Phyletic Gradualism vs Punctuated Equilibrium

The accumulation of small heritable changes within populations over time.

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Phyletic Gradualism vs Punctuated Equilibrium

#1  Postby Nautilidae » Jun 01, 2010 9:48 pm

Being one that is more ignorant when it comes to evolution, I do not know which is the favored evolutionary model: punctuated equilibrium or phyletic gradualism.

For those of you who do not know, punctuated equilibrium is the evolutionary model that models species as undergoing very little evolution for long periods of time before suddenly (on evolutionary time-scales, of course) evolving fairly rapidly.

Phyletic gradualism is the model that models new species as coming about gradually with no periods of major evolution. Life evolves gradually.

Which is the favored model and why?
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#2  Postby Sityl » Jun 01, 2010 10:00 pm

I'm not expert, but I thought various species had traits of each. For example, sharks haven't changed much, nor had trilobites over 100+ million years, but animals like cows and chickens have undergone much larger changes in smaller times, because of increased selective pressure.

I could be completely wrong though.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#3  Postby iamthereforeithink » Jun 01, 2010 10:05 pm

I'm not sure about which is considered more valid in actual biological and natural ecosystem evolution, but punctuated equilibrium works much better and explains more in the case of evolutionary theory as applied to evolution of business ecosystems and organizational behavior. Check out these papers:

Gersick, C.J.G. (1991) Revolutionary Change Theories: A Multi-Level Exploration of the Punctuated Equilibrium Paradigm, Academy of Management Review, 16(1), 10-36.

Hannan, M.T. and Freeman, J. (1977) The Population Ecology of Organizations, American Journal of Sociology, 82(5), 929-964.

Lichtenstein, B. M. (1995) Evolution or Transformation: A Critique and Alternative to Punctuated Equilibrium. Dorothy P. Moore, ed. Academy of Management Best Papers Proceedings, Madison, WI: Omnipress, 291–295.

Romanelli, E. and Tushman, M. (1994) Organizational Transformation as Punctuated Equilibrium: An Empirical Test, Academy of Management Journal, 37(5), 1141-1166.

Tushman, M. and Anderson, P. (1986) Technological Discontinuities and Organizational Environments, Administrative Science Quarterly, 31(3), 439-465.

Tushman, M. and Romanelli, E. (1985) Organizational Evolution: A Metamorphosis Model of Convergence and Reorientation, in L.L. Cummings and B.M. Staw (Eds.), Research in Organizational Behavior, Vol 7, 171-222, Greenwich, CT: JAI Press.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#4  Postby ginckgo » Jun 02, 2010 3:03 am

num1cubfn wrote:I'm not expert, but I thought various species had traits of each. For example, sharks haven't changed much, nor had trilobites over 100+ million years, but animals like cows and chickens have undergone much larger changes in smaller times, because of increased selective pressure.

I could be completely wrong though.


You are completely wrong :grin:

Trilobites changed massively throughout their main regnum from Cambrian to Devonian. The fact that they all still retained their basic morphology (cephalon, thorax, pygidium, legs plus gills on each thoracic segment, etc) is no more 'unchanging' than all vertebrates retaining four limbs, a head, a spine, etc. We are just more biased to see minor variations as bigger than in other phyla. Having said that, sharks have also varied quite a bit.

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Re: Phyletic Gradualism vs Punctuated Equilibrium

#5  Postby susu.exp » Jun 02, 2010 10:53 am

The distinction between PE and PG in the OP is somewhat sloppy (if common). A better way (IMHO) to represent them is this graph:
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Here you´ve got frequencies in which they occur plotted against rates of change, with red showing PE and blue PG. Gradualism assumes that this distribution is unimodal, whereas PE assumes it is bimodal. Gradualism is a better match to rates of change observed in the present, Punctuated equilibrium is a better match to rates of change observed through geological history.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#6  Postby Primate » Jun 18, 2010 10:19 pm

You're correct susu.exp. It depends on the span of time under consideration. Many people confuse phyletic gradualism with what Dawkins has called 'constant speedism'. Even during cases of punctuation the evolution is still gradual, just compressed. As the OP has stated, the punctuations need to be put in the context of evolutionary time scales. It all just depends on what you're trying to model.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#7  Postby Ubjon » Jun 18, 2010 10:28 pm

