Natural taxonomy in light of horizontal gene transfer

We discuss the impact of horizontal gene transfer (HGT) on phylogenetic reconstruction and taxonomy. We review the power of HGT as a creative force in assembling new metabolic pathways, and we discuss the impact that HGT has on phylogenetic reconstruction. On one hand, shared derived characters are created through transferred genes that persist in the recipient lineage, either because they were adaptive in the recipient lineage or because they resulted in a functional replacement. On the other hand, taxonomic patterns in microbial phylogenies might also be created through biased gene transfer. The agreement between different molecular phylogenies has encouraged interpretation of the consensus signal as reflecting organismal history or as the tree of cell divisions; however, to date the extent to which the consensus reflects shared organismal ancestry and to which it reflects highways of gene sharing and biased gene transfer remains an open question. Preferential patterns of gene exchange act as a homogenizing force in creating and maintaining microbial groups, generating taxonomic patterns that are indistinguishable to those created by shared ancestry. To understand the evolution of higher bacterial taxonomic units, concepts usually applied in population genetics need to be applied.