What is the term for a period of evolutionary change in which a group of organisms forms new species to fill new ecological roles?

Adaptive radiation is the correct term for a period of evolutionary change where a group of organisms diversifies rapidly, leading to the formation of new species that occupy different ecological niches. This phenomenon often occurs when a species colonizes a new habitat with varied environments, creating opportunities to exploit different resources or adapt to distinct conditions.

In adaptive radiation, the diversification is typically influenced by the availability of resources and the differing ecological pressures in the new environment. Classic examples include the evolution of Darwin's finches in the Galápagos Islands, where each species adapted to the specific food sources on their respective islands.

Other terms mentioned do not accurately describe this process. Biotic potential refers to the maximum reproductive capacity of an organism under ideal conditions, directional selection describes a type of natural selection that favors one extreme phenotype, and carrying capacity is the maximum number of individuals in a population that an environment can sustain. These concepts are related to population dynamics and evolutionary processes but do not specifically address the creation of new species in response to ecological opportunities.