NCERT Solutions: Evolution

Q1: Explain antibiotic resistance observed in bacteria in light of Darwinian selection theory.
Ans:  

• Darwinian selection theory states that individuals with favourable variations are better adapted than individuals with less favourable variation.
• It means that the environment selects those individuals with useful variation because they are better suited to survive and reproduce under the prevailing conditions.
• Antibiotic resistance in bacteria is a clear example of this process.
• In a population of bacteria, random mutations sometimes produce variants that can withstand an antibiotic such as penicillin.
• When the population is exposed to the drug (for example, when grown on an agar plate containing penicillin), bacteria that are sensitive to the antibiotic are killed, while the resistant variants survive.
• These resistant bacteria then reproduce and increase in frequency in the population.
• Over successive generations the population becomes dominated by resistant bacteria.
• In short, (i) variation arises by chance, (ii) the antibiotic acts as a selective pressure, and (iii) individuals with the favourable variation survive and pass it on, which is exactly what natural selection predicts.
• This process can be accelerated because genes for resistance may move between bacteria (for example by plasmids), but the fundamental change in population composition is driven by differential survival and reproduction.

Fig: Darwinian selection theory

Antibiotic treatment thus does not 'cause' resistance by directed change; rather it removes susceptible individuals and leaves resistant ones to multiply. This explanation fits the three key elements of Darwinian selection: (a) heritable variation exists, (b) there is differential survival and reproduction under a selective pressure (the antibiotic), and (c) the favourable variants become more common in subsequent generations.

Q2: Find out from newspapers and popular science articles any new fossil discoveries or controversies about evolution. 
Ans:  

• Dinosaurs provide evidence for evolution within reptiles and links to birds, not origin of reptiles.
• As a result of these discoveries, links to the evolution of other groups such as birds and mammals have also been made.
• In recent years, unusual fossils unearthed in China have renewed debate about the origin and early evolution of birds.
• One example is Confuciusornis, a genus of primitive, crow-sized birds from the Cretaceous period of China.
• Confuciusornis and similar fossils have features that are intermediate between non-avian dinosaurs and modern birds, and they have helped scientists refine ideas about when and how flight-related characters (such as beaks and feathers) evolved.
• Such discoveries stimulate discussion because each new specimen can change interpretations of relationships and timing in the evolutionary tree.
• Recent fossil finds often provide morphological details (for example, feathers, wing structure, or changes in skull shape) that allow palaeontologists to test competing hypotheses about evolutionary relationships.
• At the same time, controversies arise because single new specimens can be interpreted in different ways: whether they represent a new species, a juvenile form, or a regional variant.
• Reliable coverage in newspapers and popular science articles usually cites museum reports or peer-reviewed studies; students should consult those primary sources when possible.

Q3: Attempt giving a clear definition of the term species 
Ans: Species can be defined as a group of organisms that can interbreed with one another under natural conditions to produce fertile offspring. This definition emphasises reproductive unity and a common gene pool. In practice, reproductive isolation (barriers to interbreeding) helps to keep one species distinct from another.
Note: This is the biological species concept commonly used for sexually reproducing organisms. It has limitations for asexual organisms, for fossil forms, and where hybridisation occurs; in such cases other criteria (morphological similarity, ecological niche, or genetic distinctness) are also used to recognise species.

Species

Q4: Try to trace the various components of human evolution (hint: brain size and function, skeletal structure, dietary preference, etc.) 
Ans: The various components of human evolution include the following.

• Size of the brain - There has been a general increase in brain size and complexity, associated with improved cognitive abilities, problem solving, tool manufacture and more complex social behaviour.
• Body posture - A shift from a crouched, quadrupedal posture to an upright, bipedal stance occurred; changes in pelvic shape, spinal curvature and limb proportions support efficient walking and running on two legs.
• Food habits/dietary preferences - Diets shifted from mainly foraged plant foods to include more meat and, later, cooked foods; changes in teeth, jaws and digestive anatomy reflect these dietary transitions.
• Characteristics/features - Modifications in the skull (smaller jaw, flatter face), hands (improved precision grip), and sensory organs accompanied advances in tool use, communication and social organisation.

These components did not evolve in isolation. For example, changes in diet influenced tooth and jaw structure and may have supported brain enlargement by providing more energy-dense foods. Similarly, bipedalism freed the hands for carrying and tool use, which in turn influenced manual dexterity and cultural evolution.

The following table depicts the same:

Q5: Find out through internet and popular science articles whether animals other than man have self-consciousness.

Ans: Yes. Several animals show signs of self-consciousness or self-awareness. Evidence comes from behavioural studies such as the mirror self-recognition test, where an individual recognises its own reflection. Species that have shown such abilities include dolphins, great apes (for example chimpanzees, gorillas, orangutans), elephants and some corvids (for example magpies and crows). These animals display complex social behaviour, individual recognition and problem-solving skills that indicate a level of self-awareness beyond simple instinctive responses.
It is important to note that success or failure in one test does not fully define self-consciousness. Different species use different senses and may not respond to a mirror in the same way humans do. Therefore scientists use a suite of behavioural tests and ecological observations to infer self-awareness, rather than relying on a single experiment.