Natural selection can be a means of stasis just as easily as it can be a means of change.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#8  Postby susu.exp » Jun 18, 2010 11:35 pm

Well, the difference between the two curves is not merely one of time. You need some process that turns the blue curve into the red one. Gould and Eldredge in their original paper suggested the founder effect, my view is that non-selective species sorting is responsible (i.e. after speciation you have two species that are very similar in resource use. There´s a good chance that they will be sympatric within a geologically short time and if they use the same resources, one of them will go extinct. This increased likelihood of extinction is less pronounced the higher the rate of change around speciation is and thus we get the second peak in the red graph).
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#9  Postby Darwinsbulldog » Jun 19, 2010 1:57 am

The whole business seems very resistant to a general model, IMHO. Clearly, good times favour gradualism, and mass extinctions appear punctuated. But the five major mass extinctions do not even form a pattern within themselves. By what criteria do we measure extinction and recovery? Species diversity or abundance? Trophic depth? Regional recoveries or global? Why do some clades appear more plastic than others? Background extinctions compared to pulse or press events? Stability vs flexibility of GRN's? Some clades make it through a big pulse event, only to die out shortly after! Recovery rates seem to vary greatly from a few hundred thousand years to 10 million or more. :think:

Gould and Eldridge tend to produce verbal theories rather than math models. They seemed to ignore the advances in Homeobox genes despite the fact that much information can be gained from Genes networks that are still extant.

The non-randomness of the fossil record, and fossil discovery is probably biasing all results and speculations. Any "rules" seem to demand exceptions. Nevertheless, both gradualism and PE seem to be operative, but I suspect genetic data [especially how Hox genes are both highly conserved, but somewhat plastic] rather than paleontological evidence will bring more light on the issue.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#10  Postby susu.exp » Jun 19, 2010 10:38 am

Darwinsbulldog wrote:The whole business seems very resistant to a general model, IMHO. Clearly, good times favour gradualism, and mass extinctions appear punctuated.But the five major mass extinctions do not even form a pattern within themselves. By what criteria do we measure extinction and recovery? Species diversity or abundance? Trophic depth? Regional recoveries or global? Why do some clades appear more plastic than others? Background extinctions compared to pulse or press events? Stability vs flexibility of GRN's? Some clades make it through a big pulse event, only to die out shortly after! Recovery rates seem to vary greatly from a few hundred thousand years to 10 million or more. :think:


??? What does that have to do with punctuated equilibrium? There are some point you raise that have answers: Diversity rather than abundance is the criterion for extinction and recovery. There are some species that become very abundant in mass extinctions - disaster taxa - so abundance is no criterion. Regional vs. global: Global, if we´re looking at mass extinctions.

Darwinsbulldog wrote:Gould and Eldridge tend to produce verbal theories rather than math models. They seemed to ignore the advances in Homeobox genes despite the fact that much information can be gained from Genes networks that are still extant.


Gould co-authored the MBL papers which introduced some of the most important mathematical models of morphological evolution in deep time. There´s also a longish section on regulatory networks in his structure. But then again Gould was a paleontologist, not a geneticist and thus not focussed on them.

Darwinsbulldog wrote:The non-randomness of the fossil record, and fossil discovery is probably biasing all results and speculations. Any "rules" seem to demand exceptions. Nevertheless, both gradualism and PE seem to be operative, but I suspect genetic data [especially how Hox genes are both highly conserved, but somewhat plastic] rather than paleontological evidence will bring more light on the issue.


I don´t think they will.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#11  Postby Darwinsbulldog » Jun 19, 2010 3:03 pm

@susu.exp

??? What does that have to do with punctuated equilibrium? There are some point you raise that have answers: Diversity rather than abundance is the criterion for extinction and recovery. There are some species that become very abundant in mass extinctions - disaster taxa - so abundance is no criterion. Regional vs. global: Global, if we´re looking at mass extinctions.


