NCERT Class 12 Biology Ch 6 Evolution Solutions PDF

Concept used. Darwinian natural selection is the mechanism Charles Darwin proposed in 1859 (On the Origin of Species) to explain how populations change over time. It rests on four observable conditions:

• Within any population, individuals show heritable variation.
• More offspring are produced than the environment can support (overproduction).
• The resulting struggle for existence means not every individual survives or breeds.
• Variants whose traits raise their chance of survival and reproduction in that environment leave more offspring (differential survival of the fittest). Over generations these traits become more common in the population.

1. Pre-existing variation in the bacterial population.
   A bacterial colony is not genetically uniform. Random mutations during DNA replication produce variants – some carry an altered enzyme, an efflux pump, or a modified cell wall that happens to weaken the binding of an antibiotic. These mutants exist before the antibiotic is ever applied, at frequencies of roughly 10^-9 to 10^-6 per cell per generation.
2. Application of antibiotic creates a strong selective pressure.
   When penicillin (or any antibiotic) is introduced, the environment changes drastically. Sensitive bacteria are killed; the rare resistant mutants survive. This is exactly Darwin's struggle for existence: the resource (antibiotic-free niche) is limited, and only a few variants can occupy it.
3. Differential reproduction of the resistant variant.
   Resistant bacteria reproduce rapidly (E. coli divides every ∼ 20 min). Within hours the resistant genotype dominates the colony – a clear example of differential survival and reproduction. After a few generations the population is essentially all resistant.
4. Inheritance of the resistance trait.
   Resistance is encoded in DNA (often on plasmids), so it is passed to daughter cells. Plasmids can also move horizontally between species by conjugation, accelerating the spread – but the Darwinian core (variation + selection + inheritance) is intact.
5. Outcome: rapid evolution of a resistant population.
   Bacteria are a textbook demonstration of Darwinian selection because their short generation time makes the entire sequence visible within a single human lifetime. Hospitals now report MRSA (methicillin-resistant Staphylococcus aureus), VRE and XDR-TB – all evolved by the same logic.

Why this fits Darwin's theory perfectly. All four preconditions of natural selection are satisfied: heritable variation, overproduction (a single E. coli cell can yield 10⁹ descendants overnight), differential survival under antibiotic pressure, and inheritance of the favourable trait. The antibiotic does not create resistance; it merely selects for pre-existing resistant variants. This is what Darwin meant by ``nature selecting''.

Antibiotic resistance arises because rare pre-existing mutants in a bacterial population happen to be insensitive to the drug. When the antibiotic is applied, sensitive cells die while resistant cells survive and reproduce. Within a few generations the entire colony becomes resistant – a real-time demonstration of Darwinian natural selection (variation, struggle, survival of the fittest, inheritance).

Concept used. A fossil is any preserved remains, impression or trace of an organism that lived in the past, embedded in sedimentary rock. Fossils are dated by (i) stratigraphy (younger layers sit above older ones) and (ii) radiometric dating ( ¹⁴C, ⁴⁰K/⁴⁰Ar etc.). Fossils give direct evidence of evolution: they show transitional forms, extinct groups and the geological order in which life diversified.

Notable recent fossil discoveries (with sources students can look up).

1. Tiktaalik roseae (2006, Ellesmere Island, Canada). A 375-million-year-old lobe-finned fish with a flat skull, neck, ribs and wrist-like fin bones – a clear transitional fossil between fish and tetrapods, bridging the water-to-land transition. Reported in Nature 440, 757 (Daeschler, Shubin & Jenkins, 2006) and covered widely in popular science press.
2. Homo naledi (2013–15, Rising Star cave, South Africa). Over 1 500 hominin bones discovered by Lee Berger's team – a small-brained, ∼ 1.5 m-tall hominin from ∼ 300 000 years ago, that may have deliberately deposited its dead. Controversy: Did a small-brained species practise mortuary behaviour, traditionally thought unique to H. sapiens?
3. Australopithecus sediba (2008, Malapa, South Africa). ∼ 2 million-year-old hominin with a mix of Australopithecus and Homo features. Reignited the debate about which australopithecine actually gave rise to Homo.
4. Feathered dinosaurs from Liaoning, China (1990s onward). Sinosauropteryx, Microraptor, Anchiornis: theropod dinosaurs preserved with filamentous feathers in Yixian Formation lakes. They settled the question of bird origins in favour of a dinosaurian ancestry (Archaeopteryx was no longer an oddity).
5. Denisovans (2010, Denisova cave, Siberia). An entire new hominin lineage identified from a finger bone, purely by ancient-DNA sequencing. Sparked a still-running controversy about how many hominin species coexisted in Asia and how much interbreeding occurred with H. sapiens.
6. Dickinsonia as the earliest animal (∼ 558 Ma). 2018 cholesterol-biomarker work (Science 361, 1246) showed Ediacaran Dickinsonia was an animal, not a fungus or lichen – pushing back the age of confirmed Metazoa.

