I find this question difficult to formulate. Maybe you can help with that.
In plain English, I might formulate the question something like:
“What species is the smallest compared to one of its ancestor species?”
But in case that’s not clear, I’ll also put the question in the most precise and complete way I can, formulated in a way to head off possible misunderstandings and hang-ups. This will result in something difficult to read, however:
Given a species Sm, I define Vm as the average size (as measured by volume) of the members of that species.
In some species, there are specific subtypes each of which has its own characteristic size. As a matter of mere nomenclature, for the purpose of this question, I will treat such subtypes as separate species.
I’ll be talking about species’ being “descended” from each other. Subtypes should be thought of as being “descended” from those species the subtype’s own species is descended from.
I define R(Sm, Sn) as the ratio between Vm and Vn for some Sm and Sn.
The question is: What is the pair of species Sm and Sn which are such that Sn is decended through evolution by natural selection from Sm, and where also no other pair of species So and Sp, where Sp is descended from So, is such that R(So, Sp) is greater than R(Sm, Sn)?
And that clarifies the question does it? :dubious: Us simple biologists don’t respond to equations very well.
If the question is simply what species has shrunk the most compared to any ancestor of infinite age then it’s very tough to answer, but without a doubt the answer will be a plant. The reason it’s tough to answer is that we never have a sufficiently complete fossil record to determine all the ancestors of every species.
Plants change size regularly over their evolutionary history, with lineages making the transition from tree to shrub to herb and back to tree again multiple times. We might start by looking at some of the legumes or euphorbias or daisies for obvious candidates, with a great many groups seemingly able to evolve a herbaceous or tree form almost as soon as environment dictates. It would be astounding if some of the plants today that weigh just a few grams in adult form didn’t decend from trees of several tonnes at some point in their past.
And of course if we could show that some of the unicellular algae we decended from multicelluar forms (which is very likely) then we could easily have an organism that is trillions of times smaller than its ancestor.
As I said, this version is harder to read than the “plain English” version.
But I do think it is more precise in the sense that it is less likely than the “plain English” version to be misunderstood on a careful reading.
Oh shoot, I failed to specify I was thinking of animals rather than plants. But your answer is interesting nevertheless.
“We don’t know because we don’t have a complete fossil record” anwers my question, strictly speaking. I should have been even more “precise.” I should have said “What is the known pair of species Sn and Sm…” and “…no other known pair of species So and Sp…”
In other words, I am curious to know what answer can be given about that part of evolutionary history we do have access to. It was kind of ludicrous for me to ask about unknown pairs of species. And of course, it was not my intention to do so.
Once agian we run into this probelm of what level of certainty we need. I can’t think of a single species of animals where we can definitivbely say that extinct species X was an ancestor. What we usually can do however is say that the ancestor was that specie sor somehting very similar. So for example Australopithecus afarensis may not be a human ancestor, but we descended fom somehting that was very similar in size ad appearance.
If you honestly want to restrict the aswer to cases where we can say with better than 95% confidence that A is the ancestor of B then you really will be looking at cases of subspecies dwarfism.
If you want to restrict it to animals, and those species with reasonably well documented ancestry then I would immediately look at the hummingbirds. I’m not sure which if their provable ancestors was largest, but some of the earlier proto-archosaurs would have been in the 20kg range, and I suspect that many of the dino ancestors would have been in the same range. That’s a pretty substantial reduction down to a couple of grams for a hummingbird.
There are plenty of other animals that may well have shrunk more. We have fish less than a centimetre long that presumably descended from ancestors at least a metre in length. We have insects that are less than a millimetre in length that presumably descended from arthropods that were over 20 centmetres long. But the probelm is that in all such cases we can’t really definitively say that the ancestors were that large, we just assume they would have been.
Thanks for the link. When I told my wife about this question earlier today, she mentioned she saw something relevant about sloths on the Discovery channel the other day.
Also, to be clear,I specified “average size” in order to render irrelevant the fact you mention about variation within a species. Its an important fact, of course, but I think in this context its safe to pretend that “average size within a species” has something to do with a sort of a “norm” regarding that species’ size.
No. That giant species became extinct and left no ancestors of any size. There is absolutely no reason to believe that the ancestors of the extant sloth species were ever substantially larger than they are today.
