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Open access, freely available online Synopses of Research Articles Comparing Gene Trees and Genome Trees: A Cobweb of Life? DOI: 10.1371/journal.pbio.0030347 The tree of life has long served as a useful tool for genomes, Kim and colleagues estimate that HGT infi ltrates describing the history and relationships of organisms just 2% of the average microbial genome. Even when over evolutionary time. One species is represented as a relatively common, the authors conclude, HGT events branching point, or node, on the tree, and the branches do not disrupt the integrity of the tree of life, represent paths of descent from a parental node. The contributing just small bits of genetic material, tree diagram carries an implicit assumption “much like cobwebs on tree branches.” that genes are transferred vertically, from To construct both gene trees and a parent to child, and that all the genes in a whole-genome tree for the microbes, the new species come from the ancestral species. authors selected core sets of orthologous In theory, one should be able to trace the origin gene groups from the NIH database of clusters of each gene in a species back to its ancestor. of orthologous genes. (Clusters are derived by In practice, however, the ancestral gene is