Question
What is adaptive radiation? Explain with the examples of Darwin’s finches in the Galapagos Islands and marsupials in Australia.
(NEET 2021, similar pattern)
Solution — Step by Step
Adaptive radiation is the process by which a single ancestral species rapidly diversifies into many different species, each adapted to a different ecological niche. It happens when organisms colonise new habitats with many unoccupied niches and few competitors.
The key idea: one ancestor, many descendants, each specialised for a different way of life.
On the Galapagos Islands, Darwin observed about 14 species of finches that had evolved from a single ancestral finch species that colonised the islands from the South American mainland.
Each species developed a different beak shape adapted to its food source:
- Cactus finches: Long, probing beaks for cactus flowers and nectar
- Ground finches: Strong, thick beaks for crushing seeds
- Tree finches: Parrot-like beaks for gripping insects in bark
- Woodpecker finch: Uses cactus spines as tools to extract insects
The islands had empty ecological niches (no native competing bird species), so the ancestral finch population diversified to fill them. Natural selection acting on beak variation drove this diversification.
Australia separated from other continents about 50 million years ago, carrying its marsupial fauna. With no placental mammals competing (except bats), marsupials radiated into a huge variety of forms:
- Marsupial mole: Burrows underground (like the placental mole)
- Tasmanian wolf (thylacine): Carnivorous predator (like the placental wolf)
- Sugar glider: Glides between trees (like the flying squirrel)
- Koala: Tree-dwelling herbivore
- Kangaroo: Grassland grazer
Each marsupial species evolved to fill an ecological role similar to a placental mammal elsewhere — this is also an example of convergent evolution between marsupials and their placental counterparts.
Why This Works
Adaptive radiation occurs when there is ecological opportunity — empty niches waiting to be filled. This can happen after a mass extinction (opens up niches), when a new habitat is colonised (islands, isolated continents), or when a key innovation evolves (like flight in birds).
The organisms produced by adaptive radiation show homologous structures (derived from a common ancestor) that have been modified for different functions — these are called analogous to similar structures in unrelated organisms that fill the same niche.
NEET often pairs adaptive radiation with convergent evolution. Darwin’s finches are adaptive radiation (divergence from one ancestor). Australian marsupials showing similar body forms to placental mammals is convergent evolution (similar adaptations in unrelated lineages). Know both concepts and how they connect.
Common Mistake
Students sometimes confuse adaptive radiation with convergent evolution. Adaptive radiation is divergent — one ancestor diversifies into many species. Convergent evolution is when unrelated species develop similar traits independently (e.g., wings of birds, bats, and insects). The Australian marsupials show BOTH patterns: adaptive radiation among themselves, and convergent evolution when compared to placental mammals.
Another error: writing that Darwin’s finches evolved different beaks “to fill niches.” Evolution does not work with purpose or intent. Natural selection favoured individuals with beak variations that happened to be better suited to available food sources — those individuals survived and reproduced more.