Evolution Class 12 Biology NCERT Solutions

Q1EXERCISES

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

Solution

Antibiotic resistance in bacteria is a clear example of evolution by natural selection, as explained by Darwin's theory.

Variation: Within any large population of bacteria, there exists natural genetic variation. Some of this variation is due to random mutations. A few bacteria might, by chance, possess a mutation that makes them resistant to a particular antibiotic.
Selective Pressure: When an antibiotic is introduced into the environment (e.g., when a person takes an antibiotic), it acts as a strong selective pressure. The antibiotic kills the non-resistant (susceptible) bacteria.
Survival and Reproduction (Fitness): The bacteria that are resistant to the antibiotic survive the onslaught. These survivors are 'fitter' in this new environment. They continue to multiply and pass on the resistance gene to their offspring.
Evolution of Population: Over a few generations, the bacterial population changes. The proportion of resistant bacteria increases dramatically, and the population as a whole becomes resistant to the antibiotic. This is not because the bacteria 'decided' to become resistant, but because the pre-existing resistant variants were selected for by the environment.

This process, driven by human action (anthropogenic action), demonstrates that evolution is a stochastic process based on chance variations and natural selection, leading to better-adapted populations.

Q2EXERCISES

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

Solution

This question requires research from external sources. Below is a representative answer based on known discoveries and controversies.

Example of a Fossil Discovery: One significant discovery in recent years is of Homo luzonensis, found in the Philippines and announced in 2019. The fossils, dated to at least 50,000 years ago, show a unique mix of modern and ancient features. For instance, the finger and toe bones are curved, similar to earlier hominins like Australopithecines, suggesting climbing abilities. However, other features are more similar to Homo sapiens. This discovery challenges the previously simpler models of human evolution and migration, suggesting that multiple hominin species may have co-existed in Southeast Asia and that human evolution was more complex and branching than once thought.

Example of a Controversy in Evolution: The cause of the mass extinction event at the end of the Cretaceous period (about 66 million years ago), which led to the extinction of the dinosaurs, remains a topic of scientific debate.

The Impact Hypothesis: The most widely accepted theory is that a large asteroid or comet struck the Earth near the Yucatán Peninsula in Mexico, creating the Chicxulub crater. The impact would have caused massive tsunamis, earthquakes, and ejected enormous amounts of dust and soot into the atmosphere, blocking sunlight, causing a global winter, and leading to the collapse of food chains.
The Volcanic Hypothesis: An alternative or contributing theory points to massive volcanic activity in the Deccan Traps in modern-day India. This volcanism occurred around the same time and would have released huge quantities of greenhouse gases and other chemicals into the atmosphere, causing drastic climate change and acid rain.

The controversy is not about whether these events happened, but about their relative importance. Some scientists argue the asteroid impact was the primary cause, while others believe the long-term environmental stress from the volcanic eruptions was the main driver, with the asteroid impact being the final blow.

Q3EXERCISES

Attempt giving a clear definition of the term species.

Solution

A species is most commonly defined using the Biological Species Concept. According to this concept, a species is a group of organisms that can naturally interbreed with one another and produce viable, fertile offspring. Members of the same species are reproductively isolated from other such groups, meaning they cannot interbreed with members of different species to produce fertile offspring.

Key features of this definition include:

Interbreeding Population: Individuals within a species can mate and exchange genetic material, sharing a common gene pool.
Production of Fertile Offspring: The offspring resulting from their mating must be both viable (able to survive) and fertile (able to reproduce themselves).
Reproductive Isolation: There are natural barriers (geographical, behavioral, physiological, or genetic) that prevent one species from successfully interbreeding with another.

It is important to note that this definition works well for most sexually reproducing animals but has limitations. It cannot be easily applied to organisms that reproduce asexually (like bacteria) or to extinct organisms known only from fossils.

Q4EXERCISES

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

Solution

The evolution of humans from primate ancestors involved significant changes in brain size, skeletal structure, and diet.