I realize that mass extinctions, by definition, global. However, it seems that some palaeontologists in the past have talked about a "reef gap", where in fact there existed extensive reefs in some areas, and not in others(1). For sure, the diversity of surviving taxa is measure of recovery, but if and only if the population numbers are sustainable in each species. As I said before, some groups made it though a bad extinction, and petered out soon after. A thousand Koalas without Eucalyptus leaves are dead Koalas. A thousand Koalas of post-reproductive age means the species is doomed to extinction. Exaggerated small numbers in my example, but you get my point.

And if they are in low numbers, then the fossil record might not record them. So, the record may look like a PE event, in that they might look extinct locally, or another population may re-invade the extinct populations area from some refuge.

Gould co-authored the MBL papers which introduced some of the most important mathematical models of morphological evolution in deep time. There´s also a longish section on regulatory networks in his structure. But then again Gould was a paleontologist, not a geneticist and thus not focussed on them.

A citation please? I did not see much in Gould's "Structure of Evolutionary Theory"

I don´t think they will.


Perhaps you have not seen papers like the one by Hinman & Davidson (2007)? (2). They compare the Genes Regulatory Networks [GRN's] of Sea Stars and SeaUrchins, who shared a common ancestor some 500 myo in the Cambrian. Thus the LUCA of these clades must have had the same GRN as well, where the CORE is conserved and different organisms innovate and co-opt less conserved parts to satisfy their own evolutionary challenges. All methodologies have their problems of course, but palaeontologists can usable only resolve down to families or perhaps genus level, and can only work on morphologies, which is a rather crude and unreliable indicator of diversity.

In any case, real recovery seems to depend on ecological balance after the perturbations have ceased, or at least, the ecosystem can buffer those disturbances. A crude bunch of refugee species with large populations is still not out of the woods until autotrophic production is back ot capacity and there is depth to the trophic levels. Just counting mere species, or mere population numbers, is not in itself a robust measure of recovery. It depends very much who those species are, although there are sometimes gaps in niches after a recovery is said to be complete and at carrying capacity.

(1) Erwin (2008): "Extinction as the loss of evolutionary history" PNAS Vol 105, Aug 12, Supp 1. pp. 11,520-11527. See also Erwin's 2001 paper in PNAS, Vol. 98, No. 10, pp. 5399-5403.
(2) Hinman, V.F & Davidson, E.H. (2007): "Evolutionary Plasticity of Developmental Gene Regulatory Network Architecture" in PNAS, Vol 104, No 49, pp. 19404-19409.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#12  Postby susu.exp » Jun 19, 2010 7:04 pm

Darwinsbulldog wrote:I realize that mass extinctions, by definition, global. However, it seems that some palaeontologists in the past have talked about a "reef gap", where in fact there existed extensive reefs in some areas, and not in others(1). For sure, the diversity of surviving taxa is measure of recovery, but if and only if the population numbers are sustainable in each species. As I said before, some groups made it though a bad extinction, and petered out soon after. A thousand Koalas without Eucalyptus leaves are dead Koalas. A thousand Koalas of post-reproductive age means the species is doomed to extinction. Exaggerated small numbers in my example, but you get my point.


a) I did read the Erwin paper and it´s certainly interesting. But it doesn´t alter the definition of a mass extinction. There are questions concerning mass extinctions effects on the other metrics introduced (He somewhat skimps the ecospace part, which is odd considering he gave it quite a bit of room in his review paper on disparity, Erwin 2007).
b) I still don´t see how this connects to PE. PE is about the distribution of rates of morphological change, not extinction rates or the rapicity of mass extinctions.
c) Geological time. When we talk about the Permo-triassic extinction event, we do know it happened in less than 200ka. A generation more or less doesn´t even make a difference there. The simple explanation for clades going extinct shortly (that´s still geological time spans measured in millions of years) after mass extinctions is that they survive with only a few species and they go extinct as part of the background. Ammonids made it past KT with 2 species and went extinct in the danian. Did these 2 Ammonid species go extinct because of the ecological effects of KT? Probably not - there are millions of years in between. But of couse a clade containing 2 species is more likely to go extinct than a clade with 100 species or 1000 species.

Darwinsbulldog wrote:And if they are in low numbers, then the fossil record might not record them. So, the record may look like a PE event, in that they might look extinct locally, or another population may re-invade the extinct populations area from some refuge.


