Final answer:
The evolution of X4 virions signifies adaptive immune evasion and natural selection in HIV, where drug-resistant viruses survive and rapidly reproduce, altering the population's genetic makeup and demonstrating evolution as an ongoing process.
Step-by-step explanation:
The evolution of X4 virions is an example of the adaptive immune evasion mechanisms implemented by pathogens such as the human immunodeficiency virus (HIV). In the scenario given, drug-resistant viruses have survived treatment, showing that evolution is an ongoing process influenced by natural selection. These drug-resistant strains multiply, resulting in a population that consists entirely of resistant viruses. Such dynamics illustrate how HIV rapidly mutates, allowing it to change its antigens and evade the adaptive immune response, making the development of an effective vaccine extremely challenging.
Continuous exposure to the drug selects for HIV virions that have resistance, which allows them to survive and reproduce while non-resistant strains perish. This shows evolution in real-time as the genetic makeup of the virus population changes. The rapid reproduction rate of the virus facilitates these evolutionary changes, confirming that evolution, driven by natural selection, is a continuous process happening within populations of organisms, including viruses.