On Lamarck and Trees

Over at Genetic Future, Daniel MacArthur quotes Joel Parker berating as ‘embarrassing’ biologists who claim that it was Darwin, and not Lamarck, who came up with the idea of an evolutionary tree:

I have noticed many evolutionary biologists making an embarrassing mistake of falsely attributing the first use of the tree analogy to Darwin. This has occurred in numerous documentaries and on websites which I will pass on naming here. Ironically, the earliest use of the tree analogy diagram to depict evolution was published in the year of Darwin’s birth (1809) by Lamarck in his book Philosophie Zoologique (see pg 463, http://tinyurl.com/knt7vr). Lamarck even uses botanical terms (branches and rameaux) to describe the origin of animals with respect to this figure. The figure that is usually cited from Darwin’s notebook is from 1837 (http://tinyurl.com/6hs5uv), a full 8 years after Lamarck’s death. Even with our high admiration for Darwin, we should at least give credit where credit is due, and not forget that much of evolution was becoming understood before Darwin. Explaining the mechanism of natural selection was Darwin’s great contribution.

This is actually largely correct; Lamarck did have a view of evolution that involved what we would now call evolutionary branching, though it was very different from what we now know to be the case. Lamarck deserves to be read and understood as one of the first people to put together a coherent view of evolution.

However, the statement is very wrong in a number of ways. It is far from a mistake to refer to Darwin as the originator of the evolutionary tree, and those of us who do so do so not out of ignorance.

The Unified Tree Metaphor

Darwin did not invent the concept of an evolutionary tree, exactly. What he did was combine two different tree metaphors into one; a descent or pedigree tree, and a taxonomic tree.

The first is medieval or older: and a tree of descent or family tree, showed how a powerful family were related together; in this metaphor, descendants ‘branch out’ from their ancestors. Perhaps the oldest known use of this metaphor is in the bible, the book of Issiah, which describes how the Messiah will descend from Jesse, father of David:

There shall come forth a shoot from the stump of Jesse, and a branch shall grow out of his roots. (Issiah 11:1)

This lead to many medieval illustrations depicting the line from Jesse to David as a tree, with Jesse as the trunk and Jesus as the highest leaf.

Written pedigrees, tracing the descent of horses, were used from the 13th Century onwards to track an animal’s ancestry; showing that a horse was purely bred from good stock was important in selling them. In this case, the trunk may be a fine racehorse that all the purebreds are descended from, and each branch is another good horse from which a smaller sub-group of purebreds descend from.

The second type of tree metaphor is that of a taxonomic tree; a way of depicting a classification of living things; the trunk may be ‘animals’, and from that will shoot off a number of larger branches (vertebrates, arthropods, mollusks), and each branch splits into smaller branches (vertebrates splits into fish, reptiles, birds, mammals etc), and so on. According to John Wilkins, this goes back to the 18th Century, from Peter Simon Pallas’ Miscellanea zoologica Elenchus Zoophytorum in 1766.

It was Darwin, as both a taxonomist and an animal breeder, that unified these two concepts as one and the same: mammals and birds fit together on the same branch of ‘Amniota’, because they are both descended from an actual amniote common ancestor. Thus, the evolutionary tree is only new in that generalises and unifies two different tree concepts.

This unified tree concept was a big deal, and something Lamarck never came close to. He did have a concept of a tree (of sorts, see below), but then so did Pallas, and so did Issiah. There are other individuals who had inkling of common descent (notably Darwin’s grandfather, Erasmus), and people had speculated about it for a while. But Darwin was the father of the evolutionary tree; what other people had were guesses and speculations, what Darwin had was a fleshed out, fully argued and fully evidenced unification. I do not buy Joel’s idea that Natural Selection was Darwin’s major discovery: while of course it was important, to me it was the tree, and always will be the tree, that was Darwin’s major work.

