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Does Evolution Have 'Stages'?


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Does evolution have 'stages'?

(I mean outside the human mind)

 

 

As far as I'm aware there are no sudden limbs on the tree of life...they all start out as sprigs.

And it's not as if life reaches a certain point and then decides to takes a breather and stop mutating, right?

 

Seems like there may be times when lots of diversification is allowed by the environment...

and then usually (inevitably?) pruned back... but I'm wondering if it's really 'correct' to say there are 'stages'.

 

:confused:

 

Thanks.

Edited by bbrubaker
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no, there are no stages. its just a gradual change from generation to generation.

Well you could just as easily class each generation with a change as a stage. It is just need far more detail in the structures of the organisms than we have or most likely will ever have, apart from in the case of the present stage.

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Well you could just as easily class each generation with a change as a stage. It is just need far more detail in the structures of the organisms than we have or most likely will ever have, apart from in the case of the present stage.

 

But then you'd have to have clearly defined generations, and homogeneous generations. That's not going to work, either.

 

You could, perhaps, have every single individual as a "stage," with many coexisting, and the "branches" constantly weaving back together most of the time...

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But then you'd have to have clearly defined generations, and homogeneous generations. That's not going to work, either.

 

You could, perhaps, have every single individual as a "stage," with many coexisting, and the "branches" constantly weaving back together most of the time...

I was counting a stage as a further point towards which helped towards a larger step, you could only really tell what this was in hindsight though and each characteristic would have its own unique evolutionary branch at some point, however the characteristics acquired would further cause other phenotypes to develop because of them. So it would be one crazy chart.

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it's been suggested that major "leaps" can occur due to instances of genome duplication (which allows the organism lots more genes to play around with, including important developmental genes), but I've always been suspicious of this idea, mostly on account of how frequently whole-genome duplication occurs *without* corresponding "leaps".

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There are plateaus in evolution like the pre-cambrian as well as large jumps due to a particularly successful trait being passed on. There is a video that shows it really well on youtube. I'd link you too it but youtube is blocked at work. Search for "Blind Watchmaker". The author of the video wrote some evolutionary programming and used clock parts to demonstrate it (after giving the ability of clocks to reproduce :P).

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it's been suggested that major "leaps" can occur due to instances of genome duplication (which allows the organism lots more genes to play around with, including important developmental genes), but I've always been suspicious of this idea, mostly on account of how frequently whole-genome duplication occurs *without* corresponding "leaps".

 

That is true to an extent but I think you may have slightly missed the point of why it is a big leap. This idea is based on theory of protein families which have all derived from one original protein structure which got duplicated and then one of the copies was made redundant as it was no longer necessary, however it wasn't deleted and instead had the free reign to mutate without having a phenotypic effect on the organism, this means a faster rate of mutation can occur. So originally the gene is the same but it can change to whatever it wants whether good bad or neutral without effecting the organism.

 

This can also occur in a similar way but the duplication creates a dimer or tetramers which are thought to be cases in examples such as haemoglobin.

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I'm not actually referring to protein families (which can also arise from much more common single-gene duplications), but rather the (annoyingly persistent) claims that whole-genome duplication allowed a diversification of developmental genes, leading to the major innovations of most vertebrates, namely jaws and fins/limbs.

 

The problem isn't that it can't happen (clearly it did), but rather the tendency to then assert that therefore such duplications are a major driving force in the evolution of body plans while conveniently ignoring the 95% of whole genome duplications which have failed to produce any such drastic change.

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It depends on how you measure evolution. Large changes can occur in, say, the body of animal because of small changes in the genes that control development. Is this a large or small change? The physical constraints on animals means that few of these changes are likely to be successful, so is this a plateau?

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Does evolution have 'stages'?

(I mean outside the human mind)

 

 

As far as I'm aware there are no sudden limbs on the tree of life...they all start out as sprigs.

And it's not as if life reaches a certain point and then decides to takes a breather and stop mutating, right?

 

Seems like there may be times when lots of diversification is allowed by the environment...

and then usually (inevitably?) pruned back... but I'm wondering if it's really 'correct' to say there are 'stages'.

