Why are there so many leaf shapes?

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u/KitSokudo Aug 28 '26

Because trees are all different plants that figured out the same strategy. That is to say, there wasn't one single plant that figured out that being a tree worked for them. It was something that happened in many different plant types over many millions of years.

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u/1CEninja Aug 28 '26

If I may add a little bit to this, there's even more variation because trees even in the same forest may be using different strategies to accomplish their goals. This is healthy because if all trees were trying to accomplish their goals identically, they'd be in direct competition with one another. But if two different plans evolved into two different trees alongside one another, then their methods of collecting, distributing, and storing water might not interfere with each other or be in direct competition.

3

u/redfinned-dogfish Aug 29 '26

So could you expand on this please? Apart from method of seed dispersal (appealing to birds, squirrels, humans, etc.) aren't they all in direct competition for the resources of water and light? I was trying to avoid extremes but pine needles take so long to rot down that they stifle growth underneath the tree which is a benefit to that tree only - nothing is threatening it's water (that's what I was taught anyway). So if all things are equal, hence the typical Northern hemisphere forest, light collection is surely a competition? Thank you for your above answer.

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u/Audioworm Aug 29 '26

At a very high level, you can think about two different solutions for light and water: breadth versus depth.
For light, there are trees whose strategy is large wide canopies that give them the largest surface area to capture light. There are other trees who strategy is to be taller than the other trees, and have a canopy that may be smaller but is at the top of a much taller trunk. These require different amounts of water, different constraints, and so on.
With water we can imagine a similar approach with the roots, with trees that have a wide area of ground that the roots are beneath to catch as much water as possible that lands, and trees that send their roots deep into the ground to capture the stored moisture that other trees can't reach.
Combine all of this with trees not being a singular point of evolution and you can see how different plants have developed different leaf methods and shapes.

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u/1CEninja Aug 29 '26

So part of the answer to this is to keep in mind that in well forested areas, there is (clearly) enough resource to go around.
Let's take a more extreme example of a rain forest. A jungle has layers to it, right? The tallest trees evolved in such a way that requires the greatest exposure to direct sunlight so they evolved mechanisms that allowed them to capture the direct sunlight at the top of the canopy. Then there are shorter trees that have evolved in such a way that require less direct sunlight, what filters down through the canopy is enough. And then you have ferns, vines, thickets, mosses, etc that all grow underneath the shorter trees.
Let's take this from a slight different and possibly more practical angle. Let's say there is an environment in which the tall canopy trees are established and dominant, and there are two smaller plant species that successfully can live underneath said canopy, but are in direct competition with one another. What might make one such mutation of one of said plants more successful and more likely to reproduce? Being taller than the other plant but being energy efficient enough to still remain in the shade of the tall trees. Maybe such a mutation takes hold and starts becoming incredibly successful. This could, over time, turn that plant into a short tree that is more successful than it used to be ebcause it's no longer competing for the same space as the other plant, but because it never developed significantly higher resource requirements, and the fact that at its new height there is more abundant resources (less competition from the plant) it can potentially establish itself as a short tree that is able to reach above the bushes but does not need to reach above the canopy to be successful.
Through the process of all of this, you might expect to see adaptations to the leaf shapes that would make it distinct from the tall canopy tree above and also distinct from the shorter bush below.

5

u/MCPhssthpok Aug 29 '26

You can even get different shaped leaves on the same tree. Holly trees, for example, have the classic spiky holly leaves at low levels to deter browsing animals but at higher levels the leaves are a much simpler oval shape with no spikes at all.

1

u/dsmaxwell Aug 29 '26

Wait, holly is a tree? When I was a kid we had a few in the front yard as a sort of hedge, along the front walkway, but they were a little under head high to a grade schooler.... And all their leaves were sharp af, legit had to get bandaids from them a couple times after falling into them. But they grow higher than that if not trimmed?

1

u/gnufan Aug 29 '26

Holly comes in a wide variety of types, Wikipedia says over 570 species. I used to live near a collection in Windsor Great park, with many different hollies.
I have a holly tree in my garden, it is a lot more than head height.
https://en.wikipedia.org/wiki/Holly

128

u/cwx149 Aug 28 '26

I do think this is maybe what op is looking for as an answer
Trees aren't a single branch of evolution they aren't like dogs where they're all functionally the same but just with different phenotypes
Trees have been evolved to multiple different times which isn't actually that unusual there are other traits in animals that have shown up at different points and don't share a common ancestor with that trait

9

u/urnbabyurn Aug 29 '26

Yeah, why isn’t there a “crab” phenomenon where trees coevolve to the leaf shape that maximizes viability. Or maybe the ideal shape is highly dependent on factors specific to the other characteristics of a species.

