There’s no such thing as a tree (phylogenetically)

So you’ve heard about how fish aren’t a monophyletic group? You’ve heard about carcinization, the process by which ocean arthropods convergently evolve into crabs? You say you get it now? Sit down. Sit down. Shut up. Listen. You don’t know nothing yet.

“Trees” are not a coherent phylogenetic category. On the evolutionary tree of plants, trees are regularly interspersed with things that are absolutely, 100% not trees. This means that, for instance, either:

  • The common ancestor of a maple and a mulberry tree was not a tree.
  • The common ancestor of a stinging nettle and a strawberry plant was a tree.
  • And this is true for most trees or non-trees that you can think of.

I thought I had a pretty good guess at this, but the situation is far worse than I could have imagined.

CLICK TO EXPAND. Partial phylogenetic tree of various plants. TL;DR: Tan is definitely, 100% trees. Yellow is tree-like. Green is 100% not a tree. Sourced mostly from Wikipedia.

I learned after making this chart that tree ferns exist (h/t seebs), which I think just emphasizes my point further. Also, h/t kithpendragon on LW for suggestions on increasing accessibility of the graph.

Why do trees keep happening?

First, what is a tree? It’s a big long-lived self-supporting plant with leaves and wood.

Also of interest to us are the non-tree “woody plants”, like lianas (thick woody vines) and shrubs. They’re not trees, but at least to me, it’s relatively apparent how a tree could evolve into a shrub, or vice-versa. The confusing part is a tree evolving into a dandelion. (Or vice-versa.)

Wood, as you may have guessed by now, is also not a clear phyletic category. But it’s a reasonable category – a lignin-dense structure, usually that grows from the exterior and that forms a pretty readily identifiable material when separated from the tree. (…Okay, not the most explainable, but you know wood? You know when you hold something in your hand, and it’s made of wood, and you can tell that? Yeah, that thing.)

All plants have lignin and cellulose as structural elements – wood is plant matter that is dense with both of these.

Botanists don’t seem to think it only could have gone one way – for instance, the common ancestor of flowering plants is theorized to have been woody. But we also have pretty clear evidence of recent evolution of woodiness – say, a new plant arrives on a relatively barren island, and some of the offspring of that plant becomes treelike. Of plants native to the Canary Islands, wood independently evolved at least 38 times!

One relevant factor is that all woody plants do, in a sense, begin life as herbaceous plants – by and large, a tree sprout shares a lot of properties with any herbaceous plant. Indeed, botanists call this kind of fleshy, soft growth from the center that elongates a plant “primary growth”, and the later growth from towards the outside which causes a plant to thicken is “secondary growth.” In a woody plant, secondary growth also means growing wood and bark – but other plants sometimes do secondary growth as well, like potatoes in their roots.

This paper addresses the question. I don’t understand a lot of the closely genetic details, but my impression of its thesis is that: Analysis of convergently-evolved woody plants show that the genes for secondary woody growth are similar to primary growth in plants that don’t do any secondary growth – even in unrelated plants. And woody growth is an adaption of secondary growth. To abstract a little more, there is a common and useful structure in herbaceous plants that, when slightly tweaked, “dendronizes” them into woody plants.

Dendronization – Evolving into a tree-like morphology. (In the style of “carcinization“.) From ‘dendro‘, the ancient Greek root for tree.

Can this be tested? Yep – knock out a couple of genes that control flower development and change the light levels to mimic summer, and researchers found that Arabidopsis rock cress, a distinctly herbaceous plant used as a model organism – grows a woody stem never otherwise seen in the species.

The tree-like woody stem (e) and morphology (f, left) of the gene-altered Aridopsis, compared to its distinctly non-tree-like normal form (f, right.) Images from Melzer, Siegbert, et al. “Flowering-time genes modulate meristem determinacy and growth form in Arabidopsis thaliana.” Nature genetics 40.12 (2008): 1489-1492.

So not only can wood develop relatively easily in an herbal plant, it can come from messing with some of the genes that regulate annual behavior – an herby plant’s usual lifecycle of reproducing in warm weather, dying off in cool weather. So that gets us two properties of trees at once: woodiness, and being long-lived. It’s still a far cry from turning a plant into a tree, but also, it’s really not that far.

