Evolutionary trees help to convict men who knowingly infected women with HIV

In 2004, evolution itself served as a witness for the prosecution in the case of the State of Washington versus Anthony Eugene Whitfield. Whitfield contracted HIV in an Oklahoma prison, and first learned about his infection in 1992. After his release in 1995, he had more than a thousand sexual encounters with 17 different women, even fathering children with three of them. He rarely wore a condom, never told any of his partners about his infection and flatly denied it when asked. However, Whitfield did confess to various people that if he had HIV, he would give it to as many people as possible. He got his wish – five of his 17 partners became HIV-positive. Whitfield was finally arrested in 2004 and convicted on 17 counts of first-degree assault with sexual motivation, among other offences. His total sentence came to 178 years and a month. To demonstrate Whitfield’s guilt, the prosecution had to show that he had wilfully exposed women to HIV, that his five HIV-positive partners contracted their infections from him. Fortunately, David Hillisfrom the University of Texas and Michael Metzker from Baylor College of Medicine knew exactly how to do that. They had evolutionary biology on their side. Hillis and Metzker knew that HIV is a hotbed of evolution. The bodies of HIV carriers produce around a billion new virus particles every day, and their genomes change and shuffle at furious speeds. But when infections pass from one person to another, this viral variety plummets. Thousands of genetically distinct viruses might jump into a new host, but usually, only one of these managed to gain a foothold and set up a new infection. Every time it moves from host to host, HIV passes through a genetic bottleneck and that provides a massive clue about who passed an infection to whom. Armed with this knowledge, Hillis and Metzker analysed HIV genes from six anonymised blood samples – one from Whitfield and five from the women he allegedly infected. They had no way of knowing which was which. The genes revealed that the viruses were all closely related to each other, and more so than to viruses from other HIV-positive people in the local area. Using these sequences, Hillis and Metzker constructed a family tree for the viral genes (a ‘phylogeny’), to show their evolutionary relationships and, more importantly, to work out which sequences came first. The tree was damning – one group of viruses was clearly the ancestor of at least four of the others, and probably the fifth. The owner of these viruses, known only as ‘WA04cd’, must have been the source of the related infections. Once the study was complete, the prosecution revealed that the identity of this mystery carrier was none other than Anthony Eugene Whitfield. Once the defense saw the evidence, Whitfield admitted to being the source of the infections,