Live evolution controversies in the press.

• Where did Homo sapiens originate? Single-origin (Africa, ∼ 200 kya) vs. multi-regional models, complicated by new African finds at Jebel Irhoud (∼ 315 kya).
• Did Neanderthals and modern humans interbreed? Confirmed by ancient-DNA work (Svante P"a"abo, Nobel Prize 2022); ∼ 1--4% Neanderthal DNA persists in non-African humans.
• ``Hobbit'' Homo floresiensis (2003, Flores Island): a tiny hominin alive until ∼ 50 kya – species or pathological H. sapiens? Now widely accepted as a distinct species.

Major recent fossil discoveries that reinforce evolutionary theory include Tiktaalik (fish → tetrapod transition, 2006), Homo naledi (small-brained hominin with possible mortuary behaviour, 2013), A. sediba (2008), feathered dinosaurs of Liaoning, the Denisovans (2010), and biomarker evidence that Ediacaran Dickinsonia was an animal (2018). Ongoing controversies concern human origins, Neanderthal introgression, and Homo floresiensis.

Concept used. A species is the basic unit of biological classification. Several non-equivalent definitions are in current use because no single criterion fits all life. The most widely taught is Ernst Mayr's biological species concept, but students should know the alternatives because they apply to fossils, asexual organisms and bacteria.

1. Biological species concept (Ernst Mayr, 1942). A species is a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups, and produce fertile offspring. Key requirements:
   • Interbreeding – members can mate and produce viable offspring.
   • Fertile offspring – those offspring can themselves reproduce (this excludes mule, the infertile horse×donkey hybrid).
   • Reproductive isolation from other species (pre-zygotic: behaviour, timing, habitat; post-zygotic: hybrid inviability or sterility).
2. Morphological species concept. A species is a group of organisms sharing characteristic anatomical features distinct from other groups. Used for taxonomy when breeding cannot be observed (e.g. herbarium specimens, museum skeletons).
3. Phylogenetic species concept. A species is the smallest monophyletic group descended from a common ancestor – diagnosable by at least one unique derived character. Useful for molecular-tree-based classification.
4. Ecological species concept. A species is a lineage of populations that occupies a unique adaptive zone or niche. Useful when reproductive isolation is incomplete but ecological roles diverge.
5. Where Mayr's definition struggles. It fails for (i) asexual organisms (bacteria, parthenogenetic lizards) where ``interbreeding'' is undefined, (ii) fossil species where mating cannot be tested, (iii) ring species (Ensatina salamanders) where adjacent populations interbreed but the end-of-ring populations do not.

In NCERT terms: A species is a group of similar organisms that can interbreed in nature and produce fertile offspring, and that is reproductively isolated from all other such groups (biological species concept, after Ernst Mayr, 1942). Alternative definitions (morphological, phylogenetic, ecological) apply where reproductive isolation cannot be tested.

Concept used. Human evolution is the gradual change, over ∼ 7 million years, from a chimpanzee-like ancestor to modern Homo sapiens. It is documented by fossils, by comparative anatomy and by molecular data. The trends usually examined are: cranial capacity (brain size), skeletal structure (bipedalism, posture, hand), dentition and diet, and cultural artefacts (tools, fire, language).

