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NCERT Solutions · Class 12 Biology Evolution

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Exercises 6.1–6.10

  1. Exercise 6.1

    Explain antibiotic resistance observed in bacteria in light of Darwinian selection theory.

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    Antibiotic resistance is natural selection in action: the antibiotic does not create resistant bacteria, it only selects the ones that were already there.
    A colony of bacteria has built-in variation — the population is never uniform, and a chance mutation leaves a few cells able to survive the drug.
    Adding the antibiotic changes the environment. Only that part of the population which can survive the new conditions lives on; the rest are killed.
    The survivors have greater fitness under the new conditions. For Darwin, fitness means reproductive fitness — the fitter leave more progeny.
    Because microbes divide fast and can become millions of individuals within hours, the resistant variant outgrows the others in days, and the whole population comes to be resistant. In a fish or a fowl the same change would take millions of years.
    Resistance is inherited, so it has a genetic basis — this is what makes it evolution and not merely survival.
    This is evolution by anthropogenic action: our own excess use of antibiotics (like the excess use of herbicides and pesticides selecting resistant varieties) makes resistant forms appear on a scale of months or years, not centuries.
    Two cautions the chapter adds: no variant is completely wiped out, and the process is not directed — it is a stochastic process resting on chance mutation and chance events.

  2. Exercise 6.2

    Find out from newspapers and popular science articles any new fossil discoveries or controversies about evolution.

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    This exercise asks you to search newspapers and popular science magazines yourself — the chapter carries no news items, so what follows is only the material in the chapter you can use as your starting point.
    Fossil discoveries named in the chapter:
    A fish caught in South Africa in $\displaystyle 1938$ turned out to be a Coelacanth, which had been thought extinct — a lobefin, the group that gave rise to the first amphibians.
    Fossils discovered in Java in $\displaystyle 1891$ revealed Homo erectus, about $\displaystyle 1.5$ mya.
    A few man-like bones found in Ethiopia and Tanzania showed hominid features, pointing to man-like primates walking in eastern Africa $\displaystyle 3$–$\displaystyle 4$ mya.
    Controversies the chapter itself records:
    The theory of special creation (all species created as such, diversity unchanging, earth about $\displaystyle 4000$ years old) against Darwin's conclusion that the earth is billions of years old and life forms have changed.
    Why the dinosaurs suddenly disappeared about $\displaystyle 65$ mya — some say climatic change killed them, some say most of them evolved into birds, "the truth may live in between".
    Heckel's embryological support for evolution (vestigial gill slits in all vertebrate embryos) was disapproved by Karl Ernst von Baer, who showed embryos never pass through the adult stages of other animals.
    Lamarck's use and disuse of organs (the giraffe's neck) — "nobody believes this conjecture any more".
    Saltation — Hugo de Vries held that single-step large mutations cause speciation, against Darwin's gradual, small, directional variations.
    Panspermia — the idea, still a favourite with some astronomers, that life came to earth from outer space.

  3. Exercise 6.3

    Attempt giving a clear definition of the term species.

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    A species is a group of natural populations whose members actually or potentially interbreed freely among themselves and produce fertile offspring, and which is reproductively isolated from all other such groups.
    The members of one species therefore share a common gene pool — the total genes and their alleles in that population — and alleles are exchanged within the group but not outside it.
    In the chapter's own terms, a new species appears when a population's allele frequencies drift so far from the parent population's that the two no longer form one gene pool: "sometimes the change in allele frequency is so different in the new sample of population that they become a different species".
    The same idea in the chapter's bacterial example: a variant that survives the changed medium outgrows the rest and "appears as new species".
    Note the honest difficulty — this definition rests on interbreeding, so it cannot be applied to organisms that reproduce only asexually, nor to fossils, where only structure survives to be compared.

  4. Exercise 6.4

    Try to trace the various components of human evolution (hint: brain size and function, skeletal structure, dietary preference, etc.)

