Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The extensive adaptive radiation of marsupials in Australia was primarily possible due to the geographical isolation of the continent. Reason (R): The isolation prevented the entry of placental mammals, which would have otherwise occupied the available ecological niches and outcompeted the marsupia

Step-by-step solution

Both Assertion and Reason are true, and the Reason correctly explains the Assertion. Adaptive radiation requires the availability of vacant ecological niches. When Australia separated from other landmasses due to continental drift, it became geographically isolated. This isolation prevented highly competitive placental mammals from entering the continent. As a result, the pouched mammals (marsupials) faced a lack of competition from other mammals. This absence of competitors left numerous ecological niches vacant,