A drug called AZT was the first to be used against HIV. Although it initially worked, resistance developed quickly, and eventually the drug was no longer effective. Drug therapies used today, which often combine several different drugs that target different phases of the HIV life cycle into a cocktail, are much more effective over the long term than a single drug used in isolation. Please provide an evolutionary explanation for why newer therapies work better.

The evolutionary explanation for why newer HIV drug therapies work better than the initial drug AZT lies in the concept of natural selection and the evolution of the HIV virus itself.

When AZT was first introduced, it was effective at reducing the replication of the virus and slowing down the progression of the disease. However, because AZT targeted a specific phase of the HIV life cycle, the virus was able to evolve and develop resistance to the drug. This is a classic example of natural selection at work - the HIV virus that had mutations allowing it to resist the effects of AZT were able to survive and replicate, while those that were susceptible to the drug were eliminated. Over time, this led to the emergence of a population of HIV that was largely resistant to AZT, rendering the drug ineffective.

In response to this evolutionary pressure, newer drug therapies have been developed that target different phases of the HIV life cycle and often combine multiple drugs into a cocktail. This approach makes it much more difficult for the virus to develop resistance, as any individual mutation that confers resistance to one drug may not necessarily confer resistance to the others in the cocktail. As a result, the newer drug therapies are much more effective over the long term, as they are able to suppress the replication of the virus and slow down the progression of the disease without the same risk of resistance developing.

In summary, the evolutionary explanation for why newer HIV drug therapies work better lies in the ability of the virus to evolve and develop resistance to single drugs, and the development of combination therapies that make it more difficult for the virus to evolve resistance. This evolutionary understanding has been crucial in guiding the development of more effective treatments for HIV.