 

:confused:

 

Thanks.

 

Not exactly where the drift of the conversation is going, but there is a common idea in taxonomy called 'grades.' These are taxonomic groups that represent general adaptive 'steps,' or 'stages' if you like, but that maybe aren't monophyletic, i.e. descending from a common ancestor. The old families Pongidae (for the apes) and Hominidae (for humans and human ancestors) represent grades, since all apes are adaptively similar (kind of) but some are more closely related to members of the Hominidae than they are to other apes. And, more currently, the hominid/hominin genus Australopithecus may only represent a grade in human evolution instead of a monophyletic group.

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Not exactly where the drift of the conversation is going, but there is a common idea in taxonomy called 'grades.' These are taxonomic groups that represent general adaptive 'steps,' or 'stages' if you like, but that maybe aren't monophyletic, i.e. descending from a common ancestor. The old families Pongidae (for the apes) and Hominidae (for humans and human ancestors) represent grades, since all apes are adaptively similar (kind of) but some are more closely related to members of the Hominidae than they are to other apes. And, more currently, the hominid/hominin genus Australopithecus may only represent a grade in human evolution instead of a monophyletic group.

 

I know what you're talking about, though in my field we tend to talk about "key innovations / adaptations" - traits which, once they show up, allow the descendants of that population to move into an entirely new niche, diversify rapidly, etc. For instance, flight would be a key innovation for insects, or venom in colubroid snakes.

 

However, I'm not sure it really qualifies as "stages", in part because that sort of implies a very linear view of things. Sure, you could map out the stages to humans, highlighting key innovations like jaws, paired appendages, lungs, warm blood, etc., but that would require ignoring the many species that do without these traits just fine (especially ray-finned fish whose official clade motto is "I got your species diversity right here, pal!").

 

Key innovations are certainly an aspect of evolution, but I'm not sure you can really translate that to 'stages'.

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  • 1 month later...

Stephen Jay Gould and Niles Eldredge spoke extensively about their theories on punctuated equilibrium and phyletic gradualism (layman's terms: genetic leaps vs. watching the genetic paint dry). I think their viewpoint holds some merit where sudden, single-generation changes in geography or environment are concerned; to thrust a species from one extreme into another would cause drastic changes in selective pressures and quickly change the dominant phenotypes. With populations that lived under relatively static conditions, a leap in genetic mutations would not necessarily be beneficial, especially if the dominant phenotypes were already living comfortably in their environment. Competition between the species in this case would not be as important.

 

We must consider under what conditions such a drastic environmental change could occur, and what clades it could have affected. It would need to be something geologically instantaneous - a sudden change in food supply, breathable oxygen content, meteorological disturbances... Who knows. A colleague of mine (passed on now, sadly) always held the theory that the dinosaurs gave way to the mammals not directly because of the infamous comet, but because of the side-effects - a potential reduction in global oxygen supply, the nearly coincident rise of angiosperms and subsidence of gymnosperms. His feeling was that the dinosaurs simply were not able to digest the angiosperms and phased out when flowers gained dominance.

 

As for the subject question (does evolution have stages?), I don't see how evolution could progress in stages because there are small variations in every individual, to some extent. We are always changing. We'd need to be in a completely static and unchanging environment with a similarly static control on populations. It's the same reason that identical twins still grow up to be different people. We all experience different things and grow and change accordingly. I think this holds true on the genetic level, as well.

 

One of Gould and Eldredge's arguments for punctuated equilibrium was that the fossil record did not reflect gradual changes in morphology. This always blew my mind, because I've always seen just the opposite. I did my time with trilobites early in university and the genetic diversity from one age to the next is amazing. Without these morphological variances, in fact, we would not be able to correlate in the geological record (relative time, that is); a lot of geological formation correlations rely on subtle changes in trilobite, brachiopod, and foraminifera morphology, amongst many others. I think they really missed the dartboard on that one.

 

My overall problem with this - and some of the other debate topics - is that the questions are posed (presumably) for a black-and-white answer. I don't think the answer to many of our threads are that simple; to me, it seems likely that both methods of evolution played an important role.

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