Dryopithecus and Ramapithecus (approx. 15 mya):
- Skeletal Structure: Ape-like (Dryopithecus) and more man-like (Ramapithecus). They were hairy and walked like gorillas and chimpanzees.
Australopithecines (approx. 2-4 mya):
- Skeletal Structure: Walked upright (bipedalism). They were not taller than 4 feet. Hominid features were present.
- Dietary Preference: Evidence shows they hunted with stone weapons but were essentially fruit eaters.
Homo habilis (approx. 2 mya):
- Brain Size: The first human-like being, with a brain capacity between 650-800cc.
- Skeletal Structure: Showed clear hominid features.
- Dietary Preference: Probably did not eat meat.
Homo erectus (approx. 1.5 mya):
- Brain Size: Had a large brain with a capacity of around 900cc.
- Skeletal Structure: Fully upright posture.
- Dietary Preference: Probably ate meat, indicating a shift to a hunter-gatherer lifestyle.
Neanderthal man (Homo neanderthalensis) (approx. 40,000-1,00,000 years ago):
- Brain Size: Had a large brain size of 1400cc, comparable to modern humans.
- Skeletal Structure: Robust build, adapted to cold climates.
- Culture/Function: Used hides to protect their bodies and buried their dead, indicating some form of culture and abstract thought.
Homo sapiens (Modern Human) (arose approx. 75,000-10,000 years ago):
- Brain Size: Large brain with a highly developed cerebrum.
- Function: Development of complex language skills and self-consciousness.
- Culture: Pre-historic cave art developed around 18,000 years ago. Agriculture and human settlements started around 10,000 years ago.

Q5EXERCISES

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

Solution

This question requires external research. The concept of self-consciousness in animals is a major area of study in cognitive ethology.

While the textbook highlights self-consciousness as a key trait in human evolution, scientific evidence suggests that some form of self-awareness exists in other animals. The most common test for self-awareness is the mirror self-recognition test. In this test, an animal is marked with a dye on a part of its body it cannot normally see. If, upon seeing its reflection in a mirror, the animal touches or investigates the mark on its own body, it is considered to have demonstrated self-recognition.

Animals that have passed the mirror test include:

Great Apes: Chimpanzees, bonobos, orangutans, and gorillas.
Marine Mammals: Bottlenose dolphins and orcas (killer whales).
Elephants: Asian elephants.
Birds: Eurasian magpies.

While passing the mirror test is a strong indicator of self-awareness, it is not the only measure. Other complex behaviors, such as planning for the future, empathy, and tool use, also suggest advanced cognitive abilities in many species. Therefore, while human self-consciousness, with its link to complex language and abstract thought, may be unique in its depth, it is now widely believed that we are not the only species on Earth with a sense of self.

Q6EXERCISES

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

Solution

Below is a list of 10 modern-day animals and their corresponding ancient fossil ancestors or relatives, based on paleontological evidence.

Modern-Day Animal	Ancient Fossil Relative
1. Horse (Equus)	Hyracotherium (also known as Eohippus)
2. Elephant	Moeritherium
3. Whale	Pakicetus
4. Bird	Archaeopteryx
5. Human (Homo sapiens)	Australopithecus afarensis
6. Armadillo	Glyptodon
7. Crocodile	Deinosuchus
8. Rhinoceros	Paraceratherium (a giant hornless rhino)
9. Shark	Megalodon
10. Sloth	Megatherium (giant ground sloth)

Q7EXERCISES

Practise drawing various animals and plants.

Solution

This is a practical activity aimed at improving observational skills and understanding of morphology, which is crucial for studying evolution. To complete this task, one should:

Observe and Draw Figures from the Textbook: Practice drawing the figures provided in the chapter, such as:
- Figure 6.3: The homologous bones of the forelimbs in mammals (Whale, Bat, Cheetah, Human) and the homologous structures in plants (Bougainvillea and Cucurbita).
- Figure 6.5: The different beak shapes of Darwin's finches to understand adaptation.
- Figure 6.6 & 6.7: The variety of Australian marsupials and their convergent evolution with placental mammals.
- Figure 6.11: The skulls of a modern human, baby chimpanzee, and adult chimpanzee to compare developmental and evolutionary changes.
Focus on Detail: While drawing, pay close attention to the specific anatomical features that are evidence for evolution, such as similarities in bone structure (homology), adaptations for different functions (adaptive radiation), or similarities in unrelated species due to similar environments (analogy).

This practice helps in visualizing and remembering the structural evidence for evolution and the concept of descent with modification.

Q8EXERCISES

Describe one example of adaptive radiation.

Solution

An excellent example of adaptive radiation described in the chapter is Darwin's Finches on the Galapagos Islands.

Definition: Adaptive radiation is the process of evolution where different species evolve from a common ancestor, with each new species adapting to a different ecological niche or habitat.

