Final answer:
Coevolution is the process where two or more species evolve in response to each other's evolutionary changes, shaping one another's development over time.
Step-by-step explanation:
Coevolution is the process by which two interacting species evolve together, with each species influencing the trajectory of the other's evolution. This interaction often results in an evolutionary arms race where, for example, predators and prey continually adapt in response to each other's changes. A well-known example of coevolution is the relationship between flowering plants and their pollinators, where plants evolve traits such as colors and scents attractive to specific pollinators, while insects develop specialized mouthparts and senses to better exploit floral resources.
It's not merely a matter of one species evolving because of changes in another; it's also about selection pressures that ensure mutual adaptation and, in some cases, survival. Thus, option 2, "When two or more species interact, and each acts as an agent of selection," is the correct answer. Coevolution is seen in symbiotic relationships, including those that are mutualistic, parasitic, or predatory. The Red Queen Hypothesis aptly summarizes the essence of coevolution, stating that it takes constant adaptation just to maintain one's evolutionary position within a dynamic ecosystem.