W hile biologists use the study of evolutionary relationships mainly to build evolutionary trees of the world s organisms scientists are finding wider and wider uses for evolutionary trees Scientists have even used evolutionary trees to solve the mystery of whether a dentist infected his patients with HIV the virus that causes acquired immunodeficiency syndrome AIDS in humans The dentist who was himself infected with HIV had been working as usual with his patients Then some of his patients began turning up HIV positive Against a backdrop of increasing public controversy over whether health care workers with HIV pose a risk to their patients many people wanted to know had this dentist transmitted the deadly virus to his patients Adding confusion to the situation was the fact that some of the infected patients had lifestyles or habits that put them at risk for HIV infection by other means Those on both sides of the issue some saying the dentist should not be blamed others saying he was a threat argued heatedly without resolution In order to answer the question researchers took advantage of the observation that HIV s genetic material like that of most viruses evolves quickly resulting in many different but closely related strains of HIV The researchers created an evolutionary tree of the viral strains found in the dentist in each of the infected patients and in local people who were also infected with HIV but were not patients of the dentist If the dentist had infected African Patient Patient Local non person X Y patient 1 his patients then the strains of HIV found in his patients should be more closely related to his strain and less closely related to the strains from other infected people The results showed that some of the dentist s patients probably were infected by him and that some probably were not Two patients X and Y who could have been exposed to HIV as a result of their lifestyles or habits had HIV strains that were not closely related to the dentist s strain However five other patients A B C D and E were carrying strains of the virus very closely related to the strain in the dentist Moreover those people were not at risk of contracting HIV by other means So the scientists concluded that at least in the case of these five patients the dentist had infected them with his strain of the virus Local non patient 2 Local non Local non Patient Patient Patient Dentist Patient Patient patient 3 patient 4 C An Evolutionary Tree Solves the Whodunit By examining the evolutionary relationships among the strains of HIV found in a number of infected people including a dentist and his infected patients researchers were able to solve the mystery of how his patients were likely to have been infected