Evolution of marsupials and placental mammals is an example of ________.

Question

UnlockIAS · Accepted Answer

Correct — option (2), Convergent evolution. Marsupials and placental (eutherian) mammals are two separate branches of the mammal family tree, distinguished by how the young are carried: a marsupial gives birth to a very undeveloped young that completes its growth attached to a teat, usually inside a pouch, while a placental mammal nourishes the embryo inside the uterus through a well-developed placenta and gives birth to a far more advanced young. Australia drifted into isolation carrying a marsupial fauna, and those marsupials then radiated to fill the whole range of mammalian ways of life on that continent — burrower, glider, hunter, grazer, ant-eater. On the other continents the placental mammals radiated to fill exactly the same set of niches. Because similar selection pressures were acting on two unrelated stocks, the end products came out looking and behaving alike: the marsupial Tasmanian wolf against the placental wolf, the flying phalanger against the flying squirrel, the marsupial mole against the placental mole, the numbat against the ant-eater, the marsupial mouse against the mouse. Similar form arrived at independently, from different ancestry, driven by a shared way of life, is the definition of convergent evolution, and the resemblances it produces are analogous rather than homologous. This paired radiation is the standard school-syllabus illustration of convergence, which is why the Commission keyed option (2).

Why the other options are wrong — (a) Divergent evolution runs the other way: one ancestral stock gives rise to descendants that become progressively unlike each other as they adapt to different habitats, and the structures it leaves behind are homologous — the same underlying plan put to different uses, as in the forelimb of a whale, a bat, a cheetah and a human. Divergence does occur inside this example, because Australia's many marsupials all descend from a single marsupial stock; that part is adaptive radiation. But the question compares marsupials with placentals, two lineages that began apart and ended up alike. Starting together and becoming unlike is divergence; starting apart and becoming alike is convergence.  (c) Parallel evolution also produces similar features independently, but the term is normally kept for lineages that set out from a similar ancestral form and then change in the same direction, so that they stay about as alike as they began. Marsupials and placentals are separated by one of the deepest divisions inside the mammals and began from clearly different reproductive and anatomical ground plans, so the resemblance between a thylacine and a wolf is arrived-at, not inherited. Evolutionary biologists do argue about where exactly the parallel/convergent boundary lies, and some treat it as a matter of degree rather than a clean line; but the textbook usage that this paper is testing, and the label the Commission keyed, is convergence.  (d) "Independent evolution" is not one of the named patterns in the standard classification — divergent, convergent, parallel, coevolution. It describes the circumstance rather than the pattern: every case of convergence and every case of parallelism is also 'independent', so the label cannot separate one from the other and gives the examiner nothing to test. When an option in a science question is a plain-English phrase rather than a term of art, check whether it names a real category before you spend a mark on it.

Concept: Two contrasting labels sit at the centre of this topic. Homologous structures share a common underlying plan and a common ancestor, but have been put to different uses — the forelimbs of a whale, a bat, a cheetah and a human all carry humerus, radius, ulna, carpals, metacarpals and phalanges, arranged the same way and used for swimming, flying, running and grasping. Homology is the fingerprint of divergent evolution. Analogous structures do the same job and often look alike, but are built differently and come from different ancestry — the wing of a butterfly and the wing of a bird, the eye of an octopus and the eye of a mammal, the flipper of a penguin and the flipper of a dolphin. Analogy is the fingerprint of convergent evolution. The examiner's test is therefore not 'do these two organisms look alike' but 'did the resemblance come down from a shared ancestor, or was it arrived at separately'.

State PCS 2026 Q61 — Science & Technology PYQ with Answer

State PCS 2026 Q61 — Science & Technology PYQ with Answer