1. Brain size (cranial capacity). Hominid brains enlarged steadily:
   • Australopithecus (∼ 4 Ma) – ∼ 400 cc (chimp-like).
   • Homo habilis (∼ 2 Ma) – ∼ 650--800 cc; first stone tools.
   • Homo erectus (∼ 1.5 Ma) – ∼ 900--1100 cc; controlled fire.
   • Neanderthal man (∼ 100 kya) – ∼ 1400 cc; cared for sick, buried dead.
   • Homo sapiens (modern) – ∼ 1350 cc on average, but with cerebral re-organisation (large frontal lobes for planning and language).
2. Skeletal structure – emergence of bipedalism. Walking on two legs was an early adaptation:
   • Foramen magnum (the hole through which the spinal cord exits) shifted from the back of the skull (in apes) to directly below (in humans), consistent with an upright head balanced on the spine.
   • S-shaped vertebral column, broad bowl-shaped pelvis, valgus (knock-knee) angle of the femur, longitudinal foot arch and a non-opposable big toe – all aligned for striding bipedal walk.
   • Free hand from bipedalism enabled tool-making; opposable thumb refined for precision grip in Homo.
   • Body size grew: Australopithecus ∼ 1.0 m, H. erectus ∼ 1.6 m, H. sapiens ∼ 1.7 m.
3. Dental and dietary changes.
   • Earliest hominids (Ardipithecus, Australopithecus): mainly fruits and tubers, large molars, thick enamel.
   • Homo habilis: omnivorous – scavenged meat, processed plant material; jaw shortens.
   • Homo erectus: ate cooked meat, used fire (∼ 0.5–1.0 Ma).
   • Modern man: full omnivore; smaller jaw and teeth (cooking softened food, reducing the need for heavy chewing).
4. Cultural and behavioural milestones. Tool-making (Oldowan flakes by H. habilis, ∼ 2.5 Ma), control of fire (∼ 1.5 Ma by H. erectus), burial of the dead (Neanderthals, ∼ 0.1 Ma), cave painting (Bhimbetka, Chauvet, ∼ 30 kya), agriculture (∼ 10 kya), language (anatomically supported by the descended larynx and a modern hyoid, both present by ∼ 0.1 Ma).
5. Migration out of Africa. Homo erectus left Africa ∼ 2 Ma (Java, Peking). Homo sapiens originated in Africa ∼ 200 kya and migrated outward by ∼ 60--75 kya, replacing or interbreeding with archaic populations such as Neanderthals and Denisovans.

Human evolution can be traced along five interconnected components: (1) steady increase in cranial capacity (400 → 1400 cc); (2) skeletal adaptations for bipedalism (repositioned foramen magnum, S-shaped spine, bowl-shaped pelvis, arched foot); (3) dietary shift from herbivory to omnivory with smaller teeth; (4) cultural advances (tools, fire, burial, art, language); and (5) migration out of Africa ∼ 2 Ma and again ∼ 60–75 kya.

Concept used. Self-consciousness (or self-awareness) is the ability of an organism to recognise itself as an individual separate from its environment and other individuals. The standard operational test in comparative psychology is the Mirror Self-Recognition (MSR) or rouge test, devised by Gordon Gallup in 1970: a coloured mark is placed on the animal's face where it can be seen only in a mirror, and the animal is observed for self-directed mark-investigating behaviour.

1. Animals that have passed the mirror test.
   • Great apes – chimpanzees (Gallup 1970), bonobos, orangutans, and gorillas. Among hominids only humans pass it consistently from ∼ 18–24 months of age.
   • Cetaceans – bottlenose dolphins (Reiss & Marino, 2001), Asian elephants (Plotnik, de Waal & Reiss, 2006), and orcas have shown mark-directed behaviour.
   • Birds – Eurasian magpies (Prior, Schwarz & G"untürkün, 2008) pass the test, suggesting self-recognition evolved independently in birds (without a mammalian neocortex).
   • Fish – the cleaner wrasse (Labroides dimidiatus) appears to pass the mark test (Kohda et al., 2019), though this remains controversial.
2. Other behavioural indicators. Self-awareness is a spectrum, and the mirror test captures only one facet. Other indicators include:
   • Tool use with planning (crows fashion hooks from twigs; chimps strip leaves; elephants throw debris with intent).
   • Theory of mind – knowing that another agent has its own beliefs (chimps, scrub jays cache food differently if watched).
   • Metacognition – ``knowing what you know.'' Macaques and dolphins decline trials they judge hard, suggesting they recognise their own uncertainty.
   • Episodic-like memory (western scrub jays remember what, where, when of cached food).
   • Cooperative problem-solving and altruism in elephants and dolphins.
3. Brain correlates. Most MSR-passing species have a high encephalisation quotient (brain mass relative to body mass), and many have von Economo (spindle) neurons in the anterior cingulate cortex – neurons earlier thought to be unique to humans but now described in great apes, cetaceans and elephants.
4. What this implies. Self-consciousness is not uniquely human; it has evolved independently (convergent evolution) in several lineages whenever complex social or ecological cognition pays off. It exists on a continuum from minimal self-awareness (knowing one's body in space) to full reflective self-consciousness (humans).

