Final answer:
The phenomenon of the American chestnut's near extinction by the fungus Cryphonectria parasitica exemplifies how parasites can drive host populations to extinction, disrupt ecosystems, and facilitate co-evolutionary arms races.
Step-by-step explanation:
The interactions between the American chestnut and the parasitic fungus Cryphonectria parasitica best exemplify the principle that parasites can drive host populations to extinction. The American chestnut was a dominant tree in eastern North America until the chestnut blight caused by Cryphonectria parasitica led to its drastic decline and the ecological consequences that followed including changes in forest structure and nutrient cycling. This case is a particularly devastating example of how introduced pathogens can impact ecosystems by eliminating dominant plants and can even result in the extinction of host-specific species, in this case, moth species dependent on the chestnut. This historical event also supports the co-evolutionary arms race concept described by the Red Queen Hypothesis, demonstrating how a rapidly spreading pathogen can overcome the defenses of a host population.