Evolutionary trees and diversification

[delboy]

I have a perception of diversification which I'm not sure is correct. Or it might be just partly true.

 

There is often talk of a more primitive type within a group and this is the one that branches off first in a particular tree. I'm a little uneasy about why this should be because they have had an equal amount of time to evolve to the present day.

 

For example, amongst apes, the gibbons branched off first, and they resemble the ancestral ape more than other modern apes. OK, I'm thinking aloud here, I've just realised that if gibbons branched off 18 million years ago, so did 'the rest off' apes, on the other branch. But the other branch so happened to diversify more, I assume due to geographical movements and separations. I guess I'm puzzled that when a group has a lot of diversification they tend to become less like the ancestor (I might expect one of the lines could remain ancestor-like). But when a line doesn't diversify it stays closer to the ancestor in appearance whereas it could become completely different. Is this not coincidence, but the same lack of need or pressure to diversify means there is also no pressure to change?

 

The same seems to happen in primates as a whole. Lemurs have not diversified since branching off, and they more closely resemble the primate ancestor.

 

I can think of an exception to this. The platypus and echidna are considered 'primitive' mammals, but are very unlike the mammal ancestor.

 

Am I looking for a trend where there isn't one, or is there a trend but not a strict rule?

[Ophiolite]

To some extent, I think, the use of the adjective primitive is a survivor of the times when humans were thought of as being at the pinnacle, the end point, of evolution. In that regard you could say that the use of the word primitive in that way is itself primitive.

 

More properly, and more technically, primitive can be used to mean "lacking in specialisations that are present in other member of the genus, or family, or order."

[Moontanman]

 

 

I can think of an exception to this. The platypus and echidna are considered 'primitive' mammals, but are very unlike the mammal ancestor.

 

 

 

Can you give a citation for that assertion?

 

 

This animal would seem to negate your assertion.

 

http://en.wikipedia.org/wiki/Dimetrodon

 

Dimetrodon is often mistaken as a dinosaur or as a contemporary of dinosaurs in popular culture, but it went extinct around 40 million years before the appearance of the first dinosaur in the Triassic period. Generally reptile-like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than it is to any living reptilian group, though it is not a direct ancestor of any mammals.^[2] Dimetrodon belongs to a group traditionally called "mammal-like reptiles", more recently termed "stem-mammals" or "non-mammalian synapsids"^[2] because many vertebrate paleontologists today group Dimetrodon together with mammals in an evolutionary group or clade called Synapsida while dinosaurs go together with living reptiles and birds in a separate group,

 

 

At one time animals like this were in the mammal linage, not a direct ancestor but much like the platypus and echidna they are branches from the same ancestors..

Edited September 21, 2014 by Moontanman

[delboy]

To some extent, I think, the use of the adjective primitive is a survivor of the times when humans were thought of as being at the pinnacle, the end point, of evolution. In that regard you could say that the use of the word primitive in that way is itself primitive.

 

More properly, and more technically, primitive can be used to mean "lacking in specialisations that are present in other member of the genus, or family, or order."

 

Yes, I was a bit uncomfortable using the word primitive.

My thought is that when a lineage shows little branching and diversity there seems to be a trend that the species in that particular branch tend to resemble fairly closely the ancestor.

 

 

Can you give a citation for that assertion?

 

 

This animal would seem to negate your assertion.

 

http://en.wikipedia.org/wiki/Dimetrodon

 

 

At one time animals like this were in the mammal linage, not a direct ancestor but much like the platypus and echidna they are branches from the same ancestors..

 

That's a very early synapsid. I was thinking of the early mammals, possibly

 

http://en.wikipedia.org/wiki/Morganucodon

 

The platypus and echidna are very unlike it in appearance. Though perhaps not so much given the 180 million years between them.

 

If we're considering Dimetrodon, we'd have to consider it with other synapsids at the time compared to their common ancestor.

 

I'm not saying it's a rule, it just seems to be a trend. It would seem logical to me. Geographical movements and ecological pressures would cause diversification and change, whereas geographical isolation, for whatever reason, would mean a group wouldn't diversify but also that evolutionary pressure to change would be minimal. Such as lemurs in Madagascar.

[Moontanman]

 

Yes, I was a bit uncomfortable using the word primitive.

My thought is that when a lineage shows little branching and diversity there seems to be a trend that the species in that particular branch tend to resemble fairly closely the ancestor.

