Evolution of Genes and Organisms The Tree/Web of Life in Light of Horizontal Gene Transfer

摘要

Gene exchange necessitates expanding the model of the tree of life, impacts the notionof organismal and molecular most recent common ancestors, and provides examplesof natural selection working at multiple levels. Gene exchange, whether by horizontalgene transfer (HGT), hybridization of species, or symbiosis, modifies the organismaltree of life into a web. Darwin suggested the tree of life was like a coral, where livingsurface branches were supported by masses of dead branches. In phylogenetic trees,organismal or molecular lineages coalesce back to a lucky universal ancestor whosedescendents are found in current lineages and which coexisted with other, now-extinctlineages. HGT complicates the reconstruction of a universal ancestor; genes in a genomecan have different evolutionary histories, and even infrequent gene transfer will causedifferent molecular lineages to coalesce to molecular ancestors that existed in differentorganismal lineages and at different times. HGT, as well as symbiosis, provides a mech-anism for integrating and expanding the organizational level on which natural selectionacts, contributing to selection at the group and community level.