Yes – several non-human species pass the mirror self-recognition test and show other markers of self-awareness: chimpanzees, bonobos, orangutans, gorillas, dolphins, orcas, Asian elephants, Eurasian magpies, and (more controversially) cleaner wrasses. Self-consciousness is not unique to humans; it has evolved several times wherever complex social cognition is adaptive.

Concept used. A living fossil is a modern species that closely resembles its fossil ancestors and has changed little over geological time. Even for animals that are not living fossils, palaeontology has often identified an extinct or near- ancestral form. The fossil ancestor proves continuity of lineage and lets us date when the group originated.

The table below lists ten modern animals paired with a named fossil/ancestor (period and approximate age in millions of years ago, Ma).

1. Five well-documented horse ancestors. The horse line is one of the best-resolved fossil sequences in evolution: Eohippus (55 Ma, four-toed, dog-sized) → Mesohippus (38 Ma, three-toed) → Merychippus (25 Ma, grazing teeth) → Pliohippus (10 Ma, single-toed) → Equus (2 Ma, modern). Body size grew, the side toes vanished, and teeth became high-crowned for grass grazing.
2. Why Archaeopteryx is special. Discovered in 1861 (Solnhofen limestone, Germany), it has feathered wings (bird-like) but teeth and a long bony tail (reptile-like). It bridges Reptilia and Aves and is usually the textbook example of a transitional fossil.
3. Living fossils. Coelacanth (rediscovered alive in 1938 off South Africa; thought extinct for 65 Ma), horseshoe crab (almost unchanged for 200 Ma), lungfish and tuatara are textbook living fossils.

Ten modern animal–ancient fossil pairings: Equus–Eohippus; Elephas–Moeritherium; Balaena–Pakicetus; Camelus–Protylopus; modern bird–Archaeopteryx; Crocodylus–Protosuchus; Latimeria–Macropoma; Limulus–Mesolimulus; Neoceratodus–Dipnorhynchus; Rana–Ichthyostega. Each pair documents continuity across 40–400 million years.

Concept used. This question is a skills exercise, not a knowledge question. Comparative drawing of animals and plants trains the eye to spot homologous and analogous structures – a core method of evolutionary biology. Two structures are homologous if they share a common ancestor (same origin, possibly different functions); they are analogous if they perform the same function but evolved independently (different origins, similar form).

1. What to draw and what to label. Practical suggestions for the student's notebook:
   • Forelimbs of frog, lizard, bird, bat, whale and human. Label humerus, radius, ulna, carpals, metacarpals, phalanges in each. The same five bones occur in every case – proof of homology.
   • Wings of insect, bird and bat. Insect wing is a chitinous membrane; bird wing has feathers on a modified forelimb; bat wing is a skin membrane stretched between elongated finger bones. Same function (flight), different origins – analogy.
   • Modified plant stems. Bougainvillea thorn (axillary), Cucurbita tendril (axillary), Opuntia phylloclade (green, fleshy). Same axillary origin, different functions – homology in plants.
   • Modified plant leaves. Tendril of pea, spine of cactus, scale of onion, insect-catching trap of pitcher plant. All are leaves – homologous; the spine of cactus and thorn of Bougainvillea are analogous (look alike, different origins).
   • Skulls of human, baby chimp and adult chimp. Label the cranial vault, brow ridge, foramen magnum, jaw. The baby chimp skull resembles the human skull (Bolk's neoteny hypothesis).
   • Darwin's finch beaks. Four beak shapes (seed-cracker, insectivore, woodpecker-like, cactus-eater) from the same ancestor – adaptive radiation.
2. How to label scientifically. Use a sharp pencil, draw arrow leaders that do not cross, use italic for Latin binomials, and place a scale bar where applicable.
3. Why drawing helps. Forcing your hand to copy a diagram forces your eye to register details a casual reader misses – the wrist bones of a bat wing, the leaf-origin of a cactus spine, the curvature of Pakicetus's skull. Drawing is the cheapest tool in evolutionary biology.

