Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Many host-specific parasites and their hosts tend to co-evolve.
Reason (R): If the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them.
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).
Many parasites can live on only one species of host. Such a host and its parasite are locked together over evolutionary time.
Assertion (A): Many host-specific parasites and their hosts tend to co-evolve. — True. A is about what happens: the host and the parasite change together over generations.
Reason (R): If the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them. — True. R is about how it happens: each new defence of the host is answered by a new counter-measure of the parasite.
Why R explains A? Co-evolution means that the change in one species drives a change in the other. R shows that step by step: the host develops a defence, and the parasite must answer it to stay successful on the same host. The book itself joins the two with “that is”, using R to explain what co-evolution of host and parasite means. So R is the correct explanation of A.
NCERT Extract:
Population Interactions – “… Many parasites have evolved to be host-specific (they can parasitise only a single species of host) in such a way that both host and the parasite tend to co-evolve; that is, if the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them, in order to be successful with the same host species.”