Final answer:
The term for the rapid expansion and diversification of species into new ecological niches is adaptive radiation. This process often happens after mass extinction events, allowing organisms like mammals to evolve and fill vacant niches, leading to exponential growth in biodiversity.
Step-by-step explanation:
The term that refers to the relatively rapid expansion and diversification of life forms into new ecological niches is adaptive radiation. This biological process occurs when species evolve and diversify rapidly to fill empty niches, especially after events like mass extinctions or environmental changes that open new ecological opportunities. An iconic example of this is the Cambrian explosion during which a vast number of animal phyla emerged.
Adaptive radiation typically follows after a catastrophic event that has led to a mass extinction, as was the case 65 million years ago with the extinction of dinosaurs. Subsequently, mammals, including our very own ancestors, began to thrive and occupied the newly available niches, showcasing an evolutionary burst in biodiversity.
Exponential growth in species diversity can often accompany these periods of adaptive radiation, leading to an increase in biodiversity. The variety of life that develops during such periods contributes to the complexity and functionality of ecosystems across the globe.