Practising drawings of homologous structures (forelimbs, plant leaves, hominid skulls, finch beaks) and analogous structures (insect/bird/bat wings, dolphin/penguin flippers) trains your eye to interpret evolutionary evidence in anatomy. Always label the constituent parts in italics and indicate whether the similarity is by descent (homologous) or by independent evolution (analogous).

Concept used. Adaptive radiation is the evolutionary process in which an ancestral species, on entering a new geographical area or unoccupied set of niches, gives rise to several descendant species, each adapted to a different ecological role. The textbook examples are Darwin's finches of the Gal'apagos Islands and the marsupials of Australia. When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), the phenomenon is called convergent evolution – for example placental mammals in Australia exhibit adaptive radiation into forms remarkably similar to the corresponding marsupials (placental wolf vs. Tasmanian wolf-marsupial).

1. Geography. The Gal'apagos Islands lie about 1000 km west of Ecuador in the Pacific Ocean. They are of volcanic origin (no land connection to South America) and harbour many endemic species.
2. Founding event. A few seed-eating finches from South America were carried to the islands by storms, perhaps ∼ 2–3 million years ago. They found uncolonised niches – insects, seeds, fruit, cactus pulp, bark insects – and no avian competitors.
3. Diversification. Over ∼ 2 Ma the founding population diversified into 13 (some say 15) species of finches, now collectively called Darwin's finches. Each species shows a beak modified for its diet:
   • Heavy crushing beak for hard seeds (Geospiza magnirostris).
   • Slender pointed beak for insects (Certhidea olivacea, the warbler-finch).
   • Probing beak with a cactus-spine tool (Camarhynchus pallidus, the woodpecker-finch).
   • Curved beak for cactus flowers (Geospiza scandens).
4. The Darwinian mechanism. Random heritable variation in beak shape, combined with selection for whichever variant best exploited a particular food source on a particular island, drove the divergence. Peter and Rosemary Grant's 40-year fieldwork on Daphne Major showed that beak depth in Geospiza fortis measurably shifts in a single drought generation – adaptive radiation in real time.
5. Why finches are a textbook example. They satisfy every requirement: (i) a single founding ancestor; (ii) unoccupied niches; (iii) heritable variation in beak shape; (iv) selection by diet; (v) reproductive isolation on different islands; (vi) documented divergence into many species.

A second adaptive radiation – Australian marsupials.

When Australia drifted away from Gondwana (∼ 50 Ma) it carried marsupials but very few placental mammals. The marsupial line then radiated into every niche the placentals fill elsewhere: kangaroos (grazers), Tasmanian wolves (carnivores), koalas (leaf eaters), marsupial moles (burrowers), wombats (rodent-like), gliders (gliding mammals). Many of these resemble unrelated placental mammals in other continents – a textbook case of both adaptive radiation (on Australia) and convergent evolution (with placentals).

The Gal'apagos finches are the cleanest example of adaptive radiation: a single seed-eating ancestor diversified into ∼ 13 species in ∼ 2 million years, each beak shape matched to its food source (seeds, insects, cactus pulp, woodpecker-like probing). Australian marsupials are a parallel continental-scale example.

Concept used. Adaptive radiation requires many descendant species, each occupying a different ecological niche, arising from a single ancestor within a short geological time. The signature is a branching pattern of multiple coexisting forms (e.g. Darwin's finches). Human evolution must be tested against these criteria.