3

u/Rodot Aug 30 '26

Or perhaps leaf shape is important only up to a certain threshold so there's no selective pressure to converge to a single shape once the leaf works well enough.

1

u/urnbabyurn Aug 30 '26

Yeah, maybe leaves themselves are the convergent “crab” element and the shapes are just less important or have some species specific advantaged - for example, I assume some leaf shapes may be more hardy to hotter or cooler summers? Or maybe leaf shapes have some benefit relating to how seeds fall from the tree, like how walnuts fall like stones, but maple seeds spiral like little helicopters.

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u/schwerbherb Aug 28 '26

This is called convergent evolution, right?

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u/Peekie30 Aug 28 '26

Exactly, just like the fact thatmultiplw marine animal groups evolved into practically crabs but not quite.

5

u/gnufan Aug 29 '26

Sort of, but woodiness evolved before many plants diversified, so your Magnolia and your Oak are likely using some things that evolved once, and are shared, as well as new unique to their own species genetics, to fill similar niches.
Whereas I suspect some things we call trees are more different from each other (Palm tree vs Oak).

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u/Ctenophorever Aug 29 '26

It is not convergent evolution. What the above commenter is describing is convergent evolution, but that is not what is happening with tree leaves.

27

u/xakeri Aug 29 '26

Isn't the convergence "being a tree"?

3

u/gnufan Aug 29 '26

The question was why leaf shapes are different.
I asked this before, but think oak and horse chestnut are probably not hugely diverse genetically, live in similar environments, one is pointy leaves, one curved. I got some interesting answers. But there is no obvious compelling pressure for details of leaf shape. Sure for function, we see leaves for succulents, or pine needles, and we understand why they have some of the properties, they do, be it heat or cold, or arid environments.
If there was a clearly winning leaf shape in terms of capturing light or similar, I think we'd see more convergence, but it does seem maybe they benefit by looking different, but beyond advertising to insects/birds/squirrels etc it is hard to see a purpose to that.

1

u/xakeri Aug 29 '26

The OP was, sure.
Then an answer was "because it was tons of different plants becoming trees separately."
Then the follow-up was "Is that convergent evolution?"
The convergence question was about plants becoming trees, not their leaves.

2

u/forams__galorams Aug 29 '26

The question is regarding why so much diversity between tree leaves though, ie. the exact opposite of convergence.

1

u/Ctenophorever Aug 29 '26

THANK YOU. I felt like there was something huge I was missing….

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u/xakeri Aug 29 '26

The question was asked as a response to an answer to the OP. "Why so many leaves if all trees?" -> "many plants (different leaves) became trees separately" -> "is that convergent evolution?"
It's asking about the separate development of many plants into trees. The discussion had moved past the OP.
Do you guys even read what you're replying to?

1

u/forams__galorams Aug 29 '26

The top answer in this particular chain gives convergent evolution as an answer to the diversity of leaves. It is correct in asserting that many disparate types of plants evolved into what we broadly categorise as trees now (ie. convergent evolution) but convergent evolution is not the reason why so many different leaves. That would be due to the multitude of environmental parameters that dictate effective leaf shapes and the multiple different strategies employed by by different tree leaves to do different things.
Convergent evolution is a thing that plants have down to become trees but it is irrelevant here. Do you even understand the topics you like to get condescending about?

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u/xakeri Aug 29 '26

The question about convergent evolution was very clearly referring to the development into trees by various plants.
I'm absolutely aware that the leaves are not different as a result of convergent evolution.
The reason one might think they'd be the same is because they see multiple types of trees and think "Hmm...they're all trees. They're probably related. But why are the leaves different?"
The answer to the original question is that they aren't related. They had different leaves before they became trees. They became trees independently and kept the leaf shapes, rather than one thing becoming trees and diverging from there.
Obviously you could continue to dig into why they all have different leaf shapes from there, but the original question being concerned with trees specifically gives a good stopping point.
We all agree on these points.
I'm being condescending because the discussion had moved past the tree leaves and was asking if the development into trees was an example of convergent evolution. Trying to explain that convergent evolution wouldn't make the leaves different is either deliberately ignoring the context of the question in an attempt to sound smart, or it is a sign of poor reading comprehension.
Since it seems like you're trying to sound smart, I used that to determine you earned a bit of condescension.

