Evolution — NCERT Solutions

Given / Concept: Darwinian natural selection states that in any population, individuals show heritable variations. Those with favourable variations survive and reproduce better (survival of the fittest), passing the trait to offspring.

Explanation:

Step 1 – Pre-existing variation: In a large bacterial population, random mutations continuously produce a few individuals that carry a gene conferring resistance to a particular antibiotic (e.g., penicillin). These resistant variants exist even before the antibiotic is introduced.

Step 2 – Selection pressure: When an antibiotic is administered, it acts as a selecting agent (environmental pressure). The non-resistant (sensitive) bacteria are killed, while the rare resistant individuals survive.

Step 3 – Differential reproduction: The surviving resistant bacteria reproduce rapidly (binary fission), passing the resistance gene to all daughter cells. Over successive generations, the resistant type becomes the dominant form in the population.

Step 4 – Result: The entire bacterial population eventually becomes antibiotic-resistant. This is not the antibiotic causing the mutation; the mutation was already present. The antibiotic merely selected the pre-existing resistant variant.

Conclusion: Antibiotic resistance is a classic, real-time demonstration of Darwinian natural selection — variation exists in the population, the environment (antibiotic) selects the fittest variant, and that variant proliferates. This is why over-use of antibiotics accelerates the emergence of multi-drug-resistant (MDR) bacteria such as MRSA.

Note: This is an activity-based / research question. A model answer based on well-documented findings is provided below.

Recent Notable Fossil Discoveries & Controversies:

1. Homo naledi (2015, South Africa): Fossils discovered in the Rising Star Cave system by Lee Berger's team. H. naledi showed a mosaic of primitive and modern features (small brain, curved fingers, but human-like feet). Controversy: its geological age (~236,000–335,000 years) overlapped with early Homo sapiens, raising questions about coexistence of multiple Homo species.

2. Homo luzonensis (2019, Philippines): Fossils from Callao Cave suggested a new small-bodied hominin species, indicating multiple independent migrations and evolution of small body size (island dwarfism), challenging the simple linear model of human evolution.

3. Denisovans: DNA extracted from a finger bone in Denisova Cave (Siberia) revealed a distinct hominin group. Interbreeding with H. sapiens is evidenced by Denisovan DNA in modern Melanesians and Tibetans (EPAS1 gene for high-altitude adaptation).

4. Tiktaalik roseae: A transitional fossil between fish and tetrapods, supporting the fish-to-land vertebrate transition.

Conclusion: These discoveries show that human evolution was not a straight ladder but a branching bush, with multiple species coexisting and sometimes interbreeding — a continuing area of active research and debate.

Definition of Species:

A species is a group of organisms that:

1. Share a common gene pool,
2. Are capable of interbreeding among themselves under natural conditions, and
3. Produce fertile (viable) offspring,
4. Are reproductively isolated from all other such groups.

This is the Biological Species Concept proposed by Ernst Mayr (1942):

$$\text{Species} = \text{a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups}$$

Additional points:

• Members of a species share morphological, physiological, biochemical, and behavioural similarities.
• They occupy a common ecological niche.
• Reproductive isolation (pre-zygotic or post-zygotic barriers) is the key criterion.

Limitation: This definition does not apply to asexually reproducing organisms (bacteria, some plants). For such organisms, a morphological species concept (similarity in structure) or phylogenetic species concept (common ancestry) is used.

Example: All domestic dogs (Canis lupus familiaris) belong to one species because they can interbreed and produce fertile offspring, even though they differ greatly in size and appearance.

Components of Human Evolution:

Key Evolutionary Trends:

1. Brain size: Gradual increase from ~400 cc (Australopithecus) to ~1400–1600 cc (H. sapiens). Larger brain enabled complex thinking, language, and tool use.

2. Skeletal structure: Shift from quadrupedal to bipedal locomotion; changes in pelvis, femur angle, foramen magnum position (moved to base of skull), and foot arch.

3. Dietary preference: From herbivory (Australopithecus) → omnivory (H. erectus onwards). Discovery of fire allowed cooking, improving nutrition and reducing jaw size.

4. Tool use: Progression from crude stone tools (Oldowan) → Acheulean hand axes → sophisticated blades and art.

5. Language and culture: Developed in H. sapiens sapiens; enabled social organisation, agriculture (~10,000 ya), and civilisation.

Conclusion: Human evolution is characterised by increasing brain complexity, upright posture, refined tool use, and development of language — all interrelated and mutually reinforcing.

Note: This is a research/activity-based question. A model answer is provided.

Self-Consciousness in Non-Human Animals:

Self-consciousness is typically tested using the Mirror Self-Recognition (MSR) Test (developed by Gordon Gallup Jr., 1970): an animal is marked with a spot and placed before a mirror. If it touches the spot on its own body (not the mirror image), it is considered self-aware.

Animals that have passed the MSR test:

Other indicators of self-awareness:

• Chimpanzees plan for the future and deceive others.
• Elephants show grief and altruism.
• Dolphins have individual names (signature whistles).

Conclusion: Self-consciousness, once thought unique to humans, appears to exist in varying degrees in several animal species, suggesting it evolved gradually and is linked to brain complexity and social living.