Q6: List 10 modern-day animals and using the internet resources link it to a corresponding ancient fossil. Name both. 
Ans: The modern day animals and their ancient fossils are listed in the following table:

These pairings illustrate how modern groups can be traced to fossil relatives that show transitional anatomies. When researching, prefer primary sources (museum pages, peer-reviewed articles) for accurate fossil names and dates.

Q7: Practice drawing various animals and plants. 
Ans: Ask your teachers and parents to suggest the names of plants and animals and practise drawing them. You can use photographs from books, museum guides or reliable websites as references. Start with basic outlines, then add distinguishing features (leaves, beaks, limbs) and finally shade or label parts to make the drawings accurate and informative.
Suggested approach:
- Begin with simple shapes to block out body proportions.
- Add defining details (leaf venation, beak shape, limb joints).
- Label important parts and, where relevant, note adaptations (for example, wing shape or tooth type).
- Practise repeatedly and compare your drawings with references to improve accuracy.

Q8: Describe one example of adaptive radiation.
Ans: Adaptive radiation is an evolutionary process in which a single ancestral species rapidly diversifies into many new species, each adapted to a different ecological niche.

Adaptive Radation

This occurs through natural selection acting on variation as populations adapt to different environments. A classic example is the Darwin's finches of the Galápagos Islands. They are believed to have descended from a single ancestral finch that arrived on the islands. Over time, the descendants adapted to different food sources and habitats on different islands. As a result they evolved a variety of beak shapes and sizes suited to seed-eating, insect-eating or other diets. This diversification into several specialised forms from one ancestor illustrates adaptive radiation. Key points are rapid speciation, ecological opportunity (empty or underused niches) and divergent natural selection acting on traits such as beak form.

Q9: Can we call human evolution as adaptive radiation? 
Ans:   No. Human evolution cannot be called adaptive radiation.

Adaptive radiation involves a single ancestral lineage rapidly splitting into many species that occupy a wide variety of distinct ecological niches - classic examples being Darwin's finches and cichlid fishes. Human evolution does not fit this pattern for the following reasons:

While the hominin fossil record does show considerable branching - multiple species of Australopithecus, Paranthropus, and Homo coexisted at various points in time - this branching does not match the scale or ecological diversity that characterises true adaptive radiation. The various hominin species remained broadly similar in ecological role, differing mainly in features like brain size, dentition, and locomotion, rather than diversifying into widely different ecological niches the way adaptive radiation demands.

Furthermore, the overall trajectory of hominin evolution shows a dominant trend of successive anatomical and cognitive modifications - increasing brain size, bipedalism, tool use - leading progressively toward modern Homo sapiens, rather than a rapid explosion of ecologically specialised lineages.

Therefore, while human evolution was not strictly linear and did involve branching events, it lacks the two key hallmarks of adaptive radiation:

1. Rapidity of diversification

2. Ecological divergence into many distinct niches

Hence, human evolution is better described as a complex, branching but ecologically constrained progression within the hominin lineage, and does not qualify as adaptive radiation in the classical sense.

Q10: 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. 
Ans: The evolution of the horse began with Eohippus during the Eocene period and progressed through several stages marked by changes in size, limb structure and teeth adapted for grazing. Important trends include:

• Gradual increase in body size
• Elongation of head and neck
• Lengthening of limbs and feet
• Reduction of lateral digits
• Enlargement of the third functional toe
• Strengthening of the back
• Development of brain and sensory organs
• Increase in tooth complexity for grazing on grasses

The evolution of the horse is represented as

Evolution of horse

(i) Eohippus: It had a short head and neck and was relatively small. Each forelimb had four functional toes and each hind limb had three functional toes, with small splints of other toes.The molars were low-crowned and suited to a browsing plant diet. It lived during the Eocene.

(ii) Mesohippus: It was slightly taller than Eohippus and had three toes on each foot. Limbs and teeth show adaptations for more efficient running and a changing diet; it lived mainly in the Oligocene.

(iii) Merychippus: About 1 metre tall (approximately 100 cm), it still had three toes but ran largely on the central toe; the side toes were reduced and did not usually contact the ground. Its molars became higher-crowned and better adapted for grazing on grasses. It lived in the Miocene.

(iv) Pliohippus: It resembled modern horses more closely and had a single functional toe (the third) with small splints of the 2nd and 4th toes. Limb proportions and teeth were adapted for running and grazing. It lived in the Pliocene.

(v) Equus: Modern horses belong to Equus. They have a single hoofed toe on each foot, incisors for cutting grass and well-developed molars for grinding. This genus appears in the fossil record from the later Pliocene into the Pleistocene and continues to the present day.
Overall, the horse lineage shows clear adaptation from a small forest browser with many toes to a large, fast-running grazer with a single hoof adapted for open grassland habitats.