Chapter 1: A Case Study for Evolutionary Thinking: Understanding HIVOverview |
The tools and techniques of evolutionary biology offer crucial insights into matters of life and death. To justify this claim, we explore the evolution of human immunodeficiency virus (HIV). HIV is the virus that causes acquired immune deficiency syndrome (AIDS).
1.1 The Natural History of the HIV/AIDS Epidemic The worst epidemic in human history, judging by the number of deaths, was probably the "Spanish" influenza of 1918. It swept the globe in a matter of months, killing 50 to 100 million people. The second worst was likely the Black Death, an outbreak of bubonic plague that ravaged Europe beginning in 1346. In five years it took 30 to 40 of the population-perhaps as many as 28 million lives. Also worthy of mention is the New World smallpox epidemic unleashed around 1520 by European conquistadores. Its death toll is harder to reckon, but over the succeeding decades it decimated Native American populations across two continents.
Section 1.1 in your textbook goes on to compare these and other epidemics to the recent AIDS epidemic now sweeping the globe.
1.2 Why Does AZT Work in the Short Run, But Fail in the Long Run? To combat viral infections, researchers look for drugs that are capable of inhibiting enzymes special to the virus. For example, a drug that blocks reverse transcription should effectively and specifically kill retroviruses with minimal side effects. This is the rationale behind azidothymidine, or AZT.
But AZT is not a useful treatment against HIV. Refer to Section 1.2 of your textbook to learn about AZT and why it is no longer an effective treatment against HIV infection.
1.3 Why is HIV Fatal? One of the keys to becoming an evolutionary biologist is to learn "selection thinking." The idea is that evolution by natural selection, as outlined in Section 1.2, is an automatic process that simply happens whenever a population shows heritable variation in survival and reproductive success. Traits conducive to surviving and reproducing spread throughout the population; traits conducive to dying without issue disappear. If we want to understand why a particular trait is common in a particular population, a good first step is to try to understand how the trait might affect the survival and reproductive success of individuals. In this section, we apply selection thinking to a puzzling feature of HIV infections: Un-treated, they are almost always fatal.
Turn to Section 1.3 in your textbook for help in answering this question.
1.4 Why Are Some People Resistant to Infection by HIV? The principle of evolution by natural selection explains why strains of HIV have become drug resistant. It may also explain why the virus is deadly. Can the same principle clarify why some people who are repeatedly exposed to the virus do not become infected?
For researchers and physicians who are struggling to find strategies for controlling the AIDS epidemic, the existence of exposed, but uninfected, people is a ray of hope. If natural resistance to the virus exists, and if the molecular basis of this resistance can be identified, it might be possible to replicate the mechanism of resistance with new drug therapies.
Section 1.4 in your textbook tells the fascinating story of how both of these "ifs" were confirmed.
1.5 Where Did HIV Come From? We noted in Section 1.1 that AIDS was first recognized in 1981. It is a new disease for humans. Its cause, HIV, is a new pathogen. Viruses, like other organisms, only come from reproduction of their own kind. So where did the first HIV virions come from?
The first clue is that HIV's genome and life cycle are similar to those of a family of viruses that infect a variety of primates. These viruses are known SIVs, or simian immunodeficiency viruses. Like HIV, they infect their hosts' immune systems. Unlike HIV, they do not appear to cause serious disease.
A logical hypothesis is that HIV is derived from one of the SIVs, and that the global AIDS epidemic started when this SIV moved from its primate host into humans. Which SIV is the ancestor of HIV? To find out, evolutionary biologists have reconstructed the evolutionary history of the viruses in the SIV/HIV family.
Read the rest of this story in Section 1.5 in your textbook.