Write a short note on Adaptive Radiation and the Marsupials of Australia.

(N/A) Biogeographical evidence suggests that species restricted to widely separated regions often share a common ancestry. Habitat isolation likely restricted these organisms to specific geographical areas.
This can be explained through the following processes:
$1$. Adaptive Radiation: The process of evolution of different species starting from a single point in a geographical area and radiating to other geographical areas (habitats) is called adaptive radiation.
Examples of adaptive radiation:
$(i)$ Darwin's Finches: Darwin observed an amazing diversity of creatures on the Galapagos Islands. He saw small black birds that fascinated him,later known as Darwin's finches. They represent one of the best examples of adaptive radiation. He observed many varieties of finches on the same island,all of which evolved from original seed-eating finches. He explained that after originating from a common ancestral seed-eating stock,the finches radiated to different geographical areas and underwent adaptive changes,especially in their beak types. Due to these gradual alterations,some became insectivorous and others vegetarian. Living in isolation for a long time,new kinds of finches emerged that could function and survive in new habitats.
(ii) Marsupials of Australia: Another example is the Australian marsupials. $A$ number of marsupials,each different from the other,evolved from an ancestral stock,but all within the Australian island continent. When more than one adaptive radiation appears 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).