1. Did one ancestor give rise to many hominid species? Yes – the fossil record shows several genera and species: Sahelanthropus, Orrorin, Ardipithecus, several Australopithecus (A. afarensis, A. africanus, A. sediba, A. garhi), Paranthropus boisei, and the Homo clade (H. habilis, H. erectus, H. heidelbergensis, H. neanderthalensis, H. floresiensis, H. naledi, H. sapiens). At several time points (e.g. ∼ 2 Ma) three or four hominid species co-existed in Africa.
2. Did they occupy different niches? Only weakly. Paranthropus boisei (the ``robust'' australopithecine) had massive jaws and ate hard plant food – clearly a different dietary niche. But most other hominids (gracile australopithecines, early Homo) overlapped substantially in diet and habitat. There is no ``insectivore hominid'' or ``aquatic hominid'' analogous to a woodpecker-finch vs. a vampire-finch.
3. Did they survive into the present? No – and this is the decisive point. Of the dozen-plus hominid species, only one is alive today: Homo sapiens. Adaptive radiation, by contrast, produces a persisting bush of multiple coexisting forms (the 13 finches still all exist).
4. Verdict. Human evolution shows the branching stage of an adaptive radiation but not the persistence stage. Most lineages went extinct; only one ``radiated'' lineage is left. Some authors therefore call it a failed or partial radiation. Strictly, by the conventional definition (multiple coexisting forms in different niches), human evolution is not a typical adaptive radiation.

Comparison with classical examples.

• Darwin's finches. 13 coexisting species, distinct beak-driven niches, descended from one ancestor. Persistent.
• Australian marsupials. 200+ coexisting species spanning every niche from herbivore to carnivore to burrower. Persistent.
• Human evolution. A dozen species over 7 Ma, but only H. sapiens remains. Not persistent.

Strictly speaking, no – human evolution is not a classical adaptive radiation. Although several hominid species branched from a common ancestor, they did not occupy distinctly different ecological niches, and all but Homo sapiens are extinct. A true adaptive radiation (e.g. Darwin's finches) produces a persisting bush of differentiated coexisting species. Some authors describe human evolution as a branching or partial radiation that was pruned by competition and climate.

Concept used. The horse lineage (Family Equidae) is the most complete fossil sequence in vertebrate palaeontology, spanning ∼ 55 million years from the Eocene to the present. The trends across this sequence are: (i) increase in body size, (ii) progressive loss of side toes (from four to one), (iii) elongation of limbs, (iv) increase in tooth crown height (hypsodonty) for grass grazing, and (v) straightening of the back. These changes track the planet's shift from forested to grassland environments during the Cenozoic.

1. Eohippus / Hyracotherium (Eocene, ∼ 55 Ma). The ``dawn horse''. Size of a fox (∼ 0.4 m at the shoulder); four toes on the forefoot, three on the hindfoot; low-crowned teeth suited to soft browse; arched back. Forested habitat.
2. Mesohippus (Oligocene, ∼ 38 Ma). Sheep-sized (∼ 0.6 m); three toes on each foot, the middle toe enlarged and weight-bearing; still a browser. Found in North America.
3. Merychippus (Miocene, ∼ 25 Ma). Pony-sized (∼ 1.0 m); three toes, but only the middle toe touches the ground; high-crowned (hypsodont) teeth – the first grazer on the line, exploiting the Miocene grasslands. Long legs for fast running.
4. Pliohippus (Pliocene, ∼ 10 Ma). Horse-sized (∼ 1.3 m); single weight-bearing toe on each foot, with the side toes reduced to splint bones; deep-rooted hypsodont teeth; long, slender legs for sustained galloping; open-plains grazer.
5. Equus (Pleistocene to Recent, ∼ 2 Ma – 0). The modern horse and zebra. ∼ 1.6 m at the shoulder, one toe (hoof), with vestigial splint bones; permanently hypsodont molars; long mane; built for endurance running on open grassland. Domesticated by humans ∼ 5500 years ago in the Eurasian steppe.

Underlying drivers. The horse line tracks the global climate shift from warm wet Eocene forests to dry Miocene grasslands. As grasses spread and forests retreated, selection favoured grazers with tougher teeth and faster gaits. Horses evolved in North America, dispersed to Eurasia via the Bering land bridge, went extinct in North America ∼ 10 kya, and were re-introduced by Spanish settlers in the 1500s.

The horse evolved across five recognised stages from the forest-dwelling four-toed Eohippus (∼ 55 Ma) through Mesohippus, Merychippus, Pliohippus to the modern single- toed Equus. Body size grew (from ∼ 0.4 to ∼ 1.6 m), the side toes were lost, the legs lengthened, and the teeth became high-crowned to handle abrasive grass – all in step with the spread of Miocene grasslands.