This is the cover article of a recent issue of New Scientist and as this is a common debate on the forum it might be of interest. Darwinian evolution is usually portrayed as a tree of life from which every living thing originates and which splits into many branches, each branch representing a new species. Since the structure and function of DNA was discovered it has been expected that the more closely species are related the more alike their DNA ought to be. In fact this has been used as the basis for determining the relationship of one species to another.
However, this article claims that the descent of species is not exclusively vertical, which is what Darwin expected, but that species routinely swap genetic material with other species, often across huge taxonomic distances. Leaving aside the horizontal transfer of genes across microbes, which occurs to quite a large extent, it also seems to occur in plants and animals. Last year New Scientist reported that 40-50 percent of the human genome consists of DNA transferred horizontally. So some relationships are tree-like but others are more complex, more like a spider's web.
I do not think this reflects badly on Darwin as his work was mainly in the macroscopic world and DNA was unknown in his time. However, it is clear that the light (of evolution) is getting brighter as the day draws near.
