Read the passage given below and answer the following question.
Evolution by natural selection, in a true sense, would have started when cellular forms of life with differences in metabolic capability originated on earth. The essence of Darwinian theory about evolution is natural selection. 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 millions of years as life spans of these animals are in years. Here we say that fitness of B is better than that of A under the new conditions. Nature selects for fitness.
According to the passage, a new variant population of bacteria can appear within days, whereas the same change in a fish or fowl would take millions of years. Why?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The rate of appearance of new forms is linked to the life span, and bacteria divide fast.
The passage states that “The rate of appearance of new forms is linked to the life cycle or the life span” and that microbes which divide fast can become millions of individuals within hours. It then says the same change in a fish or fowl would take millions of years “as life spans of these animals are in years”.
Selection works generation by generation. A bacterium passes through many generations in a day, so a fitter variant can outgrow the rest within days. A fish or a bird passes through one generation in years.
Option (1) The rate of appearance of new forms is linked to the life span, and bacteria divide fast. — Correct. This is exactly the reason the passage gives.
Option (2) A change in the medium composition directly creates new variants in the bacterial colony. — Incorrect. The variation is already built into the colony. The change in the medium only brings out the part of the population (B) that can survive the new conditions.
Option (3) Fish and fowl show less built-in variation than a bacterial colony. — Incorrect. The passage makes no comparison of the amount of variation. The only reason it gives for the slow change is the long life span.
Option (4) Natural selection started acting on fish and fowl much later than on bacteria. — Incorrect. The passage talks about how fast the change happens, not about when selection began in each group.