The process of evolution by natural selection has generated diverse, elegant and efficacious innovations. Where such innovations have a genetic basis, they can be transmitted ‘vertically’ from parent to offspring, giving rise to a model of evolution represented as a bifurcating tree. However, it has long been recognized that genetic information can also be transferred between individuals not connected by inheritance, a process referred to as ‘horizontal gene transfer’ (HGT).Here, we define HGT as the movement of heritable genetic information that cannot be explained by vertical parent–offspring transmission.