Horizontal Gene Transfer

Horizontal gene transfer (HGT) is the movement of genetic material between organisms other than through vertical transmission from parent to offspring. In bacteria and archaea, HGT occurs through conjugation, transformation, and transduction — processes that can transfer genes across species boundaries in a single generation. HGT challenges the traditional tree-of-life metaphor by introducing reticulate connections that trees cannot represent. The acquisition of antibiotic resistance genes across bacterial species is a medically urgent example of HGT in action. HGT connects to phylogenetics as the primary reason network-based evolutionary models are replacing purely tree-based ones, and raises fundamental questions about how to define species when genes flow laterally.