CBSE Class 12 Biology NCERT Solutions: Chapter 7 - Evolution

Darwinian selection theory, proposed by Charles Darwin, explains that within any population, individuals exhibit variations. Those individuals possessing variations that are more favorable for survival and reproduction in their specific environment are more likely to thrive and pass on these advantageous traits to their offspring. This process is often referred to as 'survival of the fittest'.

Antibiotic resistance in bacteria serves as a classic example of this theory in action. When a population of bacteria is exposed to an antibiotic, most of the bacteria that are sensitive to the drug will die. However, if a few bacteria within the population possess a gene mutation that confers resistance to the antibiotic, they will survive. These resistant bacteria then multiply, leading to a new population where resistance is a common trait. The antibiotic acts as a selective pressure, favoring the survival and proliferation of resistant individuals over susceptible ones.

Therefore, the observed antibiotic resistance in bacteria is a direct consequence of natural selection, where the environment (presence of the antibiotic) selects for individuals with beneficial mutations (resistance genes).

Recent scientific literature and popular science articles frequently report on new fossil discoveries that shed light on evolutionary history. For instance, the discovery of feathered dinosaur fossils in China has provided significant insights into the evolutionary link between dinosaurs and birds. These fossils offer morphological evidence supporting the hypothesis that birds evolved from theropod dinosaurs.

Controversies in evolution often arise from new interpretations of fossil evidence or debates about evolutionary mechanisms. For example, the discovery of fossils like Confuciusornis, an early bird from the Cretaceous period, has sparked discussions about the diversity and evolutionary pathways of early avian species. Such discoveries continually refine our understanding of evolutionary timelines and relationships, sometimes challenging existing paradigms and stimulating further research.

In biology, a species is fundamentally defined as a group of organisms that share similar characteristics and are capable of interbreeding with one another in their natural environment to produce fertile offspring. This biological species concept emphasizes reproductive isolation, meaning that members of one species generally cannot successfully interbreed with members of another species to produce viable, fertile progeny.

Human evolution is a complex process marked by gradual changes in various aspects of our ancestors. Key components include brain size and function, skeletal structure (especially posture and locomotion), and dietary preferences. Here's a trace of these components through significant hominid stages:

1. Dryopithecus (Ape-like ancestor):

• Brain capacity: Not specified, but assumed to be ape-like.
• Posture: Primarily arboreal (tree-dwelling), walked similarly to chimpanzees, with arms and legs of roughly equal size.
• Diet: Likely consumed soft fruits.
• Features: Possessed canines.

2. Ramapithecus (Man-like ape):

• Brain capacity: Not specified.
• Posture: Likely more upright than Dryopithecus.
• Diet: Preferred seeds and nuts.
• Features: Had smaller canines compared to apes, with larger molars, suggesting adaptation for grinding tougher food.

3. Australopithecus africanus:

• Brain capacity: Approximately 450 cm³.
• Posture: Exhibitied erect posture and bipedal locomotion (walked upright).
• Diet: Primarily herbivorous, consuming fruits.
• Features: Hunted with stone weapons; had small canines and incisors. Lived in trees but also moved on the ground.

4. Homo habilis ('Handy man'):

• Brain capacity: Approximately 735 cm³.
• Posture: Erect posture.
• Diet: Carnivorous, likely scavenging for meat.
• Features: Possessed small canines. Notably, they were the first hominids to make and use stone tools.

5. Homo erectus ('Upright man'):

• Brain capacity: Ranged from 800 to 1100 cm³.
• Posture: Erect posture.
• Diet: Omnivorous, indicating a broader range of food sources, including hunting.
• Features: Used stone and bone tools for hunting and other purposes.

6. Homo neanderthalensis (Neanderthal man):

• Brain capacity: Approximately 1300-1600 cm³, comparable to modern humans.
• Posture: Erect posture.
• Diet: Omnivorous.
• Features: Lived in caves, used animal hides for protection, and developed the practice of burying their dead, suggesting complex social behavior and beliefs.

These stages illustrate a progressive trend towards larger brain size, more efficient bipedal locomotion, and a shift towards more varied diets, alongside the development of tool use and complex behaviors, culminating in modern humans.