Final answer:
Hosts develop defensive strategies like behavioral avoidance and immune responses to minimize parasite infections, while parasites evolve mechanisms like complex cell walls or infecting immune cells to survive within hosts. This co-evolution results in a dynamic arms race where host and parasite continuously adapt to each other's strategies.
Step-by-step explanation:
Hosts and parasites are engaged in a continuous co-evolutionary battle, with hosts evolving defensive strategies and parasites evolving offensive strategies. Hosts may develop behavioral avoidance tactics or activate immune mechanisms to prevent or minimize infections by ectoparasites. For example, to defend against invasive ectoparasites like ticks, a host species may evolve thicker skin or grooming behaviors.
On the other hand, parasites, in their quest for survival and reproduction, adapt by evading host defenses. For instance, Mycobacterium tuberculosis has developed a complex cell wall that macrophages cannot digest, enabling it to persist within the host. Stealth pathogens may use tactics such as infecting phagocytic cells to avoid destruction.
In this evolutionary arms race, the intricacies of adaptation are evident. Bacteria, protozoa, and fungi that have become parasites, have developed resistance to the host's natural defense processes. Meanwhile, ectoparasites may either become specialists, exploiting a single host species more efficiently, or generalize to counter the defenses of multiple hosts. However, a high degree of specialization may result in low performance across non-preferred hosts, while generalists may have lower performance even on optimal hosts.