I'm trying to think something through. If it were a class, I'd make a handout with these quotes and pictures from Darwin's notebooks:
a tree not good simile—
endless piece of sea weed dividing
The tree of life should perhaps be called the coral of life,
base of branches dead
The point of the exercise would be to denaturalize the familiar "tree of life" image. If Darwin himself was thinking seaweed and coral, then perhaps we too should find ways of thinking about the evolution of different forms of life in a non-vertical way. But isn't the verticality key to what makes the image of the tree of life so compelling? A tree model connotes a unity and kinship in difference but it also suggests a sort of balance, around a central upward directionality, to the history of life. Darwin's sketches and other models suggest a different story, without center, direction or balance (and that his ultimate careful endorsement of the tree of life simile wasn't about those things).
A biological tree model of the history and shared descent of species actually makes no sense. Although there is a process of natural selection in a tree's growth - as Darwin noted in Origin, "buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch" - a tree's newer branches and buds are the same species as older ones. (Most trees also grow in all directions so there's no simple correlation between how high up a branch is and its relative age either.)
Phylogenetic trees, like decision trees, are just a convenient way of representing branching relationships. But they are grafted onto distracting older ideas about trees - ideas involving unity, verticality, centrality, fullness and balance, all of them value-laden categories.
Of all these, the history of life has just one: unity. This is a biggie, of course! But you could get unity from an image of seaweed or coral, too, and a more accurate sense of the contingency of the history of life and the inaccessibility of the shared past. Indeed, choosing a tree as your model seems a good way to hide from the contingency and the loss. Even if we insist on Darwin's sense that the living branches of a tree are only part of a story which also includes all the branches which have made way for them - a unity which includes everything selected for and against - might the value-laden baggage of tree forms not still make them an unsuitable model?
In terms of the history of life itself, a network makes more sense than a tree (or coral or seaweed), and can give us the unity. Must we give up on the other ideals? We're probably better off without centrality and verticality, temptations to renewed anthropocentrism, but what about fullness and balance? Here trees might provide a usable ideal (seaweed and corals might, too). The thicket of life needs to be understood as a whole, and one to which we can contribute positively or negatively...
These are issues I'm trying to sort through for my third chapter, which will move from Darwin to the wonderful world of tree diagrams to cosmic trees. The missing step is grafting, a practice which does get different species on the same tree (if not, perhaps, apples and oranges!), and which, I think, was presupposed by many pre-Darwinian uses of tree diagrams to show full and balanced relationships. Our forebears knew, as we may not, that the vicissitudes of life don't guarantee fullness, balance, or even continuity. Flourishing is an achievement, not just natural. We need to do our part, the way a family might keep itself going by adopting children, while also perhaps invoking the aid of other agencies.
On either side of the river is the tree of life
with its twelve kinds of fruit, producing its fruit each month,
and the leaves of the tree are for the healing of the nations.
(Rev. 22:2, NRSV)