To look at it another way, as Andrew T. Groover put it:

“Obviously, in the search for which genes make a tree versus a herbaceous plant, it would be folly to look for genes present in poplar and absent in Arabidopsis. More likely, tree forms reflect differences in expression of a similar suite of genes to those found in herbaceous relatives.”

So: There are no unique “tree” genes. It’s just a different expression of genes that plants already use. Analogously, you can make a cake with flour, sugar, eggs, sugar, butter, and vanilla. You can also make frosting with sugar, butter, and vanilla – a subset of the ingredients you already have, but in different ratios and use.

But again, the reverse also happens – a tree needs to do both primary and secondary growth, so it’s relatively easy for a tree lineage to drop the “secondary” growth stage and remain an herb for its whole lifespan, thus “poaizating.” As stated above, it’s hypothesized that the earliest angiosperms were woody, some of which would have lost that in become the most familiar herbaceous plants today. There are also some plants like cassytha and mistletoe, herbaceous plants from tree-heavy lineages, who are both parasitic plants that grow on a host tree. Knowing absolutely nothing about the evolution of these lineages, I think it’s reasonable to speculate that they each came from a tree-like ancestor but poaized to become parasites. (Evolution is very fond of parasites.)

Poaization: Evolving into an herbaceous morphology. From ‘poai‘, ancient Greek term from Theophrastus defining herbaceous plants (“Theophrastus on Herbals and Herbal Remedies”).

(I apologize to anyone I’ve ever complained to about jargon proliferation in rationalist-diaspora blog posts.)

The trend of staying in an earlier stage of development is also called neotenizing. Axolotls are an example in animals – they resemble the juvenile stages of the closely-related tiger salamander. Did you know very rarely, or when exposed to hormone-affecting substances, axolotls “grow up” into something that looks a lot like a tiger salamander? Not unlike the gene-altered Arabidopsis.

A normal axolotl (left) vs. a spontaneously-metamorphosed “adult” axolotl (right.)

[Photo of normal axolotl from By th1098 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=30918973. Photo of metamorphosed axolotl from deleted reddit user, via this thread: https://www.reddit.com/r/Eyebleach/comments/etg7i6/this_is_itzi_he_is_a_morphed_axolotl_no_thats_not/ ]

Does this mean anything?

A friend asked why I was so interested in this finding about trees evolving convergently. To me, it’s that a tree is such a familiar, everyday thing. You know birds? Imagine if actually there were amphibian birds and mammal birds and insect birds flying all around, and they all looked pretty much the same – feathers, beaks, little claw feet, the lot. You had to be a real bird expert to be able to tell an insect bird from a mammal bird. Also, most people don’t know that there isn’t just one kind of “bird”. That’s what’s going on with trees.


I was also interested in culinary applications of this knowledge. You know people who get all excited about “don’t you know a tomato is a fruit?” or “a blueberry isn’t really a berry?” I was one once, it’s okay. Listen, forget all of that.

There is a kind of botanical definition of a fruit and a berry, talking about which parts of common plant anatomy and reproduction the structure in question is derived from, but they’re definitely not related to the culinary or common understandings. (An apple, arguably the most central fruit of all to many people, is not truly a botanical fruit either).

Let me be very clear here – mostly, this is not what biologists like to say. When we say a bird is a dinosaur, we mean that a bird and a T. rex share a common ancestor that had recognizably dinosaur-ish properties, and that we can generally point to some of those properties in the bird as well – feathers, bone structure, whatever. You can analogize this to similar statements you may have heard – “a whale is a mammal”, “a spider is not an insect”, “a hyena is a feline”…

But this is not what’s happening with fruit. Most “fruits” or “berries” are not descended from a common “fruit” or “berry” ancestor. Citrus fruits are all derived from a common fruit, and so are apples and pears, and plums and apricots – but an apple and an orange, or a fig and a peach, do not share a fruit ancestor.

