Exam questions · Biology · Evolution and Natural Selection
The Pentadactyl Limb
- 6 exam questions
- 22 marks
- 9 quick checks
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1 Name [3 marks]
Name the three groups of bones that are found in the limb of a human, a bat and a whale, apart from the humerus.
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Model answer
The radius and ulna, the carpals, and the metacarpals and phalanges.
Mark scheme
- Radius and ulna — 1 mark
- Carpals — 1 mark
- Metacarpals and phalanges — 1 mark
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2 State [2 marks]
State what is meant by the term pentadactyl limb.
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Model answer
A limb with five digits, which has the same basic pattern of bones in many vertebrates.
Mark scheme
- Five digits — 1 mark
- Same basic pattern of bones — 1 mark
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3 Explain [3 marks]
Explain why the human arm and the bat wing are described as homologous structures.
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Model answer
They have the same basic arrangement of bones, which is the humerus, radius and ulna, carpals and digits, but they have different functions. This is because they are inherited from a common ancestor.
Mark scheme
- Same arrangement of bones — 1 mark
- Different functions — 1 mark
- Inherited from a common ancestor — 1 mark
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4 Explain [4 marks]
Explain how the pentadactyl limb gives evidence for evolution from a common ancestor.
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Model answer
Animals as different as humans, bats and whales have limbs with the same arrangement of bones. If each had evolved separately, there would be no reason for them to share this pattern. The best explanation is that they inherited it from a common ancestor. Natural selection then changed the sizes and shapes of the bones to suit each way of life.
Mark scheme
- Same bones in different animals — 1 mark
- Would not be expected if separately evolved — 1 mark
- Inherited from common ancestor — 1 mark
- Natural selection adapted for function — 1 mark
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5 Explain [4 marks]
Explain how the bones of a bat's wing are adapted for flight.
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Model answer
The finger bones are very long and thin, so they can support a large wing surface. A thin membrane of skin is stretched between them, and this makes the wing light enough to be lifted. The thin bones also allow the wing to be folded away.
Mark scheme
- Long finger bones — 1 mark
- Support a wing membrane — 1 mark
- Light — 1 mark
- Can fold — 1 mark
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6 Evaluate [6 marks]
The wing of a bird and the wing of an insect have the same function. Evaluate whether this provides evidence of a common ancestor, and compare it with the pentadactyl limb.
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Model answer
The wings of a bird and an insect do the same job, but they have a completely different structure, because a bird's wing is made of bones, feathers and muscles, and an insect's wing is a thin membrane with veins. They are analogous structures, which evolved separately because flying is an advantage. This does not show a common ancestor. In contrast, the pentadactyl limb of a human, a bat and a whale has the same bones, despite different functions. They are homologous structures, so they provide evidence of a common ancestor. The structure, and not the function, is what shows a relationship.
Mark scheme
- Bird and insect wing have the same function — 1 mark
- Different structure — 1 mark
- Analogous, so no common ancestor — 1 mark
- Pentadactyl limb: same structure — 1 mark
- Homologous, so common ancestor — 1 mark
- Structure not function shows the relationship — 1 mark
Quick check
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1
How many digits does a pentadactyl limb have?
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C: Five
Pentadactyl means five digits.
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2
Which bone is in the upper part of the limb?
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A: Humerus
The humerus is the bone of the upper limb.
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3
What does the similar bone pattern in the arm of a human and the wing of a bat suggest?
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D: That they share a common ancestor
It suggests that they inherited the pattern from a common ancestor.
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4
What is the function of the whale's flipper?
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B: Swimming
The flipper is used for swimming and steering.
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5
Which structures are analogous?
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C: A bird wing and an insect wing
The wings of a bird and an insect have the same function but different structure and origin.
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6
Why is the horse's leg adapted for running?
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A: It is long with one digit and a hoof
It is long, with only one digit ending in a hoof, so it moves quickly.
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7
What has caused the bones of the limb to be different in different animals?
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D: Natural selection
Natural selection favoured different features in each species, according to their environment.
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8
What is divergent evolution?
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B: Related species evolving different features
It is the evolution of different features in related species from a common ancestor.
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9
A whale has a pentadactyl limb inside its flipper. What does this suggest?
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A: Whales evolved from land animals
It suggests that whales evolved from land animals with five-digit limbs.