There are no "PE events". PE is a hypothesis about the distribution of rates of morphological change.

Darwinsbulldog wrote:A citation please? I did not see much in Gould's "Structure of Evolutionary Theory"


Citation for what? The MBL papers are
Raup et al. 1973 "Stochastic models of phylogeny and the evolution of diversity",The Journal of Geology, 81, 525-542.
Raup & Gould, 1974 "Stochastic Simulation and Evolution of Morphology-Towards a Nomothetic Paleontology" Systematic Zoology, 23, 305-322.
Schopf et al, 1975 "Genomic Versus Morphologic Rates of Evolution: Influence of Morphologic Complexity", Paleobiology, 1, 63-70.
Gould et al., 1975 "The Shape of Evolution: A Comparison of Real and Random Clades", Paleobiology, 3, 23-40.
If you want page numbers for the Structure, you´ll have to wait (don´t have it here, will check on Monday).

Darwinsbulldog wrote:Perhaps you have not seen papers like the one by Hinman & Davidson (2007)? (2). They compare the Genes Regulatory Networks [GRN's] of Sea Stars and SeaUrchins, who shared a common ancestor some 500 myo in the Cambrian. Thus the LUCA of these clades must have had the same GRN as well, where the CORE is conserved and different organisms innovate and co-opt less conserved parts to satisfy their own evolutionary challenges. All methodologies have their problems of course, but palaeontologists can usable only resolve down to families or perhaps genus level, and can only work on morphologies, which is a rather crude and unreliable indicator of diversity.


It´s a cool paper, but it doesn´t seem to have any connection to punctuated equilibrium at all. And of course we can resolve species, we just tend to use genus level data to generate diversity curves, mainly because it is standard practice to exclude singletons (taxa only present in one particular time interval) - simply because they obscure the diversity signal as singletons tend to stem from extraordinary preservation, if there are 56 taxa in the danian of shoreline Y, while there are only 5 for the uppermost Maastrichtian there, but the danian has soft body preservation, while the Maastrichtian doesn´t, this does not indicate diversity went up across the KT boundary. And if you exclude singletons for species, you throw out pretty much everything. So for diversity curves genera are used, but we can resolve to species for other purposes. Only working on morphologies is not a downside if you´re looking at the distribution of rates of morphological change (though we can also work with behavioural data for instance).

Darwinsbulldog wrote:In any case, real recovery seems to depend on ecological balance after the perturbations have ceased, or at least, the ecosystem can buffer those disturbances. A crude bunch of refugee species with large populations is still not out of the woods until autotrophic production is back ot capacity and there is depth to the trophic levels. Just counting mere species, or mere population numbers, is not in itself a robust measure of recovery. It depends very much who those species are, although there are sometimes gaps in niches after a recovery is said to be complete and at carrying capacity.


Not a fan of whatever niche definition you are using there. And again, this doesn´t seem to have anything to do with PE.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#13  Postby Darwinsbulldog » Jun 20, 2010 4:22 am

@ susu.exp

Sorry, we seem to be discussing several things at once here. As I read it, Gould and Eldridge cam up with PE theory based on the fact that there are gaps in the fossil record. The traditional view was that the non-randomness of fossil preservation made it seem like there were gaps. Or that local extinctions were re-populated from refugia migrating into a region that had experienced some extinction event. Thus the transition seems "jerky"

Please bear with me, as I suspect I am being stupid. You defined PE as:

There are no "PE events". PE is a hypothesis about the distribution of rates of morphological change.


Surely the rates of morphological change are based on how when and where clades are able to adapt to environmental by playing with their [non-conserved] developmental gene complexes [or their upstream or downstream expression]. I think that this is highly germane to the discussion. An organism may look wildly different, but the underlying Bauplan is highly conservative. In mammals for example, the cervical vertebra number only seven, but humans and giraffes look very different. The genetic/molecular evidence thus has confirmed many homologies that anatomists have always known about, but by no means all. Thus you can have huge changes in morphology that is based on small [but important] changes in the Hox complexes. Thus fruit flies and humans look very different, yet at the genetic level, they share homologies.