Instead of trying to get uppity about this, may I recommend the following:

  • Acknowledge that all of our categories are weird and a little arbitrary
  • Look wistfully of pictures of Welwitschia
  • Send a fruit basket to your local botanist/plant evolutionary biologist for putting up with this, or become one yourself
While natural selection is commonly thought to simply be an ongoing process with no “goals” or “end points”, most scientists believe that life peaked at Welwitschia.

[Photo from By Sara&Joachim on Flickr – Flickr, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=6342924 ]

Some more interesting findings:

  • A mulberry (left) is not related to a blackberry (right). They just… both did that.
[ Mulberry photo by Cwambier – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=63402150. Blackberry photo by By Ragesoss – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4496657. ]
  • Avocado and cinnamon are from fairly closely-related tree species.
  • It’s possible that the last common ancestor between an apple and a peach was not even a tree.
  • Of special interest to my Pacific Northwest readers, the Seattle neighborhood of Magnolia is misnamed after the local madrona tree, which Europeans confused with the (similar-looking) magnolia. In reality, these two species are only very distantly related. (You can find them both on the chart to see exactly how far apart they are.)
  • None of [cactuses, aloe vera, jade plants, snake plants, and the succulent I grew up knowing as “hens and chicks”] are related to each other.
  • Rubus is the genus that contains raspberries, blackberries, dewberries, salmonberries… that kind of thing. (Remember, a genus is the category just above a species – which is kind of a made-up distinction, but suffice to say, this is a closely-related groups of plants.) Some of its members have 14 chromosomes. Some of its members have 98 chromosomes.
  • Seriously, I’m going to hand $20 in cash to the next plant taxonomy expert I meet in person. God knows bacteriologists and zoologists don’t have to deal with this.

And I have one more unanswered question. There doesn’t seem to be a strong tend of plants evolving into grasses, despite the fact that grasses are quite successful and seem kind of like the most anatomically simple plant there could be – root, big leaf, little flower, you’re good to go. But most grass-like plants are in the same group. Why don’t more plants evolve towards the “grass” strategy?


Let’s get personal for a moment. One of my philosophical takeaways from this project is, of course, “convergent evolution is a hell of a drug.” A second is something like “taxonomy is not automatically a great category for regular usage.” Phylogenetics are absolutely fascinating, and I do wish people understood them better, and probably “there’s no such thing as a fish” is a good meme to have around because most people do not realize that they’re genetically closer to a tuna than a tuna is to a shark – and “no such thing as a fish” invites that inquiry.

(You can, at least, say that a tree is a strategy. Wood is a strategy. Fruit is a strategy. A fish is also a strategy.)

At the same time, I have this vision in my mind of a clever person who takes this meandering essay of mine and goes around saying “did you know there’s no such thing as wood?” And they’d be kind of right.

But at the same time, insisting that “wood” is not a useful or comprehensible category would be the most fascinatingly obnoxious rhetorical move. Just the pinnacle of choosing the interestingly abstract over the practical whole. A perfect instance of missing the forest for – uh, the forest for …

… Forget it.


Related:

Timeless Slate Star Codex / Astral Codex Ten piece: The categories were made for man, not man for the categories.

Towards the end of writing this piece, I found that actual botanist Dan Ridley-Ellis made a tweet thread about this topic in 2019. See that for more like this from someone who knows what they’re talking about.

For more outraged plant content, I really enjoy both Botany Shitposts (tumblr) and Crime Pays But Botany Doesn’t (youtube.)

[Crossposted to Lesswrong.]

35 thoughts on “There’s no such thing as a tree (phylogenetically)

  1. Rahkasa

    Read this on LessWrong, absolutely delightful post! I’m intrigued by the Aridopsis that became a tree and jealous of the researchers. Do you know how long it survived and if this can be done with most/all herbaceous plants? The paper is behind a paywall. I wish I could dendronize all my favourite plants…

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    1. mskvarla36

      Couple corrections to the first figure:

      – bananas aren’t trees as the stems aren’t woody and die back after fruiting. At 40′ tall, they’re some of the tallest herbaceous plants in the world

      – there arw a whole host of extinct tree-like ferns, horsetails , lycopods, and other early diverging plants. As it’s drawn, the phylogeny suggests all ferns are herbaceous

      The tree that really drove home the idea that trees aren’t a monophyletic group for me was Dendrosicyos socotranus, which is the only Cucurbitaceae (cucumbers, squash, etc) to have a tree-like growth form. Being familiar with cucumbers, it’s so odd to see cucumber-like leaves on a tree. Also a great example of island dendronization.

