Define adaptive radiation and provide a mammal example.

• A. A slow change within a single lineage without producing new forms.
• B. Rapid diversification from a single ancestor into many forms; example: Australian marsupials such as kangaroos and koalas illustrate adaptive radiation.
• C. Convergence of unrelated species to resemble one another due to similar environments.
• D. A process where species move to new geographic areas.

Answer: (B)

Adaptive radiation is rapid diversification from a single common ancestor into many forms that occupy a variety of ecological niches. In Australia, marsupials descended from a common ancestor and quickly filled diverse roles as mammals—kangaroos and wallabies became large hopping herbivores, koalas evolved into tree-dwelling folivores, and other descendants took on different feeding and locomotion strategies. This burst of diversification tends to happen when new opportunities or environments present many empty niches, allowing descendants to expand into multiple forms in a relatively short time. The other descriptions describe processes like slow change within one lineage, convergence of unrelated species, or mere dispersal to new areas, which don't capture the pattern of one lineage branching into many forms. The mammal example here illustrates adaptive radiation well.