Homologous Structures Are Evidence For Evolution
Definition
Homologous structures
Homologous structures are anatomical features that share a similar structure and position across different species but serve different functions.
- Homologous structures have a similar structural layout, despite differences in function.
- They serve as evidence of inheritance from a common ancestor.
- They illustrate the concept of "unity of type" (shared design due to common ancestry, as coined by Charles Darwin).
A human hand (grasping), a bird wing (flying), and a whale flipper (swimming) all follow the same basic bone arrangement but are specialized for different activities.
The Pentadactyl Limb: A Classic Example
- The pentadactyl limb is the foundational structure seen in all tetrapods (four-limbed vertebrates).
- It consists of:
- Proximal Bone (single): Humerus in the forelimb, femur in the hindlimb.
- Distal Bones (paired): Radius and ulna in the forelimb, tibia and fibula in the hindlimb.
- Wrist/Ankle Bones: Carpals in the forelimb, tarsals in the hindlimb.
- Digits (fingers/toes): Metacarpals and phalanges in the forelimb, metatarsals and phalanges in the hindlimb.
How the Pentadactyl Limb Has Evolved
- Humans: Manipulation and tool use, with a strong opposable thumb.
- Bats: Elongated fingers form the framework of wings for flight.
- Whales: Shortened and flattened bones create a flipper for swimming.
- Horses: The Central digit elongates into a hoof for efficient running
- Homologous Structures share a common origin, even if their functions differ (e.g., pentadactyl limb).
- Analogous Structures serve similar functions but do not share a common origin (e.g., wings of bats and insects).
Vestigial Structures as Supporting Evidence
Definition
Vestigial structure
A vestigial structure is a reduced anatomical feature that has lost its original function but is retained as an evolutionary remnant of a common ancestor.
- Vestigial structures are reduced, often non-functional remnants of features that were functional in ancestral species.
- They reinforce evolutionary theory by showing structural inheritance even when no longer needed.
- Embryonic teeth in baleen whales (lost before adulthood).
- Tiny pelvis and femur bones embedded in whale and snake bodies.
- Human appendix (remnant of a larger herbivorous cecum).
- Define the term “homologous structure” and give one example apart from the pentadactyl limb.
- Describe the structural arrangement of the pentadactyl limb, naming the key bones in order.
