Two organisms occupying a particular geographical area (say desert) show similar adaptive strategies. Taking e

NCERT Exemplar · Q50

Q. Two organisms occupying a particular geographical area (say desert) show similar adaptive strategies. Taking examples, describe the phenomenon.

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Concept understanding — Adaptive Radiation

Let’s start with an everyday image. Imagine a single family of musicians — say, a group of siblings who all learned the same basic instrument, the piano. Over time, one sibling moves to a jazz club and starts improvising, another joins an orchestra and plays classical concertos, a third becomes a film scorer who uses electronic keyboards, and a fourth teaches music to young children using simple, colourful pianos. They all came from the same training, but each adapted their skills to a very different environment. That is the core idea of adaptive radiation, but in biology it happens over many generations, not a single lifetime.

Adaptive radiation is the process by which a single ancestral species rapidly diversifies into many new species, each adapted to a different ecological niche. The key word is rapidly — in geological time, not human time — and the driving force is that the ancestral group encounters a variety of new environments or resources, with little competition from other species.

Think of it as an evolutionary explosion. A single species arrives on an island chain, or survives a mass extinction, and finds itself in a world of empty opportunities. Different groups within that species start using different food sources, living in different habitats, or avoiding predators in different ways. Over time, natural selection favours those variations, and the groups become so different that they can no longer interbreed — they become separate species.

The most famous example, and the one your NCERT textbook will emphasise, is Darwin’s finches on the Galápagos Islands. A single species of finch from the South American mainland colonised the islands. On different islands, with different food supplies — some with hard seeds, some with soft seeds, some with insects, some with cactus flowers — the finches evolved different beak shapes and sizes. One finch developed a thick, strong beak to crack nuts; another a slender, pointed beak to probe flowers; another a parrot-like beak to tear bark. All from one ancestor, all now distinct species.

Important

Adaptive radiation is not just any diversification. It requires three conditions:

A single ancestral species.
New ecological opportunities (e.g., islands, post-extinction vacancies, new key adaptations).
Rapid speciation into forms that occupy different niches.

Another classic case is the Hawaiian honeycreepers. A single finch-like ancestor colonised the Hawaiian islands and, over millions of years, gave rise to over 50 species with beaks ranging from curved for nectar-feeding to chisel-like for wood-boring. The isolation of each island and the variety of habitats — forests, mountains, coasts — drove the radiation. …

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This is convergent evolution — the desert environment forces two unrelated plant lineages, cacti and Euphorbia, to independently evolve near-identical water-conserving body forms (fleshy stems, reduced spine-like leaves) despite sharing no recent common ancestor.

When two organisms living in the same type of habitat show similar adaptive strategies despite not being closely related, biologists call this convergent evolution — evolution converging on the same solution from different starting points, because the same environmental problem imposes the same selective pressure on whichever lineage happens to occupy that habitat.

Desert plants are the textbook case for this. A desert imposes one dominant challenge on any plant living there: survive on very little water while tolerating intense heat and radiation. Two plant families that are not closely related have each solved this problem in essentially the same way:

Cacti (family Cactaceae), native to the deserts of the Americas, have thick, fleshy, water-storing stems; their leaves are reduced to spines, cutting the surface area available for water loss; and the stem itself has taken over the plant's photosynthesis.
Euphorbia species (family Euphorbiaceae), native to the deserts of Africa, independently show the very same fleshy, water-storing stem, the same reduction of leaves to spines, and the same shift of photosynthesis onto the stem.

These two families share no close common ancestor — whatever ancestor they do share long predates either lineage evolving succulence. Neither "borrowed" the trait from the other; instead, the desert environment itself selected for the same combination of features in both lineages independently, because any variant that stored water and minimised its transpiring surface had a survival advantage in that specific habitat, regardless of its ancestry. …

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Q.Two organisms occupying a particular geographical area (say desert) show similar adaptive strategies. Taking examples, describe the phenomenon.

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