Definition
Adaptive radiation = rapid diversification of a single ancestral species into many descendant species, each adapted to a different ecological niche.
Classic Example 1: Darwin's Finches (Galapagos Islands)
Ancestor: One species of finch from mainland South America colonized the Galapagos.
Result: 14+ species with different beak shapes:
| Beak Type | Food Source | Island Condition |
|---|---|---|
| Large, thick beak | Hard seeds (ground finch) | Seed-rich ground |
| Long, thin beak | Insects in bark crevices | Insect-rich trees |
| Long, curved beak | Cactus flowers/fruit | Cactus-dominated island |
| Small beak | Small seeds | Mixed environments |
| Woodpecker-like beak | Insects in wood (uses cactus spine as tool) | Insect-rich trees |
Process: Island isolation (allopatric) → different selection pressures → beak shape divergence → eventual reproductive isolation → separate species.
Classic Example 2: Australian Marsupials
| Marsupial | Ecological Equivalent (Placental) | Niche |
|---|---|---|
| Kangaroo | Deer/Antelope | Grazing herbivore |
| Koala | Slow arboreal mammal | Tree-dwelling folivore |
| Wombat | Badger/Groundhog | Burrowing herbivore |
| Tasmanian devil | Wolverine/Hyena | Scavenger/predator |
| Marsupial mole | Mole | Burrowing insectivore |
Process: One ancestral marsupial in Australia (isolated continent) → diversified to fill all available niches → adaptive radiation.
Key Insight for NEET
Both Darwin's finches AND Australian marsupials = adaptive radiation. The Galapagos example also involves allopatric speciation (island isolation). When NEET asks "which is adaptive radiation," look for: one ancestor → many species → different niches.