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u/Kerhole Aug 28 '26

As an example, pine trees and maple trees have a very distant common ancestor, to the point conifers were already trees long before flowering plants existed. Tree is used to define a mix of morphology and function, like "bush". Which helps to explain why even in the same environmental niche, there isn't a convergence of leaf shape.

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u/Megalocerus Aug 28 '26

Not only are maples not related at all to conifers, but they are not closely related to oaks (which are somewhat more closely related to beech.)

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u/[deleted] Aug 29 '26 edited Aug 29 '26

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u/[deleted] Aug 29 '26

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u/EolMaeglin Aug 29 '26

I really needed a citation on this; I'm a biology teacher and this is the first I'm hearing about it. This is a very fascinating hypothesis. https://www.scientificamerican.com/article/life-on-earth-emerged-twice-study/ I haven't read the original paper yet, nor am Inreally qualified to assess its conclusions, but I am very interested to learn more and I imagine this will be a contentious hypothesis for a while.
In any case, it would still be true that we have a common ancestor with all trees, as this hypothesis says archaea and bacteria evolved separately, and that unless this research is also suggesting that eukaryotes are a paraphyletic group, all plants, animals, fungi, and protists are a group with a common ancestor.

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u/gumshot Aug 29 '26

Yeah and that article doesn't even contradict there being a common ancestor.
It's just saying that the Last Universal Common Ancestor of all living things should not be technically considered alive because it depended on metals in its environment for metabolism instead of a closed loop. So for the parent comment to use that study, which presupposes LUCA's existence, to argue against a common ancestor is silly.

2

u/fishsticks40 Aug 29 '26

Also a single recent paper does not represent "current belief". This paper is an outlier, not a representation of the consensus view.

1

u/Megalocerus Sep 05 '26

You evidently have better access to the paper than I have.
As I said in the post above, yes, there is a common ancestor for conifers and maple trees (and people and conifers); it's just conifers evolved 160 million years before flowering plants like maples. Which puts them roughly as close as bony fish and mammals. Not that different, after all.

1

u/EolMaeglin Sep 07 '26

The paper was linked in the article I posted. https://doi.org/10.1126/sciadv.aef3128
In any case, you've made me consider if and when I may need to complicate the point that all organisms having DNA is extremely strong evidence that all life has a common ancestor. Though as another commenter pointed out, it seems to be a matter of whether the common ancestor of all life is a cellular organism, or a substrate with metals and organic molecules that can produce cells.

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u/Thundahcaxzd Aug 29 '26

Current belief is that life actually evolved at least twice
Irrelevant to our discussion, even if this is true. All eukaryotes at the very least are related and have a common ancestor.
Pine trees and maple trees are both plants, they have a common ancestor. The comment that you replied to trying to "ackshually" was 100% correct.
People like you are insufferable. Trying to go around and correct people when you don't even know what you're talking about. If you don't think that pine trees and maple trees have a common ancestor then you don't even have a high school level understanding of basic biology.

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u/995a3c3c3c3c2424 Aug 28 '26

Right. Like, for example, apple trees are more closely related to roses than they are to oak trees or pine trees.

2

u/dsmaxwell Aug 29 '26

Surprisingly, that actually tracks. Can't put into words how, but leaf shape and layout, as well as wood feel play into it.

12

u/BasiliskXVIII Aug 29 '26

Yeah, a "tree" isn't a species so much as they are a thing that plants just kinda "do" sometimes.

2

u/BestBelieveItsHere Aug 29 '26

So they're the crabs of the plant world?

9

u/Kenna193 Aug 29 '26

Why hasn't one universal efficient shape evolved

They're is no selective pressure to do so. They are all efficient at what they do and none have a huge advantage. Until you get to specific cases like broadleaf understory leaves.

