Abstract: Series 99, Lecture 6

The Harvey Lectures Series 99 (2003—2004)

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Lecture #6: Thursday, April 15, 2004 — Time and Location

Comparative Microbial Genomics and the Tree of Life

W Ford Doolittle, PhD

Fellow and Director, Canadian Institute for Advanced Research Program in Evolutionary Biology
Canada Research Chair, Department of Biochemistry and Molecular Biology

Dalhousie University

Halifax, Nova Scotia

The consensus “Universal Tree of Life” depicts three “domains” — Bacteria, Archaea and Eukarya — diverging in two sequential branchings from a single last universal common ancestor (LUCA), which might have lived more than 3.8 billion years ago. It is based primarily on phylogenetic analyses of sequences of genes encoding small subunit ribosomal RNA (SSU rRNA). However, phylogenetic analyses of other genes often support other relationships, and comparative prokaryotic (bacterial and archaeal) genomic data show that transfer of genes across species (and even domain) boundaries has been a major force in evolution, indeed part of the history of all (or almost all) genes in all genomes. Thus there can be no unique tree of genomes, there was no unique LUCA, and web-like patterns and population processes may provide better models for genome evolution than trees and speciation events. What the three-domain Universal Tree actually represents becomes a subject for debate, and will be the focus of this lecture.