Lamarck, Darwin and the Tree of Life

There has been some interesting discussion in the blogosphere as to whether Lamarck beat Darwin to the Tree of Life metaphor:

• http://scienceblogs.com/geneticfuture/2009/08/lamarck_beat_darwin_to_the_tre.php
  
• http://www.genetic-inference.co.uk/blog/?p=505
  

I am due to speak on this subject at the upcoming Society for General Microbiology meeting in Edinburgh, so I was intrigued to read these posts.

Here is my own quick-fire rather disorganised contribution to the argument, quickly cribbed off my Powerpoint slides, with a shameless inattention to sources:

It seems that Pallas did indeed come up with this metaphor in words before Lamarck and Darwin, even though he didn't draw a tree. This is what he wrote in 

Elenchus Zoophytorum (1766):

At omnium optime Arboris imagine adumbraretur Corporum organicorum Systema, quae a radice statim, e simplicissimis plantis atque ani- malibus duplicem, varie contiguum proferat truncum, Animalem & Vegetabilem; Quorum prior, per Mollusca pergat ad Pisces, emisso magno inter haec Insectorum laterali ramo, hinc ad Amphibia; & extremo cacumine Quadrupedia sustineret, Aves vero pro laterali pariter magno ramo infra Quadrupedia exsereret.

“But the system of organic bodies is best of all represented by an image of a tree which immediately from the root would lead forth out of the most simple plants and animals a double, variously contiguous animal and vegetable trunk; the first of which would proceed from molluscs to fishes, with a large side branch of insects sent out between these, hence to amphibians and at the farthest tip it would sustain the quadrupeds, but below the quadrupeds it would put forth birds as an equally large side branch.”

Augustin Augier in 1801 deserves credit for this description of a phylogenetic tree:

‘‘A figure like a genealogical tree appears to be the most proper to grasp the order and gradation of the series or branches which form classes or families. This figure, which I call a botanical tree, shows the agreements which the different series of plants maintain amongst each other, although detaching themselves from the trunk; just as a genealogical tree shows the order in which different branches of the same family came from the stem to which they owe their origin.’

see http://www.bio.sdsu.edu/faculty/archibald/Archibald08JHBonline.pdf for a depiction of Augier's tree. In fact, this article in general is an excellent source of information on the whole issue of evolutionary trees and in particular draws attention to a paleontological chart from 1840 that looks a lot like an evolutionary tree.

Chambers in his Vestiges of the Natural History of Creation has something that looks a bit like an evolutionary tree, although it is probably best described as a developmental tree. Follow this link: http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=A2&pageseq=215

Note that Darwin's first depiction of an evolutionary tree is arguably not the oft-quoted "I think" figure, but these "coral of life" depiction that appears a few pages earlier in his notebook.

http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=CUL-DAR121.-&pageseq=27

And finally, Wallace hit on the metaphor independently. One can dispute whether his tree-like diagrams on the classification of birds really count as evolutionary trees, but his verbal description in the article he published in 1855 clearly pre-date Darwin's published descriptions of the tree of life and add the appealing analogy of the human vascular system:

On the law which has regulated the Introduction of New Species. Annals and Magazine of Natural History, including Zoology, Botany, and Geology 16: (September): 184-196.
“We are also made aware of the difficulty of arriving at a true classification, even in a small and perfect group;—in the actual state of nature it is almost impossible, the species being so numerous and the modifications of form and structure so varied, arising probably from the immense number of species which have served as antitypes for the existing species, and thus produced a complicated branching of the lines of affinity, as intricate as the twigs of a gnarled oak or the vascular system of the human body. Again, if we consider that we have only fragments of this vast system, the stem and main branches being represented by extinct species of which we have no knowledge, while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have to place in order, and determine the true position each originally occupied with regard to the others, the whole difficulty of the true Natural System of classification becomes apparent to us.”

Interestingly, Wallace is now buried under a huge fossilised tree:

 [NB we all know it is really a huge fossil phallus ;-) ]

But the last word goes to Darwin, with his melodic prose:

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth... As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications. 

And for those of you who like reggae, try those "beautifiul ramifactions" Jamaican-styleee...

Of trees of life and straw men

I am trying to write a grant proposal, but cannot help but get distracted by all this discussion (stemming from a recent new Scientist article) about Darwin supposedly being wrong about the tree of life. All the new evidence about the role of horizontal gene transfer in bacteria and even in unicellular eukaryotes is of course all very interesting. But to try and depict this as a Darwin-was-wrong argument strikes me as misleading and risks casting Darwin into the role of a straw man. In fact, Darwin was ignorant of the existence of bacteria until late in his life and as far as I am aware never commented on their evolution. 

But more than that, he never said any so simplistic as "all life follows a tree-like pattern of evolution right back its origin and that's that!". 

The first point to note is that Darwin was always quick to point out the caveats and counter-arguments of any given proposition. Secondly, his discussion in The Origin of the tree of life sets the "universe of discourse" at the taxonomic level of Class, rather than universally applying to all life. 

Here is what he actually wrote, with my emphasis added:

From Chapter 4

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth... As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications. 

(And even Dawkins can't top the elegance of Darwin's prose here!)

[Link]

From the final chapter:

Therefore I cannot doubt that the theory of descent with modification embraces all the members of the same class. I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype. But analogy may be a deceitful guide.
[link]

Darwin's wasn't wrong about the tree of life—he accepted that the evidence before him was limited and it was unsafe to generalise it to the whole of life. Give the poor man--or should that be "straw man"--a break!

Of figs and fusions: Darwin's great tree of life rehabilitated?

In one of his notebooks, Darwin sketched an iconic figure of the tree of life, prefaced by a pregnant "I Think" (see adjacent image). Darwin also wrote some of his finest prose about the tree of life in the Origin of Species:

"The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth... As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications."

But in recent years there has been a great deal of fuss about whether it is still acceptable to talk about a tree of life and whether we should instead be talking about a ring of life or net of life, because there appear to have been many fusions between different lineages, particularly early in life's history, in addition to the lineage splittings that underlie a traditional branching phylogeny.

And, yeah, I get the point, if your idea of a tree is similar to the  tree drawn by Darwin's associate Haeckel, a boring old deciduous tree from northern Europe or North America, with a single trunk and no fusions between trunks or branches.

But for me the tree simile still "largely speaks the truth"--you just have to broaden your idea of what a tree looks like! 

Here are some images of a fig tree that I snapped while on a recent holiday in Sicily. Here you can see what appear to be rampant fusions between trunks and branches*. To me this suggests that Darwin's simile is fine, so long as you think fig tree, not oak tree!

*OK, I realise that in fact these apparent fusions between branches are the result of aerial roots arising from branches that then grow downloads, but for a simile to work, it doesn't need to be accurate in every detail!