Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Thursday, August 25, 2005

The tapestry of life

There's to be a colloquium on what is technically called "anastomosing evolution", or "lateral transfer", but which is better expressed as the "tapestry of life" here. This is the idea, nicely summed up in an image on Carl Zimmer's Loom, of the entanglement of the tree across branches, formed by lateral genetic transfer, hybridisation, endosymbiotic capture (where parasites and food organisms end up as part of the host organism and are reproduced along with it - our intracellular organelles, such as the mitochondrion, are examples), and so on.

The evolutionary tree metaphor still mostly holds. At the organism level, apart from the hybrids that form massively in plants and occasionally in animals and who knows how often in fungi, bacteria, algae and so on, species tend to split, and so larger scale groups tend to diverge over time. But we are finding, as is often the case in learning something new, that what we had down as the basic pattern of life is not all that neat.

Another example of Hull's Law: There is nothing so strange that some organism somewhere doesn't do it. Intuitions are unreliable, and teaching models are just for teaching...

I love the word "anastomosing". It is applied to the tiny capillaries in your body that turn red blood into blue blood that gets sent back to the lungs for reoxygenation. Phylogeneticists also use the terms "reticulation", "tokogeny" (for recombination at the sexual level), and "polytomy" for when three or more species cannot be distinguished in terms of branching order. Hybridisation causes polytomies ("many cuts") because the historical information about order is lost, although we cna perhaps retrieve it by using gene trees instead of taxon trees.

It has taken quite a while for reticulating (from retarius, Latin for net) evolution to be taken seriously, although everyone has known about hybridisation since Aristotle. Even Linnaeus recognised it, the famous Peloria case. So this is an interesting area to keep an eye on.