One bone pattern supports common ancestry
Definition
Pentadactyl limb
A vertebrate limb built from the same basic pattern of one upper bone, two lower bones, wrist or ankle bones and five digits.
- A pentadactyl limb has the same underlying plan of one upper bone, two lower bones, wrist or ankle bones, and five digits.
- Humans, bats, whales and many other vertebrates share this arrangement even though their limbs perform different functions.
- These are homologous structures, which means they have the same basic structural origin but may have different functions.
- The shared pattern is best explained by inheritance from a common ancestor.
- Natural selection then modified the ancestral limb for grasping, flying, swimming, walking or running in different environments.
Common Mistake
Similar function alone is not evidence of homology; the underlying arrangement and evolutionary origin must also match.
Differences show divergent evolution
- The bones are not identical in size or shape because different selection pressures favoured different movements.
- In a bat, elongated digits support a wing membrane, while in a whale shortened and broadened bones form a flipper.
- The mixture of a shared plan and specialised proportions supports divergent evolution from a common ancestral structure.
- This anatomical evidence agrees with evidence from fossils and genetic comparisons.
Example
A whale flipper and a human arm look different externally, but both contain a humerus, radius, ulna, wrist bones and digits in the same order.
Exam technique
A complete answer links the same bone pattern to common ancestry, then links different proportions to adaptation for different functions.
Self review
- What is a pentadactyl limb?
- What is a homologous structure?
- How does the limb plan support common ancestry?
- Why are the bones different in different animals?
