Read the given passage carefully and answer the questions that follow:-
During his journey, Darwin went to Galapagos Islands. There he observed an amazing diversity of creatures. Of particular interest, small black birds later called Darwin's Finches amazed him. He realised that there were many varieties of finches in the same island. All the varieties, he conjectured, evolved on the island itself. From the original seed-eating features, many other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches. This process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. Darwin’s finches represent one of the best examples of this phenomenon. Another example is Australian marsupials. A number of marsupials, each different from the other (Figure 6.6) evolved from an ancestral stock, but all within the Australian island continent. When more than one adaptive radiation appeared to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution. Placental mammals in Australia also exhibit adaptive radiation in evolving into varieties of such placental mammals each of which appears to be ‘similar’ to a corresponding marsupial (e.g., Placental wolf and Tasmanian wolf-marsupial). The essence of Darwinian theory about evolution is natural selection. Nature selects for fitness. One must remember that the so-called fitness is based on characteristics which are inherited. Hence, there must be a genetic basis for getting selected and to evolve. Another way of saying the same thing is that some organisms are better adapted to survive in an otherwise hostile environment. Adaptive ability is inherited. It has a genetic basis. Fitness is the end result of the ability to adapt and get selected by nature.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): A variant population of bacteria that can use a new feed component may appear as a new species within days, while the same change in a fish or fowl would take millions of years.
Reason (R): The rate of appearance of new forms is linked to the life cycle or the life span.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A): A variant population of bacteria that can use a new feed component may appear as a new species within days, while the same change in a fish or fowl would take millions of years. — True. A compares how fast new forms appear in bacteria and in animals.
Reason (R): The rate of appearance of new forms is linked to the life cycle or the life span. — True. R gives the rule behind the speed of evolution.
Why R explains A? Bacteria divide fast and become millions within hours, so many generations pass in a few days and the variant that can use the new feed soon outgrows the rest. Fish and fowl have life spans in years, so the same number of generations takes millions of years. The difference in A comes directly from the difference in life cycle stated in R, so R is the correct explanation of A.
NCERT Extract:
Biological Evolution – “… The rate of appearance of new forms is linked to the life cycle or the life span. Microbes that divide fast have the ability to multiply and become millions of individuals within hours. A colony of bacteria (say A) growing on a given medium has built-in variation in terms of ability to utilise a feed component. A change in the medium composition would bring out only that part of the population (say B) that can survive under the new conditions. In due course of time this variant population outgrows the others and appears as new species. This would happen within days. For the same thing to happen in a fish or fowl would take million of years as life spans of these animals are in years.”