UPSC Prelims 2024 Q28: Coevolved Tree–Insect Pollination — Explained
Which one of the following shows a unique relationship with an insect that has coevolved with it and that is the only insect that can pollinate this tree?
- Fig
- Mahua
- Sandalwood
- Silk cotton
Answer: (a) Fig
Statement-wise explanation
- a: correct — Each fig species (genus Ficus) can be pollinated only by its own tiny fig wasp, from the family Agaonidae, and usually by just one or a few wasp species. The fig's flowers are sealed inside a closed, hollow receptacle called the syconium. A pollen-carrying female wasp squeezes in through a small opening called the ostiole, pollinates the flowers and lays her eggs in some of them. The fig cannot set seed without the wasp, and the wasp cannot breed anywhere but the fig. This is an obligate mutualism, and it has coevolved over about 75 million years.
- b: incorrect — Mahua (Madhuca longifolia) is pollinated mainly by fruit bats (pteropodid bats), not by one insect. Its flowers suit bats: they are dull-coloured, open at night and have a strong scent. The same bats also spread its seeds. There is no single coevolved insect partner.
- c: incorrect — Sandalwood (Santalum album) is insect-pollinated, but many kinds of insects do the job. Honey bees (Apis species), hoverflies, butterflies and small wasps all visit its small, nectar-bearing flowers. It needs pollen from another tree (cross-pollination), but no single insect is its only pollinator.
- d: incorrect — Silk cotton (Bombax ceiba, the red silk-cotton or semal tree) is pollinated mostly by birds and bats. Its large, bright red flowers have plenty of dilute nectar and bloom in the dry season. This fits bird pollination, not a relationship with one specific insect.
How to eliminate
This question is solved mainly by knowing the fact, but two clues in the wording help. First, 'coevolved' and 'the only insect that can pollinate' describe an obligate, one-to-one mutualism, and the textbook example is the fig and the fig wasp. Second, look at the flowers. Mahua and silk cotton have large, showy or night-scented flowers, which are typical of bat and bird pollination, so neither depends on one insect. Sandalwood's small open flowers are visited by many insects. A fig's flowers are hidden inside a closed fruit-like structure, which only a specialist could reach. The fact to remember: figs are pollinated only by fig wasps (Agaonidae).
Know more
Coevolution happens when two species shape each other's evolution through a close interaction. In pollination it can produce obligate mutualisms, where neither partner can reproduce without the other.
How the fig–fig wasp cycle works
- A fig's flowers line the inside of a closed receptacle called the syconium.
- A female wasp carrying pollen enters through the narrow ostiole and usually loses her wings on the way in.
- She pollinates some flowers and lays eggs in others. The flowers she lays eggs in become galls, which feed her larvae.
- Wingless males hatch first, mate with the females and chew exit holes. The females then fly out carrying pollen to a new fig.
- Adult wasps live only about 1.5–3.5 days at 26 °C. At higher temperatures their lives get even shorter, so warming threatens this partnership.
Why figs matter ecologically There are over 750 Ficus species. Different fig trees fruit at different times, so ripe figs are available all year. Up to 70% of rainforest birds and mammals eat figs. This makes figs a keystone resource: losing the wasps would cause a collapse that runs right through the forest food web.
Other well-known specialist pairs
| Plant | Pollinator | Nature of link |
|---|---|---|
| Fig (Ficus) | Fig wasp (Agaonidae) | Obligate; brood-site mutualism |
| Yucca | Yucca moth (Tegeticula) | Obligate; moth packs pollen, larvae eat some seeds |
| Orchid Angraecum sesquipedale | Hawk moth with very long proboscis | Specialised; long nectar spur |
The trees in the other options and their pollinators
| Tree | Main pollinators |
|---|---|
| Mahua | Fruit bats |
| Silk cotton (semal) | Birds and bats |
| Sandalwood | Bees, flies, butterflies (generalist) |
Recent research shows that the fig–wasp pairing is not always strictly one-to-one. Wasps sometimes shift to a new host fig, and some fig species share pollinators. The rule of 'one fig, its own wasp' still holds broadly.
NCERT Concepts to Revise
- Ecosystem services: Pollination is a core ecosystem service. The question tests which tree depends on one coevolved insect to deliver it.