While walking in the woods this fall and noticing the leaf litter, I notice both Red Oak and White Oak leaves and began to ponder their differences.
Clearly, these two tree species exist in the same environment (I am in Wisconsin), yet they have slight variation between their leaf forms. The Red Oak leaves have pointy tips and the White Oak leaves have rounded tips.
I assume the two species arose out of some common ancestor. But I cannot think of any obvious explanation as to why these two shapes would evolve differently. Some common reasons I am aware of would be:
Geographic isolation. I suppose this could have happened and then the two species returned to the same environment after the separation was removed. Glaciation maybe?
Sexual selection. Does this even happen with plants?
Environmental “fit”. I can’t quite convince myself that pointy tips or rounded tips result in any survival benefits.
The same question could be posed with various Maple species. I consider myself relatively familiar with how evolution works, but I’m kind of stumped on this.
I am not an evolutionist, but I have a couple of comments
1.) “Sexual Selection” of a sort does happen with plants, if by that you mean evolution of structures and features that enhance reproductive success. The clever designs of orchids that drop pollen on bees, heck, the development of flowers at all, along with nectar, is an innovation meant to lure in pollen vectors like bees to transmit pollen from one plant to another. The designs and colors of flowers to make then obvious and attractive is entirely a reproduction-based development
2.) That said, I doubt if the leaves of maples or other trees have any relation to this. The fact that, for each species, somer characteristic shape evolved and has stabilized says that such a shape had some evolutionary advantage over prior shapes, and that the current shape is pretty good for its current situation (or else there’d be other mutant variants around, succeeding just as well). Evolutionary changes occur because of random mutations in the genes, and are the result of small changes in the existing gene, rather than a wholesale redesign from scratch – evolution doesn’t give you the optimum shape of something, just the moist successful one given the starting material and possible mutations. If two maple species have leaves that are pretty close to the same, you can blame a common ancestor. Somewhere the lines diverged because each had some mutation that wasn’t fatal , but was actually slightly beneficial, so it was retained. You don’t have to go positing different environments (although that could, of course, be the cause for a mutation being beneficial) – you could have two mutations existing under the same circumstances and continuing to refine to obtain better and better results, until you had two full-fledged species that were no longer completely interfertile, with different leaves. I suspect physical separation would help refine the species by preventing mixing of the characteristics before the species were fully defined, but it might not be necessary.
Evolution by natural selection works by having random variations in organisms. Some of those turn out to have advantages, such that they out-survive the others. But the others die out only if they have significant dis-advantages. Many random changes are just neutral – they don’t provide either significant evolutionary advantages or dis-advantages. So they just breed on, eventually being recognized as a different species.
That’s likely the case with leaves of red vs. white oaks. Both pointed tips and rounded tips are ‘good enough’ for the tree to survive and reproduce. So they do so.
Evolution doesn’t have any hierarchy.
Both humans and mosquitoes are ‘good enough’ for their niche in life, so both continue to survive & breed.
In this particular case, I would agree with this. There are certainly features of leaves that have adaptive value. For example, in the tropics many leaves have “drip tips” so they shed rain rapidly to avoid overgrowth by algae and mosses on the leaves. Other leaf designs are good for rapid growth. Some leaves have a surface coating of hairs or fuzz to deter insects.
But in the case of the minor variations between different species of oaks and maples, each of the designs may be selectively neutral, or close enough. Any one of the designs gets the job done. The differences arose by random chance when the ancestors of each species were isolated from one another, and haven’t been selected against since.
In general it is thought that geographic separation is necessary for speciation to take place, but there are a few mechanisms in plants that could allow speciation to occur even in the same locality.
It may be worth looking at whether there is any place that their ranges don’t overlap - one may be better at resisting wind, shedding rain, conserving moisture or dealing with more intense sunlight than the other - or it could be that those attributes (or something like them) mattered more to the ancestors of one species in some local conditions that are now past history - such as ice ages or sustained droughts.
Or as others have already said, it could just be chance that is fairly neutral to selection.
I would add a hugely important factor that is nearly always overlooked: genetic drift.
That is, simply, change that persists simply because there’s no reason for it not too. A mutation pops up conferring a slightly different shape that works just as well as the old shape. It’s not selected against, so it persists in the population. Eventually, due to random chance, it goes to “fixation”, meaning that it’s the only version of the gene left. This is particularly likely in a small population.
I’ve heard surprisingly fierce debates between scientists over whether drift or selection is a more powerful force in evolution.
I didn’t think of the reason of: “eh, it just happens sometimes.”
I guess I was laboring under a false expectation that a random mutation in a single individual (in this case, a tree) that conferred no advantage would simply die out without spawning a large and successful new species.
In other words, I thought genetic drift would only have a meaningful effect if the drift resulted in a selective advantage.
And as was mentioned by more than one responder, perhaps there once was geographic isolation that allowed the speciation to “take hold” and persist.
Thank you. I am more knowledgeable now than I was a few hours ago.
Genetic drift is by definition not based on selective advantage but on random chance. You might be interested in the various types of speciation, such as sympatric speciation (as alluded to by Colibri earlier.)
Thanks for the explanatory links. I learned that genetic drift is not (necessarily) a result of mutations. And I learned these various pathways to speciation (allopatric, peripatric, parapatric and sympatric) though I confess I am still murky about the how and why of sympatric speciation.
It is increasingly clear I have an incomplete (and perhaps outdated) understanding of evolutionary processes (despite devouring Dawkins, but never read original Darwin).
I’m no expert, but I believe a major adaptive value of leaf shape in temperate trees (like most oaks) is that they effectively shed wind by rolling or folding when the wind blows hard. A mature oak in full leaf-out has a LOT of surface area and anything that lowers wind resistance will be a major factor in keeping the tree upright.
Obviously, there are lots of different shapes that work, but there aren’t many temperate full-height trees with large, stiff, round or square leaves.
It’s clear that Darwin had an incomplete understanding of evolutionary processes. And I don’t think reading Dawkins would have helped him …
Darwin is interesting enough to read anyway. If nothing else, when you read someone writing “Darwin didn’t say that”, you can form your own opinion about the accuracy of the statement.
Specific to leaves, I would think that leaf-scattering structures would be the primary driver. Look at the maple “helicopters” as an extreme example. The helicopters have a very strong structure, but it’s not woody. It’s more like a very thick/thin leaf. As it happens, maple leaves are also thicker and stronger veined than most (though less so than magnolias). Thick leaves use a lot more resources to catch the same amount of sun.
Did the very successful helicopter scattering structure evolve and just happen to include DNA instructions which affected leaf thickness?
Well, maybe. When we’re talking about evolution, we often speak of how a particular trait leads to more or less reproductive success, and different traits do certainly contribute to that, but there’s also a large dose of just plain luck. Maybe this tree was more successful than that tree because it had some beneficial trait… but maybe it was more successful just because the dice happened to favor it. Now, if a trait really does have some significant benefit or drawback, then over the long run, eventually the odds will win out, just like a skilled poker player will, in the long run, beat a novice… but if the difference isn’t that significant, or the time scales not so long, any particular variant might end up being widespread.
EDIT: Oh, and whenever the topic of Mus musculus comes up, we’ll remember your particular expertise.
Probably not. Winged seeds (known as samaras) have evolved in many different kinds of trees with vastly different leaf types (like Ash trees, which have compound leaves and winged seeds).
The leaves of deciduous trees necessarily must be designed for rapid growth. Because of this, the leaves of deciduous trees in general are thinner than those that keep their leaves year round (especially notable in the tropics).
