The tree of life, it seems, is no more than a first approximation, and must be supplanted by the web of life.
This week's New Scientist contains a fascinating article which explains that, due to horizontal gene transfer, the evolution of life does not necessarily possess a tree-like, branching structure, but can also contain horizontal links, when different species exchange genetic material, and loops when two different species coalesce. The structure of living species, then, is not so much dendritic as reticulate.
Horizontal gene transfer (HGT) applies primarily to microbial species, rather than multicellular eukaryotic species such as plants and animals. Whilst microbes undergo self-reproduction rather than sexual reproduction, they are capable of exchanging genetic material during their individual lifetimes, which can be incorporated into their genomes, and transferred to their descendents through self-reproduction. It's been known for a while that microbial evolution is strongly influenced by HGT, but it was thought that this web-like structure was confined to the base of the tree of life. In multicellular eukaryotes, HGT appears to be difficult because the germ cells are separate and protected; even if a virus infects a subsystem of the body, and inserts its genetic material into the cells there, that genetic material will only affect the function of the subsystem, and will not be inherited by the descendants of the multicellular organism. The New Scientist article, however, suggests that there is evidence of prolific HGT amongst plants and animals.
This is interesting in itself, but immediately made me wonder if HGT has a cosmological analogue. If we combine braneworld cosmology and cosmological natural selection, then we obtain a scenario in which self-reproducing universes not only evolve by natural selection, but in which they collide and interact. Perhaps such interactions would permit physical information to be exchanged between different universes, altering the parameters of physics in the colliding universes. Lee Smolin's proposal for cosmological natural selection suggests only that the parameters of physics undergo random mutations when a new universe is born inside a black hole. Whilst Smolin's scenario produces a tree-like structure to the population of evolving universes, what we might dub horizontal cosmogenic transfer (HCT) produces more of a web-like structure.
Horizontal Cosmogenic Transfer Horizontal Gene Transfer Tree of Life Web of Life