So at the genetic level, it looks more like gradualism rather than PE. Further, these genetic changes seem to pre-date the appearance of morphological change as seen in the fossil record.

Citation for what? The MBL papers are
Raup et al. 1973 "Stochastic models of phylogeny and the evolution of diversity",The Journal of Geology, 81, 525-542.
Raup & Gould, 1974 "Stochastic Simulation and Evolution of Morphology-Towards a Nomothetic Paleontology" Systematic Zoology, 23, 305-322.
Schopf et al, 1975 "Genomic Versus Morphologic Rates of Evolution: Influence of Morphologic Complexity", Paleobiology, 1, 63-70.
Gould et al., 1975 "The Shape of Evolution: A Comparison of Real and Random Clades", Paleobiology, 3, 23-40.
If you want page numbers for the Structure, you´ll have to wait (don´t have it here, will check on Monday).


Many thanks for those, I have some reading to do! :thumbup: :cheers: :cheers:
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#15  Postby susu.exp » Jun 20, 2010 2:32 pm

Darwinsbulldog wrote:@ susu.exp

Sorry, we seem to be discussing several things at once here. As I read it, Gould and Eldridge cam up with PE theory based on the fact that there are gaps in the fossil record. The traditional view was that the non-randomness of fossil preservation made it seem like there were gaps. Or that local extinctions were re-populated from refugia migrating into a region that had experienced some extinction event. Thus the transition seems "jerky"


Well, a short note on gaps in the fossil record. A gap is the time between the first appearance of a clade and the first appearance of its sister clade. These are explained through the spotty record and the fact that character evolution takes some time after the split - so one clade can usually be identified in older fossils than the sister clade.
What G&E noted were two things, one is the general absence of species to species transitional forms (there are exceptions to this, but they are rare). The other - and more crucial one really - is that there is stasis. We find some species that do not change a lot through large chunks of geological time. A lot of index fossils fit that bill, biozones defined through the occurance of a specific species lasting for more than 1Ma. So you´ve got a lot of species that remain morphologically stable for extended periods, but then change rapidly (you find them by the 1000s, that´s why they are index fossils - if they weren´t common, they´d be of little use - so not finding the transition means it must take a short time compared to the time they remain almost unchanged). Eldredge had worked on Trilobites where this is rather striking. The main support for PE is stasis. A splotchy record can explain a lack of transitional forms, but if there is an excellent record of stasis there´s no good explanation for why it should turn bad just when the interesting bit happens. Then you find things like the Steinheimer Becken, where the interesting thing does happen (neat transitional forms for the snails of Steinheim), or the snails of Kos, or... And in each of these you have an extraordinary temporal resolution, laminated lake sediments where you can pretty much count through annual layers. It all happens in a few 1000 years. So the combination of these things - big changes that show up as gradual on a scale measured in 100s of years and next to no changes on a scale measured in millions - is what propted PE. Both were well known, but the novelty of the PE paper was that it connected the two for the first time into a view on the distribution of rates of morphological change.

Darwinsbulldog wrote:Surely the rates of morphological change are based on how when and where clades are able to adapt to environmental by playing with their [non-conserved] developmental gene complexes [or their upstream or downstream expression]. I think that this is highly germane to the discussion. An organism may look wildly different, but the underlying Bauplan is highly conservative. In mammals for example, the cervical vertebra number only seven, but humans and giraffes look very different. The genetic/molecular evidence thus has confirmed many homologies that anatomists have always known about, but by no means all. Thus you can have huge changes in morphology that is based on small [but important] changes in the Hox complexes. Thus fruit flies and humans look very different, yet at the genetic level, they share homologies.


Indeed. But there we may enter into another can of worms, that for some reasons tends to enter discussions on PE (mainly because Gould wrote some things about this as well - constraints of baupläne and ontogenetic effects one of his key areas). It´s an interesting area to look into - where cutting edge science meets with von Baers law and Fishers geometrical argument (changes in HOX genes affect early development, leading to large changes in adult form, the larger a change is the less likely it is to be adaptive [Fisher], thus HOX genes tend to be highly conserved and so is early development [von Baer]). But such changes are compatible with both PE and PG. In the graphs they´d end up on the far right (where the PE and PG curves don´t differ much). The second peak in the PE graph does not correspond to fundamental changes in the baupläne, it does correspond to fast changes of features within them. If you look at the giraffe neck vertebra, Giraffa first appears about 11.6Ma ago and the containd species have short necks. The first long necks appear about 1.8Ma ago and Giraffa camelopardalis does not further increase the neck lenght. Now, the time span during which this happened is probably some 100ka long, but compared to 10Ma with no significant changes that´s rather short (1% of the time span). So that´s where PE comes in.

