Chapter 1: a case for evolutionary thinking: understanding hiv. Hiv: is the virus that causes acquired immune deficiency syndrome (aids) Evolutionary biology devoted to understanding two things: how populations change through time following modifications in their environment, how new species come into being. 1. 1| the natural history of the hiv/aids epidemic. What is hiv: an intracellular parasite that afflicts cells in the human immune system, two phases in life cycle, extracellular form of a virus is called a virion or virus particle. The virus moves from one host cell to another: intracellular is the parasitic phase. In summary others: transcription errors produce mutations in the reverse transcriptase gene, the mutant virions pass their reverse transcriptase genes, and thus their azt resistance or susceptibility to their offspring. Heritable traits that lead to survival and reproductive abundance spread in populations. Heritable traits that lead to reproductive deficit disappear.