Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Placental mammals in Australia evolved into varieties each of which appears to be similar to a corresponding marsupial, as seen in the placental wolf and the Tasmanian wolf.
Reason (R): A number of marsupials, each different from the other, evolved from an ancestral stock within the Australian island continent.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Adaptive radiation is the evolution of different species in one geographical area, starting from a point and spreading into different habitats. When more than one adaptive radiation takes place in the same isolated area, and the groups come to look alike, NCERT calls it convergent evolution.
Assertion (A): Placental mammals in Australia evolved into varieties each of which appears to be similar to a corresponding marsupial, as seen in the placental wolf and the Tasmanian wolf. — True. A is about a likeness between two different groups, the placental mammals and the marsupials. Figure 6.7 pairs them up, such as the mole with the marsupial mole and the wolf with the Tasmanian wolf.
Reason (R): A number of marsupials, each different from the other, evolved from an ancestral stock within the Australian island continent. — True. R is about the adaptive radiation of marsupials alone, in which one ancestral stock gave rise to many different marsupials.
Why R does not explain A? R tells us how the marsupials became different from one another. It says nothing about the placental mammals, which went through an adaptive radiation of their own. The two groups look alike because each adapted to similar habitats, which is convergent evolution. The spread of the marsupials from their ancestral stock does not make a placental wolf look like a Tasmanian wolf, so R is not the correct explanation of A.
NCERT Extract:
What is Adaptive Radiation? – “… 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). (Figure 6.7).”
Figure 6.7 Picture showing convergent evolution of Australian Marsupials and placental mammals