A good comparison states the same feature for both groups. The vertebrate classes make a clear practice set because they differ on four features.
This lesson is part of biodiversity in the SPM Biology subject guide. It helps if you already know how to use a dichotomous key.
What features separate the vertebrate classes?
| Class | Body covering | Breathing | Reproduction | Body temperature |
|---|---|---|---|---|
| Fish | Wet scales | Gills | Lay eggs in water | Varies with surroundings |
| Amphibians | Moist skin | Lungs and skin | Lay eggs in water | Varies with surroundings |
| Reptiles | Dry scales | Lungs | Lay eggs on land | Varies with surroundings |
| Birds | Feathers | Lungs | Lay hard-shelled eggs | Constant |
| Mammals | Fur or hair | Lungs | Give birth to live young (most) | Constant |
Learn the rows by the feature, not the class. “Which classes breathe with gills?” is easier to answer from a column than from a list of paragraphs.
Worked example: a two-sided comparison
Question (original): Compare amphibians and reptiles.
A full answer uses the same criteria on both sides.
| Feature | Amphibians | Reptiles |
|---|---|---|
| Skin | Moist, no scales | Dry, covered with scales |
| Eggs | Laid in water, soft | Laid on land, leathery shell |
| Breathing | Lungs and skin | Lungs only |
Written as a sentence: “Amphibians have moist skin without scales, whereas reptiles have dry skin covered with scales.”
A common mistake: a one-sided comparison
| Answer | Status |
|---|---|
| Amphibians have moist skin. | Weak: no feature stated for reptiles |
| Amphibians have moist skin, whereas reptiles have dry scales. | Strong: both sides, same feature |
Always include a linking word such as “whereas” or “but”, and name the same feature for both groups.
Explaining the reason behind a feature
Some questions ask why a feature helps. Link the feature to a need.
- Dry scales reduce water loss, which helps reptiles live on land.
- Feathers trap air, which helps birds keep a constant body temperature.
State the feature, then the advantage, then the link to the habitat or function.
Check yourself
Compare birds and mammals using two features. Then explain one advantage of a constant body temperature.
Answer
Covering: birds have feathers, whereas mammals have fur or hair. Reproduction: birds lay hard-shelled eggs, whereas most mammals give birth to live young.
Advantage: a constant body temperature keeps enzymes working at a steady rate, so the animal can stay active even when the surroundings turn cold, such as at night.
Check that both sides use the same feature in each sentence.
What to study next
See why this variety matters in explaining the importance of biodiversity with evidence, then practise with the biological data interpretation trainer.
If you want a teacher to mark your comparison answers, see online one-to-one Biology tuition.