 

 

That's a very early synapsid. I was thinking of the early mammals, possibly

 

http://en.wikipedia.org/wiki/Morganucodon

 

The platypus and echidna are very unlike it in appearance. Though perhaps not so much given the 180 million years between them.

 

If we're considering Dimetrodon, we'd have to consider it with other synapsids at the time compared to their common ancestor.

 

I'm not saying it's a rule, it just seems to be a trend. It would seem logical to me. Geographical movements and ecological pressures would cause diversification and change, whereas geographical isolation, for whatever reason, would mean a group wouldn't diversify but also that evolutionary pressure to change would be minimal. Such as lemurs in Madagascar.

 

 

Are you asserting that platypus and echidna are unchanged from their original form? this seems unlikely, there were at one time more varieties of these animals but now we have only there two, we used to have far more marsupials than we do now, I'm not understanding your assertion.

[delboy]

 

 

Are you asserting that platypus and echidna are unchanged from their original form? this seems unlikely, there were at one time more varieties of these animals but now we have only there two, we used to have far more marsupials than we do now, I'm not understanding your assertion.

 

I was actually citing platypus and echidna as an exception to the trend. My assertion (if strict) would be that, since monotremes have diversified far less than the rest of mammals, they should relatively closely resemble the first mammal. I'm saying this clearly isn't true, so I would consider it a trend rather than a strict rule. Though they are certainly closer than whales or bats. I think the examples amongst primates were better ones.

 

I'm only suggesting it's a trend, which is perhaps not very scientific

[Moontanman]

 

I was actually citing platypus and echidna as an exception to the trend. My assertion (if strict) would be that, since monotremes have diversified far less than the rest of mammals, they should relatively closely resemble the first mammal. I'm saying this clearly isn't true, so I would consider it a trend rather than a strict rule. Though they are certainly closer than whales or bats. I think the examples amongst primates were better ones.

 

I'm only suggesting it's a trend, which is perhaps not very scientific

 

 

I'm not trying to be insulting but you have no idea what you are talking about... Bats and whales diversified at the about the same time primates did and they are placentals just like me and you, at one time there were several other lineages of mammals other than the placentals who were quite diverse and if you include synapsids you have to include animals like dimetrodon, they were quite a diverse group as well and there were other more or less mammals that were also diverse. If placentals had made it to australia we would see so few marsupials you would be saying the same thing about them... To some extent it is the luck of the draw, some animals are better at competing and others just had the bad luck to exist when a mass extinction hit. I see no trends other than those that can survive do... Under other circumstances we might still have isolated populations of dimetrodon like animals...

 

Think of turtles the only living representatives of anapsids and they predate dimetrodon and it's relatives by quite a margin...

[delboy]

 

 

I'm not trying to be insulting but you have no idea what you are talking about... Bats and whales diversified at the about the same time primates did and they are placentals just like me and you, at one time there were several other lineages of mammals other than the placentals who were quite diverse and if you include synapsids you have to include animals like dimetrodon, they were quite a diverse group as well and there were other more or less mammals that were also diverse. If placentals had made it to australia we would see so few marsupials you would be saying the same thing about them... To some extent it is the luck of the draw, some animals are better at competing and others just had the bad luck to exist when a mass extinction hit. I see no trends other than those that can survive do... Under other circumstances we might still have isolated populations of dimetrodon like animals...

 

Think of turtles the only living representatives of anapsids and they predate dimetrodon and it's relatives by quite a margin...

 

I'm not making myself clear.

Just consider apes. c18 million years ago there was a split in the early apes. One branch didn't diversify much and led to gibbons. The other group diversified more (presumably due to greater geographical movements) and led to all other apes. I believe that gibbons look more like the 18 million year old common ancestor than any ape on the other branch.

Same goes for all primates. 65/70 million years ago they split. The side that didn't diversify led to lemurs and they look more like the common ancestor than any other extant primate I think.

 

I think I'm seeking a better way of explaining the old phrase that might have said that lemurs are primitive primates. They are not but they have the more primitive features. I'm also wondering why isn't there a primate on the other branch with primitive features. I assume it's because evolutionary branching leads to change. The lemur has had exactly the same amount of time to evolve but has changed much less - presumably due to less branching (or fewer pressures that cause branching).