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  2. SE

    All fruit does share a fruit ancestor! Angiosperms produce fruit by definition. It might just not look like what we call fruit. The winged seeds of maples are fruit, for example. So are acorns. (Also, there are no trees/woody monocots. A few more peccadillos, but those are the big ones. But I think your writing style is very engaging and I’m refreshed by your enthusiasm for systematics.)

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    1. Brooks Moses

      Whether the statement that “there are no trees / woody monocots” is true depends on whether one considers palm trees to be trees, or bamboo to be woody. That feels somewhat like splitting hairs, since palm trees are definitely trees in a landscaping sense, and bamboo is a pretty common wood for several construction purposes such as flooring, spoons, and cutting boards.

      On the other hand, it is true that palm trees don’t usually produce a material that can be used like wood (trying to cut a palm trunk is, itself, a bit like splitting tightly-packed hairs), and bamboo plants seem rather an edge case of “treelike”, so monocots that are both trees and woody are somewhat rare, and they’re not what we think of as “normal” instances of the category.

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      1. SE

        Sorry, I meant that they are not woody in the botanical sense. They don’t have ‘real’ secondary growth the way that non-monocots do. I was aiming for botanical accuracy. There is a botanical definition of wood, so I don’t think it’s ‘splitting hairs’ to point that out.

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  4. peterisp

    The comment “the common ancestor of flowering plants is theorized to have been woody.” stuck with me as a potential explanation – essentially, perhaps “the mechanism of treeness” is something that all/most flowering plants have, it’s just turned off/regulated away and not expressed, so there’s an easy path for evolution to turn it back on in favorable circumstances instead of re-developing all the required gene functionality.

    I.e. the question is if really “on Canary Islands, wood independently evolved at least 38 times!” or it was “re-evolution” (which is substantially different from independent convergent evolution) where there are 38 instances when plants floated to an environment where a mutation knocking out one or few “woodiness development inhibitors” was beneficial, so it happened.

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  6. Dan Warren

    “Send a fruit basket to your local botanist/plant evolutionary biologist for putting up with this, or become one yourself”

    How does one become a fruit basket? Asking for a friend.

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  7. Benoît Côté

    Grammar: “in becoming”? —> “some of which would have lost that in become the most familiar herbaceous plants today”

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    1. David Marjanović

      Probably “and become”. Native speakers generally pronounce “in” and “and” as just “n”.

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  8. Edgar Mauricio Lemos

    Wait wait… why are “whales are mammals” and “spiders are not insects” grouped together with “hyenas are felines”??? As far as I know, the first two statements are true and the last is false… what am I missing?

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  10. David Marjanović

    Best description of Amborella I have yet seen.

    Hyenas aren’t felines, or even just felids, but they are feliforms, i.e. closer to cats than to dogs (or bears, weasels, seals, skunks, red pandas…).

    Based on the way Greek grammar works, I vote for “dendrization” and “poization”. Poaceae are the grasses, though.

    For an actually anatomically simple plant, try duckweed.

    mammal birds

    Well, bats. They’re birds in the Bible…

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    1. David Marjanović

      More things I forgot:

      There are mistletoe trees in Australia. They parasitize water from the roots of other trees, but nothing else.

      Prototaxites was probably not tree-shaped at all. It seems to be a misinterpretation of rolled-up liverwort mats, from a time when liverwort mats covered the land.

      First, what is a tree? It’s a big long-lived self-supporting plant with leaves and wood.

      Pine trees don’t have leaves! Their needles are twigs (“cladodia”). Cut one through, put it under a microscope, and you’ll see it immediately.