This is why the question is so hard to answer defintitively. When we find giant fossils related to later species we can not simpy assume that the species became dwarfed over time. As with the sloths we may simply be looking at a giant branch of the same tree. Just because we find a Tyrannosaur fossil that does not allow us to conclude that the ancestors of birds were ever that large. Rather the Tyrannosaurs seem to be a giant branch of that particular tree that lived alongside the ancestors of birds, and contemporary bird ancestors never weighed more than a few kilograms.
As we go further and further back and our fossil record becomes ever poorer it becomes increasingly difficult to deal with this problem. As a result it becomes impossible to say with any certainty whether a giant animals was truly an ancestor to later smaller relatives.
It’s not a problem. Just pick a level of certainty and go with it. If you suspect the level you picked might issue in an answer which I wouldn’t have found helpful, then make note of the level you picked, in order to give me a “heads up.” No problems at all.
If you don’t know whether your answer is likely to be helpful or not, then either note that fact or just ignore it. Whatever is fine.
I thought we had some ideas, about this. I didn’t think any of it was “definitive” but I thought there were some pairs where it is okay to say “We think very probably this species is an ancestor of that one.” If that’s not the case, no problem. The spirit of the question I was trying to ask is still answerable in terms of what you go on to say next.
Thanks, that’s the kind of thing I was thinking about.
Not really. Imagine if in the future they have equine animals. It would be impossible to say based on fossil evidence whether it descended from a horse or a zebra or a donkey. OTOH it really wouldn’t matter because in terms of habit and appearance and size all those animals are essentially the same.
The same goes for those species where we have a good idea of ancestry. We can’t say with certainty whether the fossils we have are “the” ancestor, or simply a co-existing closely related species. But it really doesn’t matter. If birds aren’t desceded form Euparkeria specifically they are certainly descended from something that was very similar in terms of size and apperaance.
Also, it’s my understanding that oysters don’t grow nearly as large as they once did, due to harvesting by humans. Colonial American records describe Oysters from Chesapeake Bay the size of dinner plates. Maybe they would grow that large again if we stopped harvesting them, but who knows? We have been putting some serious pressure on them to reach sexual maturity at a smaller size.
Would this ever happen? I don’t mean cooperative colonial multicellular organisms but true differentiated multicellular organisms in which different types of cells are derived from core “stem” cells and cannot survive autonomously.
OK, taking into account (or rather, brushing conveniently aside) the problems of uncertainty, suppose we accepted as given that the smallest avian ancestor of all modern birds was probably no smaller than a crow and perhaps as big as a chicken, then the bee hummingbird, weighing in at a couple of grammes, might be a suitable candidate.
Only animals have stem cells. Algae are protists. All cells are totipotent and all cells are perfectly capable fo surviving independently in appropriate environments by smply reverting to an undifferentiated form.
Has this ever happened? It’s hard to say. Certainly the unicellular stages of many multicelluar algae can exists for infinite generations in unicelluar form. And certainly many unicellular algae today show uncanny similiarities to the spores of multicellular alage, but uncertainty remains as to whether this is because they regressed from multicelluar forms, because they multicellualr forms evolved from the unicellular or because they share a common ancestor.
Unfortunately fossils don’t really allow su to solve those sorts of problems, though advances in genetic analysis will eventually.
That’s a possibility. (I suspect the earliest dinosaurian ancestors of the lineage that led to birds were considerably larger than chickens, though.) Other candidates would be the smallest shrews and bats, which are in a similar size range. The early ancestors of mammals, the synapsids, were fairly substantial beasts.
What about the Diprotodon ? It became extinct fairly recently (in the last 50,000 years) and it was related to the wombat and koala. They were as large as a small elephant - koalas and wombats are much smaller (although wombats can weigh 70 pounds).
There are many examples of extinct Australian megafauna that have smaller sized counterparts, such as the echidna and kangaroo. It is theorized that the arrival of humans caused the extinction of the larger species, allowing the smaller ones to survive.
The Diprotodon left no descendents - the wombat and koala are not descended from it. Likewise, I am not aware that living kangaroos or echidnas are descended from earlier larger forms.
If you suspect the level you picked might issue in an answer which I wouldn’t have found helpful, then make note of the level you picked, in order to give me a “heads up.” No problems at all.