Q4. Which observations about populations does NCERT call factual, as the basis of natural selection?
Natural resources are limited; populations are stable in size except for seasonal fluctuation; members of a population vary, no two being alike; and most variations are inherited (NCERT page 119). Beyond the NCERT text: in the general model, populations need not stay constant and not every difference is inherited; heritable variation affecting reproductive success is the essential requirement (Wikipedia, “Natural selection”, facts in our words).
Q5. In NCERT’s bacterial example, why does population B replace population A?
After the medium changes, only B can survive the new conditions; it outgrows the others, so its fitness is better than A’s under the new conditions: nature selects for fitness (NCERT page 118).
Q6. A bacterium divides every 40 minutes. How many generations does it pass through in one day?
24 × 60 ÷ 40 = 36 generations (our arithmetic; NCERT page 118 links the rate of evolution to the life span).
Q7. Why is evolution described sometimes as a process and sometimes as a result?
Describing the story of the world, we treat evolution as a process; describing the story of life on earth, we treat it as a consequence of a process called natural selection. NCERT says we are still not very clear which way to regard them (page 119). Beyond the NCERT text: evolution is population change across generations, and can also mean its historical outcome; natural selection is one mechanism of that change, alongside mutation, recombination, drift and gene flow (the factors NCERT lists on page 121).