Branching

I would even challenge that Lamarck really used much of a ‘tree’ metaphor at all. I think he mostly just inherited tree language from other individuals, without the ‘branching tree’ insight really adding much to his thinking. Joel’s evidence, that Lamarck talked about ‘branches’, does not mean that Lamarck is using it in any particularly insightful way.

For instance, Lamarck does use ‘branch’ when he talks about groups that diverge evolutionarily:

[aquatic animals] were divided into three branches by reason of the diversity arising in their habits

But he also uses it to talk about groups that he believes come into being separately, and never experience evolutionary divergence:

The series of animals begins with two branches where the most imperfect animals are found

Note that Cuvier, who never believed in anything approximating evolutionary divergence, used the term “embranchments” to describe the 4 major groups of animals. It seems clear to me that both Lamarck and Cuvier are using this ‘branch’ in the same way we’d talk about the local branch of our bank; it was a common term used to describe groups of animals, and does not have any deeper conceptual connotations of the branching evolutionary tree that Darwin put forward.

What Lamarck did think

Lamarck’s view, however, was not simple, it was not stupid, and it was a perfectly valid evolutionary theory that deserves to be acknowledged as important. Lamarck did believe in genuine phyletic branching; he had definite ideas of how a large group can split into two.

He had a relatively accurate idea of speciation, in which a number of individuals from of a species will migrate elsewhere, and the new environment causes new adaptations to develop. However, he only mentioned this in passing, and did not consider it an important factor in creating new species (he believed that most new species came about as the result of spontaneous generation).

The diagram that Joel refers to (which you can see here) from Philosophie Zoologique does indeed show how Lamarck believed that both aquatic and land-dwelling mammals evolved from reptiles, and in the text he discusses how the the split occurred depending on whether the reptiles were in water or on land while they increasing their perfection.

But what is missing is any idea of actual common ancestry; each whale and cow species evolved from a reptile species, but each species was the result of a separate upward trajectory along the branching path, rather than any true family, or evolutionary, tree.

More Information

For more information on what Lamarck really believed, I can strongly recommend Enst Mayr’s essay Lamarck Revisited.

Edit: John Wilkins has a detailed post about the first use of the taxanomic tree metaphor. Also, he was responsible for the correction above (turned out Pallas wrote more than one thing in 1766!).

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5 Responses to On Lamarck and Trees

  1. People attribute common descent to Darwin primarily because they hate Darwin and Darwinism, for Darwin and Darwinism is natural selection, a doctrine whose implications are disturbingly brutal, and when applied to humans, horrifyingly politically incorrect. So they pick up something else, almost at random, and call it Darwinism.

    It is perfectly clear, and not an all controversial, that Lamark and earlier thinkers proposed the tree of life – that animal species were related through common ancestors, and that the seeming gaps were the result of extinctions.

    If he proposed multiple parallel origins, he would not have needed to explain the gaps. That he explained gaps as due to extinctions, makes it clear his tree is intended as a history, as development over time from a single root, or a very small number of roots, one root for all veterbrates, as a history, not as a scheme of classification.

    Darwin’s contribution, as he makes perfectly clear, was natural selection. If you read Darwin, he does not treat common descent as a big new idea, but as something that is widely suspected, something that has long been in the air. Darwin tells us what his big new ideas are:

    Darwin’s big new idea was natural selection, and all of the important and violently controversial ideas that follow from natural selection, such as sexual selection resulting in sex roles and sexually specific behavior, sociobiology resulting in an innate sense of property and intuitions about rights, races as the precursor of speciation, and so on and so forth.

    If a idea about biology can get you beaten up and denied tenure, it is Darwinism. If it cannot, it is not. Common descent will not get you into trouble, therefore is not Darwinism.

  2. @James A. Donald

    Wow that’s some pretty incoherent stuff you’ve got there. Biologists are denied tenure for believing in Natural Selection? Lamarck believed in common descent? Darwin did not treat common descent (‘like confessing to a murder’) as a big new idea? Sex roles follow from sexual selection?

    I see you are an adherent of the “making stuff up” school of argument.

  3. whos lamarck and linnaeus?

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