I have read an interesting article by Graham Lawton recently published in The New Scientist (21 January 2009): Why Darwin was wrong about the tree of life, in which it is suggested that a process called HGT (Horizontal Gene Transfer), initially seen as a minor phenomenon in species evolution, mainly affecting microbia and other simple forms of life, has turned out to be much more relevant than first thought, affecting other types of species and being identified as the cause of some important events in evolution. The exchange of genetic material between different species can explain many aspects of DNA variation that remain unexplained in the context of the traditional tree-of-life model. It is suggested that the whole process of biological evolution should be seen as a much more complex phenomenon: rather than the clear, straight lines of the typical genetic tree, it looks more like a web-structure. Some researchers think that the idea of genetic trees in biology should be abandoned; other researchers think that the tree model can still be used in biology, combining it with HGT. In any case, this current debate in biology resembles some of the things I have said about language trees and language change in this blog. Let's see an excerpt from Lawton's article, where he quotes John Dupré, a philosopher of science. These are Dupré's words: "Our standard model of evolution is under enormous pressure. We're clearly going to see evolution as much more about mergers and collaboration than change within isolated lineages". Something like this could also be said about historical linguistics. Some biologists even use the word hybridization to refer to some aspects of the new model. Note: in the picture you can see a drawing made by Charles Darwin in one of his notebooks (1838), which is actually the first depiction ever made of a "Tree of Life", an essential element in evolution theory. I have taken the picture from The Complete Work of Charles Darwin Online: link.