Description of Darwin's Finches:

Origin: Darwin conjectured that all the varieties of finches he found on the Galapagos Islands evolved from an original, single ancestral species of seed-eating finch that had migrated to the islands.
Radiation and Adaptation: Starting from this single point, the finches spread (radiated) across the different islands and adapted to the available food sources. This led to the evolution of multiple new species.
Evolutionary Change: The most significant adaptation was in their beaks. Over time, natural selection favored finches with beak shapes and sizes best suited for their specific diet. For example:
- Some finches retained the original seed-eating features.
- Others developed slender, sharp beaks to become insectivorous (insect-eaters).
- Still others developed beaks suited for eating fruits or different types of vegetation.

This diversification from a single ancestor into multiple species with different beak morphologies and feeding habits is a classic illustration of adaptive radiation.

Q9EXERCISES

Can we call human evolution as adaptive radiation?

Solution

No, human evolution is generally not considered a classic example of adaptive radiation. Here is a comparison to the definition:

Adaptive Radiation involves the rapid evolution of many diverse species from a common ancestor to fill various distinct ecological niches. For example, Darwin's finches evolved different beaks to specialize in eating different types of food (insects, seeds, cactus).

Human Evolution, on the other hand, shows a different pattern:

Lack of Niche Specialization: While different hominin species like Australopithecus, Homo habilis, and Homo erectus existed, they did not radiate to fill a wide array of distinct ecological roles in the way that finches or Australian marsupials did. The evolutionary path appears more linear, with later species often replacing earlier ones, rather than coexisting by specializing in different niches.
Generalization, not Specialization: The key to human success has been generalization, not specialization. Humans (Homo sapiens) did not evolve into distinct biological species adapted for different environments. Instead, they adapted through culture, technology, and intelligence. We use tools, clothing, and fire to live in diverse habitats from the arctic to the desert, rather than evolving different biological forms for each.

Therefore, while the human lineage did produce several species over millions of years, it does not fit the model of a single ancestor rapidly diversifying into multiple species to occupy a variety of empty ecological niches. The pattern of human evolution is better described as a sequence of evolving species with increasing brain size and intelligence, culminating in one highly adaptable, global species.

Q10EXERCISES

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.

Solution

The evolution of the horse (Equus) is a classic story of evolution documented in the fossil record. It shows clear trends over millions of years, primarily in response to a changing environment from forests to open grasslands.

The main evolutionary stages are:

Hyracotherium (or Eohippus) - 'Dawn Horse'
- Time: About 55 million years ago (Eocene epoch).
- Size: Small, about the size of a fox or small dog.
- Habitat: Lived in forests.
- Anatomy: Had an arched back, four toes on the front feet and three on the hind feet. Its teeth were small and low-crowned, adapted for browsing on soft leaves and fruits.
Mesohippus - 'Middle Horse'
- Time: About 35 million years ago (Oligocene epoch).
- Size: Larger than Hyracotherium, about the size of a sheep.
- Habitat: Lived in a mix of forests and open areas.
- Anatomy: Had three toes on all feet, with the middle toe being larger and bearing more weight. The side toes were smaller. Its teeth were more developed for chewing tougher vegetation.
Merychippus - 'Ruminant Horse'
- Time: About 17 million years ago (Miocene epoch).
- Size: Larger, about the size of a pony.
- Habitat: Lived in grasslands.
- Anatomy: Considered the first true grazer. It stood on its middle toe, and the side toes were very small (vestigial) and did not touch the ground. Its teeth were high-crowned, adapted for grinding tough, abrasive grasses.
Pliohippus - 'More Recent Horse'
- Time: About 5 million years ago (Pliocene epoch).
- Size: Similar to a modern horse.
- Anatomy: The first one-toed horse. The two side toes were reduced to splint bones on the side of the leg, a feature seen in modern horses.
Equus - Modern Horse
- Time: About 4 million years ago to the present.
- Anatomy: Includes modern horses, donkeys, and zebras. They are large, fast-running animals with a single toe (hoof) on each foot and high-crowned teeth adapted for a grazing diet.

Overall Trends in Horse Evolution:

Increase in overall body size.
Reduction in the number of toes from four to one.
Lengthening of legs and face.
Increase in the size and complexity of teeth (from low-crowned to high-crowned) for grazing on grasses.

EvolutionClass 12 Biology NCERT Solutions

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

Solution

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

Solution

Attempt giving a clear definition of the term species.

Solution

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

Solution

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

Solution

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

Solution

Practise drawing various animals and plants.

Solution

Describe one example of adaptive radiation.

Solution

Can we call human evolution as adaptive radiation?

Solution

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.

Solution