What relationship best describes co-evolution?

Co-evolution refers to the process where two or more species exert selective pressures on each other, leading to adaptive changes that influence their evolution over time. This relationship typically occurs when species interact closely, such as through predation, herbivory, or symbiosis.

In this context, adaptive changes arise in one species as a direct response to specific traits or adaptations of another species. For instance, if a predator evolves more efficient hunting strategies, the prey species might respond by developing better camouflage or defensive mechanisms. This ongoing interaction can lead to significant evolutionary changes in both species involved.

The other options represent different evolutionary concepts. While competing species developing similar traits can occur through processes like competitive exclusion, it does not capture the reciprocal nature of co-evolution. Mutually beneficial interactions, such as those observed in mutualism, may involve co-evolution but are not exclusive to the definition of co-evolution itself. Lastly, diversification due to geographic barriers pertains to speciation processes rather than the direct interaction-driven adaptations characteristic of co-evolution. Thus, the response to another species' evolutionary changes highlights the essence of co-evolution, making it the most fitting answer.