Others have given great answers. I wanted to point out that the “Red Queen hypothesis” (Wikipedia link) is relevant here. In Lenski’s experiment, conditions were intentionally kept pretty much constant, but in the real world

> species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species.… the effective environment of any homogeneous group of organisms deteriorates at a stochastically constant rate. …the evolutionary progress (= increase in fitness) of one species deteriorates the fitness of coexisting species, but because coexisting species evolve as well, no one species gains a long-term increase in fitness