Molecular clocks and evolutionary relationships: Possible distortions due to horizontal gene flow

Summary This paper discusses recent evidence suggesting that genetic information from one species occasionally transfers to another remotely related species. Besides addressing the issue of whether or not the molecular data are consistent with a wide-spread influence of horizontal gene transfer, the paper shows that horizontal gene flow would not necessarily preclude a linear molecular clock or change the rate of molecular evolution (assuming the neutral allele theory). A pervasive influence of horizontal gene transfer is more than just consistent with the data of molecular evolution, it also provides a unique explanation for a number of possibly conflicting phylogenies and contradictory clocks. This phenomenon might explain why some protein clocks are linear while the superoxide dismutase clock is not, how the molecular data on the phylogeny of apes and Australian song birds are not necessarily in conflict with those based on morphology, and, finally, why the mycoplasmas have an accelerated molecular clock.