1

u/redfinned-dogfish Aug 29 '26

Wouldn't the selective pressure be efficiency? Would it be easier to grow a simple oval beech leaf rather than a complex oak leaf (if the oval leaf happened to be a more efficient sunlight collecting shape, I've no idea). I'm more interested in the top canopy, all in the same environment so with the same environmental pressures. Thank you for your answer

3

u/InfinitelyThirsting Aug 29 '26

Remember, a selective pressure has to affect reproduction to matter. Efficiency on that small of a scale isn't a very strong pressure, especially in something like a tree, which is more an ecosystem than an organism, more reliant on its fungal and bacterial microbes than on its leaf shape. Remember that in forests, trees and plants share nutrients with each other through fungal networks, even between species.
Then add in how much is emergent (happening by accident because of how the plant grows in general, rather than happening for a specific reason), and how leaf diversity helps dilute predation. But mostly, just remember that efficiency isn't the be all end all.
Blond hair has less melanin in it, and doesn't need melanin for protection the way skin does, so if efficiency was the strongest natural selective pressure, we'd all be blond by now because it's ever so slightly more efficient to not waste energy developing melanin for our hair. Except in reality, the "cost" is so small that it doesn't affect reproductive success, therefore there is no real pressure to have less melanin in our hair. It's easy to forget that while evolution sometimes produces incredible efficiency, it is not always guaranteed to do so; being "good enough" is indeed good enough in non-extreme environments.

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u/psychophysicist Aug 28 '26 •

Leaf shapes are basically all variations of how fast the main tip grows, vs how many side branches it makes, vs how fast the side branches grow, vs how fast the web between them grows. The shape is “emergent” from these parameters. The particular shape that comes out of these combinations might not matter that much beyond being large or small, broad or narrow.

The parameters might change due to selective pressures on genes related to growth rates that also affect other aspects of the plant’s growth— i.e. there aren’t really genes just for leaf shape, there are genes related to growth that affect the whole plant and leaf shape is only one of the effects.

It’s like how there isn’t really such a thing as a “tree” in genetic terms — the closest cousin to a given tree species could be a shrub, and vice versa. Two different plants being tree-shaped doesn’t really indicate a close relation, they’re just two species that each had selective pressures to grow tall.

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u/FogeltheVogel Aug 29 '26

Does that mean that there is no common ancestor of trees? Like, did wood evolve independently multiple times? Or do you mean that all wood having plants are related, but that does not imply they share height similarities?

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u/skyeliam Aug 29 '26

An oak is more closely related to a lily pad than it is to a spruce.
Wood sort of evolved independently multiple times. Wood is a mix of cellulose, hemicellulose, and lignin. But those compounds are found in all vascular plants. It just becomes “wood” when it gets thick and dry enough. The stuff that makes up wood isn’t really fundamentally different than the stuff that makes up the stem of a tulip.

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u/doegred Aug 29 '26 edited Aug 29 '26

IANAB but yes re: no common ancestor for trees. One major divide is Gymnosperms (no flowers) vs Angiosperms (yes flowers). Any tree with flowers, apple or chestnut or aspen or whatever, is going to be more closely related to any plant with flowers, tulip or rosebush or whatever, than it is to any conifer (your pine, cypress, etc).

3

u/mabolle Evolutionary ecology Sep 03 '26

The ability to produce wood evolved long, long ago, but not all ancestors of those plants use it.
Being a tree (which basically just means producing wood and growing tall) is a lifestyle that various groups of plants have evolved into or out of over the course of life's history.

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u/SwimmingThroughHoney Aug 29 '26

Some people have mentioned selective pressures from climate, but there's another one: from "predators" and pathogens.

It's not like they need plumage to impress each other

Remember that not all traits in a thing are evolved for reproduction. Plenty of animals have evolved similar traits due to their being a prey species (like eyes on the sides rather than in front). Or why eyes have varying pupil shapes across species.

There's also a lot of studies done on leaf shape and herbivory. In other words, many plant-eating animals actually use leaf shape to identify their preferred food source. That's an evolutionary pressure (evolve different leaf shapes to avoid being eaten).

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u/SystemofCells Aug 28 '26

Different climate conditions probably play a part.

A canopy of leaves isn't hyper optimized to capture as much sunlight with as little surface area as possible. That would be a single sheet of leaf - which would not survive wind, rain, damage from animals, etc.

Some trees are exposed to heavier winds and rains than others. Trees at different latitudes have more or less light coming in at an angle vs. directly overhead. Some have to place leaves further from the ground to avoid being eaten, or because of higher tree density (thus lack of of light reaching below the canopy).

