Horizontal gene transfer

Horizontal gene transfer (HGT), also called lateral gene transfer, is the movement of genetic material between organisms by means other than direct parent-to-offspring inheritance. In contrast to vertical transmission — the standard Darwinian mechanism — HGT allows genetic information to cross species boundaries, potentially in a single generation.

HGT is the dominant mode of genetic exchange in bacteria and archaea. The mechanisms include transformation (uptake of environmental DNA), transduction (bacteriophage-mediated transfer), and conjugation (direct cell-to-cell transfer via plasmids). The resulting pattern of microbial evolution is not a tree but a network — shared ancestry between genomes reflects both vertical descent and lateral exchange, and reconstructing either requires distinguishing the two. Phylogenetic methods designed for strictly tree-like evolution produce systematically misleading results when applied to organisms with extensive HGT.

Antibiotic resistance is the most consequential contemporary instance of HGT: resistance genes originating in soil bacteria have spread across unrelated clinical pathogens through plasmid transfer, rendering formerly reliable antibiotics ineffective. The speed of this horizontal spread — measurable within years rather than millennia — makes it incommensurable with the timescales of vertical evolution and is the primary reason why antibiotic resistance constitutes a genuine evolutionary emergency.

HGT also occurs, at lower rates, in eukaryotes, including animals. Endogenous retroviruses — integrated remnants of ancient retroviral infections — constitute approximately 8% of the human genome. Some have been domesticated: the syncytin genes, essential for mammalian placental development, are of retroviral origin. Evolution, at the molecular level, is opportunistic and does not respect categorical boundaries between self and other.

The conception of evolution as descent with modification from a common ancestor requires qualification: at the molecular level, the genome is also an archive of successful invasions. The tree of life is a tree only for organisms; for genes, it has always been a network.