MPSC 2026 Prelims Paper-I · Question 61 of 100

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).

• (1)Divergent evolution — 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.
• (3)Parallel evolution — 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.
• (4)Independent evolution — "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.

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'.

Adaptive radiation is the process behind both halves of this question: a single stock, arriving in a place with many empty ways of life, diversifies rapidly into a set of specialised descendants. Darwin's finches on the Galapagos are the classic case, where beak shape tracks diet across the island group. Australia is the other classic case, and it is unusual because the same exercise ran twice on two different continents with two different starting stocks. The result is a natural experiment: when the two radiations are laid side by side, the matching pairs show that a way of life shapes a body, whatever lineage the body came from. That is why this example appears in the syllabus under convergence rather than under adaptive radiation alone.

• Convergent evolution — unrelated or distantly related lineages independently arrive at similar form or function because they face similar selection pressures. The structures it produces are analogous: same job, different build, different ancestry.
• Divergent evolution — one ancestral stock gives rise to descendants that grow progressively unlike each other as they adapt to different habitats. It produces homologous structures: same underlying plan, different function, shared ancestry.
• Marsupial-placental look-alike pairs: Tasmanian wolf (thylacine) and placental wolf; flying phalanger and flying squirrel; marsupial mole and placental mole; numbat and ant-eater; marsupial mouse and mouse.
• The reproductive difference that separates the two groups: marsupials deliver a very immature young that completes development attached to a teat, usually in a pouch; placental mammals nourish the embryo in the uterus through a placenta and deliver a far more developed young.
• Adaptive radiation — rapid diversification of one ancestral stock into many forms occupying different niches. Darwin's finches of the Galapagos and the marsupials of Australia are the two standard examples.

• Reading 'similar-looking' as 'closely related'. The examiner is testing the direction of change, not the degree of resemblance: apart-then-alike is convergence, together-then-unlike is divergence.
• Answering 'divergent' because Australian marsupials clearly diverged among themselves. They did — but the question pairs marsupials with placentals, which is the convergent comparison.
• Treating 'parallel' and 'convergent' as interchangeable. Parallel is reserved for lineages starting from a similar ancestral condition and changing in the same direction; convergent covers distant lineages arriving at the same solution.

State services and UPSC both favour the analogy/homology discrimination, usually as a one-line 'which of the following pairs is analogous' or 'X and Y are an example of ______'. The commonest formats are: a named pair of organs (wing of bat and wing of butterfly) to be classed; a named pair of organisms, as here; or a two-statement pair asking whether homology implies common ancestry. Learn the standard example lists in both directions — given the label, name a pair, and given a pair, name the label — because the question can be set either way round and the option sets are usually the same four terms.

No directly related past PYQ was found.

• practice — not a real PYQ

  The wings of a butterfly and the wings of a bird perform the same function but are built on entirely different plans. Such structures are described as :

  • (a)Homologous organs
  • (b)Analogous organs
  • (c)Vestigial organs
  • (d)Atavistic organs

  Answer(b) Analogous organs — same function, different structural plan and different ancestry. Analogy is the evidence for convergent evolution, whereas homologous organs (the forelimbs of whale, bat, cheetah and human) share a common plan and a common ancestor and are the evidence for divergent evolution.

• practice — not a real PYQ

  The diversification of a single ancestral stock into a number of different forms, each adapted to a different habitat within one geographical area, is called :

  • (a)Adaptive radiation
  • (b)Convergent evolution
  • (c)Genetic drift
  • (d)Saltation

  Answer(a) Adaptive radiation — the process shown by Darwin's finches on the Galapagos, where beak shape tracks diet, and by the marsupials of Australia, which spread from one stock into burrowing, gliding, hunting and grazing forms.