[AndrewC]

I was accused of "ignoring" numbers I don't recall seeing. I guess they are not there. Anyway HT as a mechanism seems to be readily and frequently used. This process involves not just fragments of genes or whole genes, but is possible with groups of genes.

Evolution of coenzyme B12 synthesis among enteric bacteria: evidence for loss and reacquisition of a multigene complex.

Lawrence JG, Roth JR.

Department of Biology, University of Utah, Salt Lake City 84112, USA.

We have examined the distribution of cobalamin (coenzyme B12) synthetic ability and cobalamin-dependent metabolism among enteric bacteria. Most species of enteric bacteria tested synthesize cobalamin under both aerobic and anaerobic conditions and ferment glycerol in a cobalamin-dependent fashion. The group of species including Escherichia coli and Salmonella typhimurium cannot ferment glycerol. E. coli strains cannot synthesize cobalamin de novo, and Salmonella spp. synthesize cobalamin only under anaerobic conditions. In addition, the cobalamin synthetic genes of Salmonella spp. (cob) show a regulatory pattern different from that of other enteric taxa tested. We propose that the cobalamin synthetic genes, as well as genes providing cobalamin-dependent diol dehydratase, were lost by a common ancestor of E. coli and Salmonella spp. and were reintroduced as a single fragment into the Salmonella lineage from an exogenous source. Consistent with this hypothesis, the S. typhimurium cob genes do not hybridize with the genomes of other enteric species. The Salmonella cob operon may represent a class of genes characterized by periodic loss and reacquisition by host genomes. This process may be an important aspect of bacterial population genetics and evolution.

[general_re]

Anyway HT as a mechanism seems to be readily and frequently used. This process involves not just fragments of genes or whole genes, but is possible with groups of genes.

I don't think anyone would argue that it doesn't happen - the evidence seems clear that it does. However, "frequently" is a relative term that may or may not turn out to be appropriate as we learn more about the genomes of various organisms.

For example, human beta-hemoglobin shows a high degree of correspondence with beta-hemoglobin in chimps, a somewhat lower degree with beta-hemoglobin in llamas, and a still-lower degree of correspondence with beta-hemoglobin in chickens (guess what I was doing this morning). Horizontal transfers seem unlikely to have sufficient explanatory power to resolve this, particularly when those relationships dovetail nicely with what we already know from cladistic studies.