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    Human evolution is traced through four changing components — brain size and function, skeletal structure, dietary preference and culture — over roughly the last $\displaystyle 15$ million years.
    (a) Brain size and function
    Homo habilis, the first human-like being, the hominid — brain capacity $\displaystyle 650$–$\displaystyle 800$ cc.
    Homo erectus, about $\displaystyle 1.5$ mya (fossils from Java, $\displaystyle 1891$) — a large brain of about $\displaystyle 900$ cc.
    Neanderthal man, in the near east and central Asia between $\displaystyle 1,00,000$–$\displaystyle 40,000$ years back — brain size $\displaystyle 1400$ cc.
    Modern Homo sapiens, arising during the ice age $\displaystyle 75,000$–$\displaystyle 10,000$ years ago — the successful story of man with language skills and self-consciousness.
    (b) Skeletal structure
    About $\displaystyle 15$ mya Dryopithecus and Ramapithecus were hairy and walked like gorillas and chimpanzees; Ramapithecus was more man-like, Dryopithecus more ape-like.
    About $\displaystyle 3$–$\displaystyle 4$ mya, man-like primates walked in eastern Africa (bones from Ethiopia and Tanzania). They were probably not taller than $\displaystyle 4$ feet but walked upright — the hominid features begin here.
    Two mya the Australopithecines lived in East African grasslands.
    The skulls compared in the chapter make the point: the skull of a baby chimpanzee is more like the adult human skull than like the adult chimpanzee's skull.
    (c) Dietary preference
    Australopithecines hunted with stone weapons but essentially ate fruit.
    Homo habilis probably did not eat meat.
    Homo erectus probably ate meat.
    Agriculture came about $\displaystyle 10,000$ years back, after which humans grew their own food and settlements started.
    (d) Culture and spread
    Neanderthals used hides to protect their body and buried their dead.
    Homo sapiens arose in Africa, moved across continents and developed into distinct races.
    Pre-historic cave art developed about $\displaystyle 18,000$ years ago — one such set of paintings is at the Bhimbetka rock shelter, Raisen district, Madhya Pradesh.
    After agriculture, the rest is the human history of growth and decline of civilisations.

  5. Exercise 6.5

    Find out through internet and popular science articles whether animals other than man has self-consciousness.

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    Yes — a few animals other than man do show self-consciousness: the great apes, bottlenose dolphins, Asian elephants and some corvids are the accepted cases. The list is short, and each case rests on a specific test rather than on general cleverness.
    From reference sources beyond this chapter (the question sends you to them): the standard test is the mirror self-recognition (MSR) test, devised by Gordon Gallup in 1970. A mark is placed on the animal's body where it can be seen only in a mirror; if the animal touches the mark on its own body instead of reaching at the reflection, it is taken to recognise the image as itself.
    Species reported to pass MSR: chimpanzee, bonobo and orangutan; bottlenose dolphin; Asian elephant; the Eurasian magpie, the first non-mammal to pass; and, in recent and still-debated work, the cleaner wrasse, a small fish.
    Species that consistently fail: monkeys, dogs, cats and most birds — they behave towards the reflection as though it were another individual. Failing is not proof of having no self-awareness: a dog lives largely by smell, so a purely visual test may simply be the wrong test for it, which is the main criticism of MSR.
    Other lines of evidence pointing the same way: scrub jays and magpies re-hide their food when another bird has watched them cache it, and elephants and apes show targeted helping and consolation of distressed companions — behaviours that need an animal to separate itself from others and to model what others know.
    What this chapter itself gives you is one line: the most successful story of evolution is that of man, with language skills and self-consciousness. It names no other self-conscious animal and describes no test for it, so the species above come from outside the book.
    Keep two ideas apart while you read. The chapter's remark that "Mammals were more intelligent in sensing and avoiding danger at least" is about intelligence — solving problems and dodging danger. Recognising oneself as a self is self-consciousness, and it needs evidence of its own.

  6. Exercise 6.6

    List $\displaystyle 10$ modern-day animals and using the internet resources link it to a corresponding ancient fossil. Name both.

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    Ten modern animals with a named fossil relative:
    Horse (Equus) — Eohippus (Hyracotherium), the small four-toed browser of the Eocene.
    Elephant (Elephas, Loxodonta) — Moeritherium, a pig-sized Eocene proboscidean.
    Human (Homo sapiens) — *Homo habilis*, the first of our genus to make stone tools.
    Birds — *Archaeopteryx*, which had feathers and wings but also teeth and a bony tail.
    Coelacanth (Latimeria) — *Macropoma*, a Cretaceous coelacanth of the same order.
    Horseshoe crab (Limulus) — *Mesolimulus*, of the Jurassic Solnhofen limestone.
    Camel (Camelus) — *Protylopus*, a rabbit-sized Eocene camelid of North America.
    Whales (Balaenoptera) — *Ambulocetus*, a four-limbed Eocene whale that could still walk.
    Nautilus (Nautilus pompilius) — *Orthoceras*, a straight-shelled Palaeozoic nautiloid.
    Crocodile (Crocodylus) — *Protosuchus*, a small Triassic crocodylomorph.
    Pair each on the SAME lineage. Latimeria is a coelacanth, so its fossil relative is another coelacanth such as Macropoma — not the rhipidistian lobefins, which sit on the branch leading to the four-limbed land vertebrates instead.

  7. Exercise 6.7

    Practise drawing various animals and plants.

