 18 mm. What is the mode?

More practice: on your own

1

Winters in an area become much colder. A population of rabbits has some with thicker fur and some with thinner fur. Predict what will happen over many years. Explain.

2

Why do conservation programs try to keep populations large and connected?

3

Ten pea pods hold 4, 5, 5, 6, 6, 6, 7, 7, 8 and 6 peas. Find the range and the mode.

4

Most store bananas are one variety grown as copies. Why do scientists worry about this?

5

In the potato example, why did Field B recover but Field A did not?

Challenge

1

Zoos breed endangered animals to help save them. Why do they carefully choose which animals to pair, instead of letting any two animals breed?

2

A farmer saves seeds only from her single biggest pumpkin every year. Give one benefit and one risk of this.

3

A class measured 30 beans and found a range of 9 mm. Another class measured 30 beans of a different kind and found a range of 3 mm. Which sample shows more variation in length? Can you say for sure which kind of bean is "better at surviving"? Explain.

Check your understanding

Quiz

1.

Diversity within a species means...

2.

Why does variety help a species survive a new disease?

3.

In the 1840s, potato crops in Ireland failed mainly because...

4.

A population shrinks from 10 000 to 50 animals. What usually happens to its genetic diversity?

5.

Bean lengths: 11, 13, 13, 15, 16 mm. What is the range?

6.

Which is an example of an inherited trait?

For parents and teachers

Big idea: members of a species vary. That variety (genetic diversity) gives a species a better chance of surviving changes such as new diseases or a changing climate, because some individuals have traits that help them survive.

How to help at home:

  • Do the bean-measuring activity and make the line plot together. Ask: "If only the longest beans could grow next year, what would happen?"
  • Compare apples or potato varieties at the grocery store and talk about why it's good that farmers grow many kinds.
  • Compare hand spans or heights in the family to see variation in people.

Safety: keep dried beans away from small children (choking), and don't eat them raw.

Common difficulties:

  • Mixing up variety within a species with variety among species.
  • Thinking individuals change their traits on purpose to survive. A population changes over generations because individuals with helpful traits survive and have more young.
  • Range: remember it is largest minus smallest, not just the largest number.