Horizontal gene transfer in eukaryotic algal evolution

Although seemingly innocuous, the power of something as simple as a drop of water, given a few millions of years over which to act continually, is immediately and awe-inspiringly obvious to someone peering over the edge of the Grand Canyon. Acting over a similarly immense time period but on vastly different scales, this massive effect from a weak, but constant, force embodies how natural selection has been able to direct the evolution of once primitive, mal-adapted biological structures to the remarkable and almost inconceivably diverse molecular machines found within extant organisms. This idea of natural selection as a slow-and-steady workhorse was central to Charles Darwin's evolutionary synthesis, as epitomized in his oft-repeated précis ``Natura non facit saltum,'' Nature does not make leaps. Darwin would not live to see the discovery of genes as the vessel of inheritance and random mutation as the propagator of change, although these breakthroughs would serve to reinforce his prescient ideas.

To the contrary, the discovery of horizontal gene transfer (HGT) as a significant evolutionary driver may require an addendum to the Darwinian synthesis. A growing body of evidence indicates that many organisms, particularly prokaryotes, can and do make evolutionary leaps by sharing genes with one another, thereby opening a back door to an adaptation or ability that was already fine-tuned within another organism. Once thought to be an explanation of last resort when the data were not robust enough to give unambiguous results, with the recent availability of a wealth of whole-genome data, HGT has not only become respectable but has emerged as a central force in the evolution of many different prokaryotes (1–3). Of course, this idea came as no major surprise to many bacterial geneticists, who for decades have been selecting prokaryotes for their ability to take up and express exogenous …