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    This is a drawing practice exercise, not a question with a written answer. Work through the chapter's own figures, drawing each one neatly and labelling it.
    Beaks of Darwin's finches — several small black birds of the same size, differing only in the beak: a heavy, blunt seed-eating beak, a slender pointed insectivorous beak, and a vegetarian form. Label each with its food habit and mark them all as one island's stock.
    Homologous forelimbs of whale, bat, cheetah and human — draw the four limbs side by side, same bone order in each: humerus, then radius and ulna, then carpals, metacarpals and phalanges. Shade the matching bone the same colour across all four so the common plan shows.
    Homology in plants — the **thorn of *Bougainvillea* and the tendril of *Cucurbita***, each drawn in the axil of a leaf, to show two structures in the same position doing different work.
    Moths on a tree trunk — two panels: (a) an unpolluted trunk covered with almost white lichen, the white-winged moth hidden on it and the dark moth conspicuous; (b) a soot-blackened trunk, the dark (melanised) moth hidden and the white moth conspicuous.
    Miller's apparatus — a closed flask of \(\displaystyle CH_4\), \(\displaystyle H_2\), \(\displaystyle NH_3\) and water vapour with two electrodes for the electric discharge, joined to a boiling water flask and a condenser, and a trap at the bottom labelled amino acids.
    Skull comparison — three skulls side by side: adult modern human, baby chimpanzee, adult chimpanzee, labelled to show that the baby chimpanzee's skull is nearer the adult human's than the adult chimpanzee's.
    Australian marsupials — the radiating set (marsupial mouse, mole, anteater, wolf, and so on) drawn around a single ancestral stock at the centre.

  8. Exercise 6.8

    Describe one example of adaptive radiation.

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    Darwin's finches of the Galapagos Islands are the classic example of adaptive radiation.
    Adaptive radiation is the process of evolution of different species in a given geographical area, starting from a point and literally radiating out to other areas of geography, that is, to other habitats.
    On his voyage Darwin reached the Galapagos Islands and found an amazing diversity of small black birds — many varieties of finch living on the same island.
    He concluded that all these varieties had evolved on the island itself, from one original stock.
    The ancestral stock was seed-eating. From it many forms with altered beaks arose, which allowed them to become insectivorous and vegetarian finches — the beak fitting each variety to a different food and habitat.
    A second example is the Australian marsupials: a number of marsupials, each different from the other, evolved from a single ancestral stock, all within the Australian island continent.
    Australia also carries a second radiation of placental mammals, each of which looks similar to a corresponding marsupial (placental wolf and the Tasmanian wolf-marsupial). When more than one adaptive radiation occurs in the same isolated geographical area like this, it is called convergent evolution.

  9. Exercise 6.9

    Can we call human evolution as adaptive radiation?

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    No — human evolution cannot be called adaptive radiation.
    Adaptive radiation means many different species evolving from one ancestral stock in a given geographical area and radiating out into different habitats — as the finches did on the Galapagos and the marsupials did in Australia.
    Human evolution is not a fan of species but a single line of descent: Dryopithecus and Ramapithecus → Australopithecines → Homo habilis → Homo erectus → Neanderthal man → Homo sapiens, one stage replacing the one before.
    Only one species has survived out of it. Homo sapiens arose in Africa and moved across continents, but developed only into distinct races — and races are not different species; all human beings still share one gene pool.
    The changes traced are in brain size, upright posture, diet and culture, not a splitting into many forms each specialised for a different habitat.
    Man does occupy almost every habitat on earth, but he reached them by moving and by culture — language, tools, hides, agriculture — not by evolving into a separate species in each one. Radiation of habitat without radiation of species is not adaptive radiation.

  10. Exercise 6.10

    Using various resources such as your school Library or the internet and discussions with your teacher, trace the evolutionary stages of any one animal, say horse.

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    The chapter does not trace this — it names horse evolution as one of the "special stories of evolution" and says you will learn about it in higher classes. The exercise itself sends you to the library, the internet and your teacher.
    What the chapter does give you: there were in South America mammals resembling horse, hippopotamus, bear and rabbit; when continental drift joined South America to North America, these animals were overridden by North American fauna. This is the only remark in the chapter that touches the horse.
    What to trace, from reference sources beyond this chapter — the usual sequence is Eohippus (Hyracotherium) → Mesohippus → Merychippus → Pliohippus → *Equus*, the modern horse.
    Follow four trends along that line rather than memorising names: steady increase in body size; lengthening of the limbs and of the neck; reduction in the number of functional toes, from four on the forefoot down to a single hoofed toe; and molars changing from low-crowned browsing teeth to high-crowned grinding teeth as the diet shifted from soft forest leaves to hard grassland grass.
    Read those trends the way the chapter reads all evolution — as natural selection on inherited variation as forests gave way to open grassland, not as the animal striving to change.