NCERT · Class 12 · Biology · EvolutionQuestion 2 (a) Define Adaptive Radiation. Explain it with the help of two suitable examples. (b) Differentiate between Homologous organs and Analogous organs with suitable examples, highlighting how they support the concept of biological evolution.
Step-by-Step Solution
Part (a): Adaptive Radiation
Definition:\nAdaptive radiation is an evolutionary process in which a single ancestral species evolves into a variety of different forms that adapt to different ecological niches. It occurs when a species enters a new environment with unexploited resources or vacant ecological spaces.
Examples:
- Darwin's Finches of Galapagos Islands:
- Charles Darwin observed that an original seed-eating finch species migrated to different islands of the Galapagos archipelago.
- Over time, in response to different available food sources (seeds, insects, cactus fruit, nectar), their beaks underwent structural modifications.
- This led to the evolution of many distinct finch species from a common progenitor.
- Australian Marsupials:
- A variety of marsupials (e.g., kangaroo, koala, Tasmanian wolf, marsupial mole) evolved from a single ancestral marsupial within the isolated Australian continent.
- Each adapted to a specific ecological niche, demonstrating adaptive radiation.
Part (b): Difference Between Homologous and Analogous Organs
| Feature | Homologous Organs | Analogous Organs |
|---|---|---|
| Anatomical Structure | Same basic anatomical structure and embryonic origin. | Different basic structure and embryonic origin. |
| Function | Perform different functions in different organisms. | Perform similar functions in different organisms. |
| Type of Evolution | Represents Divergent Evolution (common ancestor adapted to different habitats). | Represents Convergent Evolution (unrelated species adapted to similar environments). |
| Examples | Forelimbs of Human, Cheetah, Whale, and Bat (all have humerus, radius, ulna, carpals, metacarpals). | Wings of Birds and Wings of Butterflies/Insects. |
| Botanical Example | Thorns of Bougainvillea and Tendrils of Cucurbita (both are modified stems). | Sweet potato (root modification) and Potato (stem modification). |
Significance in Biological Evolution:
- Homologous organs prove that organisms sharing common ancestry have adapted to different functional requirements in diverse habitats (Divergent Evolution).
- Analogous organs demonstrate that organisms from different evolutionary lineages can independently evolve similar phenotypic traits when adapting to similar environmental pressures or ecological niches (Convergent Evolution).
💡 Study Guide: This question tests core syllabus concepts from Evolution. For formulas, key summaries, and mock exam reference guides, read the full Evolution Revision Notes.