So at the genetic level, it looks more like gradualism rather than PE. Further, these genetic changes seem to pre-date the appearance of morphological change as seen in the fossil record.

I note a typo in the literature list. The last one should be:
Gould et al., 1977 "The Shape of Evolution: A Comparison of Real and Random Clades", Paleobiology, 3, 23-40.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#16  Postby Darwinsbulldog » Jun 20, 2010 5:06 pm

@susu.exp

:cheers: :cheers:

[My printer will overheat, if I keep on printing out your gems. :clap: ]
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#17  Postby David M » Jun 21, 2010 3:25 pm

Nautilidae wrote:Being one that is more ignorant when it comes to evolution, I do not know which is the favored evolutionary model: punctuated equilibrium or phyletic gradualism.

For those of you who do not know, punctuated equilibrium is the evolutionary model that models species as undergoing very little evolution for long periods of time before suddenly (on evolutionary time-scales, of course) evolving fairly rapidly.

Phyletic gradualism is the model that models new species as coming about gradually with no periods of major evolution. Life evolves gradually.

Which is the favored model and why?


Both are correct as mechanisms.

The evidence shows that some species have evolved gradually until they reach the point of speciation while some species undergo rapid evolution to the point of speciation.

The jury is still out on what the exact balance is but it looks like its somewhere around the 50/50 mark as to whether speciation was a result of gradualism or PE rapid evolution.

It is important to note that Gould etc did not exclude gradualism completely and even Darwin did not propose gradualism as the only way.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#18  Postby Berthold » Jul 19, 2010 5:28 pm

David M wrote:...and even Darwin did not propose gradualism as the only way.

Dawkins, too. He wrote one essay in which he said (of course, more elaborately :grin: ),"Well, duh, we knew that all the time." Gould, in response, accused him of somwhat misrepresenting the idea of PE (reducing it to just the occurrence of different speeds of evolution) in this essay.

Edited to add: It's "Puncturing Punctuationism", Chapter 9 of "The Blind Watchmaker"
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#19  Postby Sityl » Jul 29, 2010 3:04 am

ginckgo wrote:
num1cubfn wrote:I'm not expert, but I thought various species had traits of each. For example, sharks haven't changed much, nor had trilobites over 100+ million years, but animals like cows and chickens have undergone much larger changes in smaller times, because of increased selective pressure.

I could be completely wrong though.


You are completely wrong :grin:

Trilobites changed massively throughout their main regnum from Cambrian to Devonian. The fact that they all still retained their basic morphology (cephalon, thorax, pygidium, legs plus gills on each thoracic segment, etc) is no more 'unchanging' than all vertebrates retaining four limbs, a head, a spine, etc. We are just more biased to see minor variations as bigger than in other phyla. Having said that, sharks have also varied quite a bit.

/rant


Sorry if I implied that there were no changes to trilobites, but they WERE around for 300 million years. While there may have been changes, they stayed pretty much the same for a pretty damn long time. If you compare that to the changes in cows and chickens in a few thousand years of concentrated domestication, the difference is huge. That being said, I still don't know how to answer the OP's question.
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Re: Phyletic Gradualism vs Punctuated Equilibrium

#20  Postby Darwinsbulldog » Jul 29, 2010 8:58 am

DB wants to make an apology and retraction. After going from being an avid fan of the late Professor Gould, I realize that I have been a little too overcritical of him. Hence this has made me a little too critical of PE than is warranted by the facts. Reading some evo-devo books, especially by Carroll and others, has made me realize how minor changes to Hox gene clusters can lead to really huge-looking morphological change. Which just goes to show, pre-conceived ideas, or dogma, have no place in science-ever!
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