Homologous & analogous structures | Heredity & Evolution | Biology | Khan Academy

Introduction to Evolutionary Structures

• Evolutionary biology examines how structures in different species can be similar or different and what this implies about their relationships.

Similar Structures, Different Relations

• Examples of Structures:

  • Shark fins vs. dolphin flippers: Both aid in swimming but not closely related.

  • Bat wings vs. eagle wings: Both for flying, yet not closely related.

• Main Idea: Just because structures look alike and perform the same functions doesn’t mean they have a common evolutionary origin.

Homologous Structures

• Definition: Structures that are anatomically similar in different species but may serve different functions due to common ancestry.

• Examples of Homologous Structures:

  • Bat wings, dolphin flippers, and human hands

    • Bone Structure: Shares significant similarities in bone structure despite having different functionalities (flying, swimming, grasping).

    • Similarities in bone number and types emphasize common ancestry.

• Key Concept: Homologous structures reveal divergent evolution—the same structure adapted for different purposes.

Analogous Structures

• Definition: Structures that serve similar functions but arise from different evolutionary paths and have different anatomical structures.

• Examples of Analogous Structures:

  • Bat wings vs. eagle wings:

    • Bat wings (skin over bones) differ from eagle wings (feathers attached to the arm).

    • Conclusion: Not homologous; evolved from different ancestors.

  • Dolphin flippers vs. shark fins:

    • Dolphin flippers are bony; shark fins are made of cartilage, indicating separate evolutionary paths.

• Key Concept: Analogous structures showcase convergent evolution—unrelated species develop similar traits due to adapting to similar environments or functions.

Key Differences Between Homologous and Analogous Structures

Homologous Structures

• Common Features:

  • Same structure, different functions.

  • Evidence for divergent evolution.

  • Example: Human and chimpanzee hands.

Analogous Structures

• Common Features:

  • Different structures, similar functions.

  • Evidence for convergent evolution.

  • Example: Shark fin and dolphin flipper.

Conclusion

• Understanding these evolutionary concepts helps clarify the relationships between different species:

  • Look beyond mere appearance and function to underlying structures and ancestry.

  • Recognize that evolution can yield diverse adaptations (divergent evolution) or result in similar adaptations through different pathways (convergent evolution).