Horizontal gene transfer (HGT) moves genes between distantly related organisms without vertical inheritance, especially common in prokaryotes. HGT can rapidly introduce metabolic innovations and pathogenicity factors, causing sudden phenotypic shifts. Evidence of HGT complicates phylogenetic reconstruction and challenges tree-of-life assumptions.
From your study of molecular evolution, you know that DNA sequences change over time through mutation, drift, and selection, and that comparing sequences reveals evolutionary relationships. Vertical inheritance — parent to offspring — is the default assumption: genes flow down the tree of life. Horizontal gene transfer (HGT) breaks this assumption entirely. In HGT, an organism acquires genes not from its parent but from a completely unrelated organism, sometimes one that diverged billions of years ago.
Three main mechanisms drive HGT in prokaryotes. Transformation occurs when a bacterium takes up free DNA from its environment — for instance, DNA released by a dead neighboring cell. Transduction happens when a bacteriophage (a virus that infects bacteria) accidentally packages host DNA and delivers it to a new bacterial cell. Conjugation involves direct cell-to-cell contact through a pilus, transferring a plasmid or even chromosomal DNA from donor to recipient. Each mechanism moves genetic material sideways across lineages rather than vertically through reproduction.
The consequences of HGT are enormous. Consider antibiotic resistance: a gene encoding a beta-lactamase enzyme that destroys penicillin can jump from a harmless soil bacterium into a pathogenic Staphylococcus in a single transfer event, instantly conferring resistance. This is far faster than waiting for the pathogen to evolve resistance through point mutations. HGT can also introduce entirely new metabolic capabilities — some bacteria have acquired photosynthesis genes, nitrogen fixation genes, or toxin-production genes from distantly related donors, fundamentally reshaping their ecological roles.
For phylogenetics, HGT creates a serious problem. If you build a tree from a gene that was horizontally transferred, that gene's tree will not match the species tree. One gene might place a bacterium near cyanobacteria while another places it near proteobacteria, not because the species has two ancestors but because different genes have different histories. This is why molecular evolution in prokaryotes is often better described as a web of life or a network rather than a strictly branching tree. Recognizing when a gene's phylogeny conflicts with the species tree is one of the primary methods for detecting HGT events, and it forces biologists to think carefully about which genes are reliable markers of organismal relationships and which have been "borrowed" from elsewhere.