[Delta1212]

Assuming there is a trend, it may be the result of the fact that diversification is the result of numerous varying selection pressures on sub-populations. If a particular population does not diversify, it is likely because there is some selection pressure to maintain the form it has and preventing any off-shoots from flourishing.

 

That situation is certainly more likely to result in an organism that resembles its ancestors stretching back across many generations than a situation that promotes extensive diversification.

 

I'm not sure this is a real trend but there is certainly a mechanism that would lead to both preserving ancestral traits and lack of further speciation, so I wouldn't be surprised to find some examples where those two qualities are correlated.

[Moontanman]

 

I'm not making myself clear.

Just consider apes. c18 million years ago there was a split in the early apes. One branch didn't diversify much and led to gibbons. The other group diversified more (presumably due to greater geographical movements) and led to all other apes. I believe that gibbons look more like the 18 million year old common ancestor than any ape on the other branch.

Same goes for all primates. 65/70 million years ago they split. The side that didn't diversify led to lemurs and they look more like the common ancestor than any other extant primate I think.

 

I think I'm seeking a better way of explaining the old phrase that might have said that lemurs are primitive primates. They are not but they have the more primitive features. I'm also wondering why isn't there a primate on the other branch with primitive features. I assume it's because evolutionary branching leads to change. The lemur has had exactly the same amount of time to evolve but has changed much less - presumably due to less branching (or fewer pressures that cause branching).

 

 

You are still not seeing the big picture, lots of animals in those lineages did diversify but are simply not present today.

 

This video while lengthy does explain that animals were at one time more diverse than they are today. At one time we shared this earth with several species of human, today there is only one.

 

[delboy]

 

 

You are still not seeing the big picture, lots of animals in those lineages did diversify but are simply not present today.

 

This video while lengthy does explain that animals were at one time more diverse than they are today. At one time we shared this earth with several species of human, today there is only one.

 

I still think you don't see what I'm getting at. Delta1212 seems to.

I do understand about extinctions.

 

I suppose I'm really asking if there's a link between diversification and evolutionary change. Or looking at it the other way, that lack of diversification tends to lead to less change. I seem to see this within primates. Unless you are suggesting that the extinct forms derived from the lemur ancestor were once as diverse as the rest of the primates. Which I doubt very much.

[Moontanman]

 

I still think you don't see what I'm getting at. Delta1212 seems to.

I do understand about extinctions.

 

I suppose I'm really asking if there's a link between diversification and evolutionary change. Or looking at it the other way, that lack of diversification tends to lead to less change. I seem to see this within primates. Unless you are suggesting that the extinct forms derived from the lemur ancestor were once as diverse as the rest of the primates. Which I doubt very much.

 

 

That is exactly what I am suggesting... in fact i would say that diversity is less now than then...

[delboy]

 

 

That is exactly what I am suggesting... in fact i would say that diversity is less now than then...

 

So after the lemur/rest of primates split, the rest of primates have produced things as varied as humans, chimpanzees, gorillas, tarsiers, marmosets, baboons, gibbons, etc. You're suggesting that extinct species branching from the lemur line were just as varied. Can you link me to the evidence please.

[Ophiolite]

Are humans, chimpanzees and gorillas varied?

[Moontanman]

 

So after the lemur/rest of primates split, the rest of primates have produced things as varied as humans, chimpanzees, gorillas, tarsiers, marmosets, baboons, gibbons, etc. You're suggesting that extinct species branching from the lemur line were just as varied. Can you link me to the evidence please.

 

 

I linked you to a video that explicitly stated that very thing. At this point in time mammals consist of four varieties, at one time there were many, primates are the same way, I honestly don'y get what you are trying to say...

[CharonY]

 

For example, amongst apes, the gibbons branched off first, and they resemble the ancestral ape more than other modern apes. OK, I'm thinking aloud here, I've just realised that if gibbons branched off 18 million years ago, so did 'the rest off' apes, on the other branch. But the other branch so happened to diversify more

 

Gibbons are unusual in many ways, but if anything they diversified faster than other groups. In less than 2 mill years four genera were formed that represent the extant species (Carbone et al. 2014 Nature 513, 195–201). Then, if we compare great apes (Hominidae) we got four genera with a total of about nine species. Gibbons (Hylobatidae) sub-divide to Hoolock, Hylobates, Nomascus and Symphalangus with more than twenty species.

So I do not see that the premise holds any water.

Edited September 23, 2014 by CharonY