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  11. Emilio Río Rodríguez

    This is somewhat similar to what birds do; let’s see:

    -Other than emus and cassowaries (we don’t quite know if emus are gracile grassland cassowaries, or if cassowaries are stocky forest emus; they are descendant of animals akin to Emuarius tho), ratites are all convergent. They lost flight and evolved into cursorial large forms independently of each other; tinamous are deeply nested within them, and as a rule, no ratite/paleognath clade is the sister group of its closest geographic relative.

    -Grebes and flamingos form a very solid group. Loons are the closest relatives of a pair of groups: One formed by tubenoses and penguins, and the other formed by storks, pelicans and their relatives (more on that later). Cranes and rails are likely allied with plovers (which include gulls, which in turn include auks). Neither of those 3 groups is particularly close to each other.

    – Herons, ibises, and specially the shoebill and hamercop storks, are more closely related to pelicans than to storks. In fact, they are more closely related to pelicans than what were understood as pelican relatives (cormorants and shags, gannets and boobies, frigatebirds and darters).

    -Swifts are nested within what were undertood to be nightjar relatives, closely related to hummingbirds. Swallows are related to warblers. Very distantly related.

    -It is often said that falcons aren’t close to other raptors; it is true, falcons are related to parrots and songbirds (and seriemas slighty more distantly), while hawk-like raptors (incluiding New World vultures) and owls are subsequent outgroups of a lineage that leads to rollers, woodpeckers and allies. However, those 2 big groups are close to each other, and it is plausible that their common ancestor was raptorial, and the convergence happened between parrots & songbirds and rollers & woodpeckers, LOSING that raptorial lifestyle.

    -Speaking of raptors, vultures evolved 3 times. New World vultures ended up being close to hawk-like raptors after all, but, within the hawk-like raptors, the Old World vultures represent 2 groups: Griffons and the like are close to snake eagles, while the palm nut, Egyptian and bearded vulture are close to honey buzzards. And secretary birds of course are convergent with seriemas; no terror bird forms on the eagle side thus far tho.

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  12. jaynsand

    Bananas are trees? Speaking solely from a few years’ residence in South America, they grow a sturdy green woodless stalk that rots away after the bananas are harvested, sprouting a new stalk the next year. No permanent woody structure at all. What am I getting wrong?

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  15. warnimct

    Interesting take on the evolution of different plants from common ancestors and how gene expression can make a closely related plant look a lot different than it’s relative. The chicken/ dinosaur and closely related species is a good example of how this works in the animal world. Could scales and feathers be a difference of gene expression?

    Bananas aren’t a tree but are grasses and related to ginger. The fruit is also a berry.

    Pineapples are bromeliads and the fruit grows from the middle of the plant on a stalk (the spot on the bottom is where it is attached). Also a berry.

    Honeysuckle is a broad category and can be a tree/ shrub form whereas some are herbaceous vines.

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  21. sev

    There doesn’t seem to be a strong tend of plants evolving into grasses, despite the fact that grasses are quite successful and seem kind of like the most anatomically simple plant there could be – root, big leaf, little flower, you’re good to go. But most grass-like plants are in the same group. Why don’t more plants evolve towards the “grass” strategy?

    There are some grasslike plants not in Poaceae, for certain definitions of ‘grasslike’. Some are still in Poales, like rushes and cattails; others aren’t, like wild onions (Amaryllidaceae) and quillworts (which aren’t even angiosperms).

    But the trait that makes these plants “grasslike” isn’t just “root, big leaf, little flower”; to be grasslike, a plant has to look like a grass, which seems in some sense harder than looking like a tree. Clover, creeping phlox, chickweed, and ground ivy are like grasses in the sense that they’re groundcovers, but they don’t have the right leaf shape to be properly grasslike. Poaization stricto sensu seems more like “mapleization” than like dendrization; is there not a dendrization-like trend of plants evolving into groundcovers?

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  22. JAB

    So grasses have this ability to regenerate under constant grazing pressure from herbivores by having their growing tips protected. Without this amazing habit, we would not have had meat as part of human diet. If trees were grazed to the extant of grasses ( growing tips) they may not survive.

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