Which scenario best illustrates adaptive radiation?

• A. Rapid diversification of a lineage into many forms occupying different niches; example: after the dinosaurs' extinction, placental mammals diversified into many lineages (bats, primates, cetaceans).
• B. Evolution of a single trait across all species.
• C. Convergent evolution among unrelated groups.
• D. Extinction of a lineage due to environment.

Answer: (A)

Adaptive radiation is rapid diversification of a lineage into many forms that fill a variety of ecological niches. This happens when a group encounters new opportunities or environments and can evolve different adaptations to exploit them. The described scenario fits perfectly: after the dinosaurs disappeared, placental mammals quickly diversified into many lineages—bats, primates, whales and other forms—each adapting to a distinct way of life. That burst of variety shows how a single lineage can radiate into many shapes and functions to occupy different ecological roles.

In contrast, evolving a single trait across all species isn’t about branching into many forms or niches. Convergent evolution involves unrelated groups developing similar features because they face similar environments, not rapid diversification within one lineage. Extinction reduces diversity rather than producing a burst of new forms.