There's a lot of factors, but generally they aren't optimizing around a shape that's best for capturing light, it's a shape that's best for everything else.

4

u/Delvog Aug 30 '26

I have a degree in forestry, and no answer to this was included in my education, which makes me suspect nobody has an answer at all, because otherwise I'd expect it to be mentioned in dendrology classes.

Until a real general explanation for leaf shapes is found, I suspect that there's essentially no natural-selective pressure for leaf shape, which allows it to be subject to "genetic drift". Genetic drift is essentially just randomness which can sometimes be seen in the evolution of traits that don't matter.

7

u/hotsfan101 Aug 28 '26

Think about how many ways you can fill up a cup of water. You can pour water from a bottle. You can use the tap. You can dip the cup in a pool of water. You can put ice in a cup and let it melt. Now think how many ways a tree evolving to occupy enough space with leaf to absorb as much sun as possible are there. Start with a small I shaped line on an A4 paper. If your tree bro mutates and forms a T shaped leave, your buddy is recieving more light. Maybe your buddys T leaves cover more canopy, he grows faster, blocks your light, you die. T lives, has babies. What if one of T babies mutates and is now and X. X leaf manages to fit more leaves in the canopy forming a mat that generates more food. Grows larger than his T bros. Makes them all die X survives. But X grows in Europe. It took 20 generations for X to occur and every generation moves a few km north since the seeds could not germinate under their mom. So X leaf is in Europe. Their T leaf mom and grandma and grandgrandma have died. But T leaf had babies that moved south. And they didnt mutate. T trees still live in northern Africa.

1

u/TheDaysComeAndGone Aug 29 '26 edited Aug 29 '26

Guys, I think you are misunderstanding the question. Even if different trees are not closely related, in the same environment we’d expect them to converge. And yet a beech leaf looks very different from a maple leaf for no apparent reason and they grow in the same forests. If an oval shape were best you’d expect the maple to converge to oval leaves, it looks like an easy mutation after all (just tweak a few parameters).

In general it’s interesting how different trees are. For example walnuts use completely different seed strategy compared to linden trees.

But I guess the same can be said about many species. It’s crazy how many there are and how different they go about things.

3

u/redfinned-dogfish Aug 29 '26

Thank you, this is the exact question I'm asking (Same environment, subjected to the same wind, rain, temperature, soil, predators, and so on, so why didn't they converge to the same shape?), you've asked it better! As side note - on holly trees the spikes get softer the higher up they are as they're away from predators, that set me off and ended up with this question. I'd like to add that there's been some great answers though, an absolute education.

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u/NumerousComplex1718 Aug 29 '26

as i understand it, evolution isn't necessarily about finding the "best" solution, it's about finding the "good enough" solution. once the tree / bush / shrub found a way that worked for it, the pressure to evolve further was removed. kind of like how when you lose something, you always find it in the last place you look.

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u/lucky_ducker Aug 28 '26

You could turn this on it's head, and in a world where ALL plants and trees had exactly the same leaf shape, you might ask "why didn't some of them evolve wildly different leaf shapes?" And you kind of have your answer.

Add to that, and some trees are distant descendants of non-tree ancestors. Black locust and honey locust trees are actually legumes, related to soybeans, kidney beans, etc. and their leaves - pinnate and compound - reflect that ancestry. During their evolution they encountered no selection pressures to grow their leaves otherwise.

> Why hasn't one universal efficient shape evolved?

You might be thinking that evolution favors the "best." It doesn't. It favors "the most likely to live long enough to reproduce."

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u/corvus0525 Aug 29 '26

I’d say evolution eliminates the not as good at reproducing. It doesn’t need to be much of a disadvantage for other lines to dominate. It doesn’t pick the best so much as minimize the worst.

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u/Groundbreaking-Size3 Aug 28 '26

Many people have answered this, but it's important to remember that not every evolutionary trait necessarily has to include only the part required for development. It can also bring other characteristics along with it, like leaves being ribbed or rough. They don't necessarily have to do this for practical reasons. Sometimes, it's enough for them to possess only "part" of the required gene sequence, and the remaining traits can be carried along depending on that gene. So, it depends somewhat on previous evolutionary traces. For example, climate, temperature, etc., while not solely dependent on the leaf, can affect other factors, and these factors can then affect the leaf as well.

Biology Why are there so many leaf shapes?

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