Beyond linear sequence comparisons: the use of genome-level characters for phylogenetic reconstruction.

The first whole genomes to be compared for phylogenetic inference were those of mitochondria, which provided the first sets of genome-level characters for phylogenetic reconstruction. Most powerful among these characters has been the comparisons of the relative arrangements of genes, which has convincingly resolved numerous branch points, including those that had remained recalcitrant even to very large molecular sequence comparisons. Now the world faces a tsunami of complete nuclear genome sequences. In addition to the tremendous amount of DNA sequence that is becoming available for comparison, there is also a potential for many more genome-level characters to be developed, including the relative positions of introns, the domain structures of proteins, gene family membership, the presence of particular biochemical pathways, aspects of DNA replication or transcription, and many others. These characters can be especially convincing owing to their low likelihood of reverting to a primitive condition or occurring independently in separate lineages, thereby reducing the occurrence of homoplasy. The comparisons of organelle genomes pioneered the way for using such features for phylogenetic reconstructions, and it is almost certainly true, as ever more genomic sequence becomes available, that further use of genome-level characters will play a big role in outlining the relationships among major animal groups.