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Author | Topic: Does Neo-Darwinian evolution require change ? | |||||||||||||||||||||||||||||||||||||||||||
Dr Adequate Member (Idle past 315 days) Posts: 16113 Joined: |
This is what I'm asking: given what we know of genetics, and what we know of selection, if we observe any given species (even in an unchanging environment), will we automatically observe macro-evolution if we stay long enough (except in maybe some very statistical rare cases) PS I never liked the terms macro and micro evolution. Me neither. The question you pose is an interesting one. How much evolution would we see in an old species in a perfectly stable environment? well, adaptive evolution would stop, and genetic drift will go on happening. Now it should be pointed out that a variation which is neutral is not necessarily without consequences. When we use Darwinian algorithms to solve a problem, the solution goes to a local optimum (which we hope will be the global optimum) and stays there. this would suggest on the face of it that this is what would happen in real life. But in such algorithms the characters on which the program is operating are usually relevant ones. If we're trying to produce a design for the acoustics of a concert hall (to take an example at random) then what we're evolving is the shape. If we made the color one of the characters evolving, then being irrelevant to the fitness of the solution, it would vary at random. Now, the existence of cryptic sibling species sharing the same environment shows that such variations can produce speciation. In a way this is interesting, and in a way it isn't. What about variations which have a visible effect on the phenotype? That's more tricky to think about. Are there, in fact, variations which would be both obvious and selectively neutral? It has been suggested that we can see this in ammonites. The shape of their shells can be more or less smooth or ribbed or bumpy, and certainly varies over time which is why they make such nice index fossils. and yet there doesn't seem to be a pattern or direction to the variation. It could be, however, that we are ignorant of the underlying causes. We might speculate that this is a form of Red-Queen's-Race style adaptation which we could understand if we correlated it with the shape of the teeth of their major predators. Or perhaps it is a side-effect of some adaptive change. Maybe they were changing their biochemistry to try to get one step ahead of some bacterial parasite, and this just happened to change their external form as an incidental consequence.
It brings up two issues. First, living fossils, where a species appears in the fossil record millions of years ago, dissapears, and is found alive and almost identical today. Such a group is more accurately described as a Lazarus taxon, and they're usually not "almost identical" after a long period of time.
Second, one of the two great trends in the fossil record, according to Gould, is stasis (the other being the sudden appearance of new species). This is what we would expect. Once a species has evolved to fit its environment, there's no selective pressure any more, and if the sort of visible morphological neutral drift I've been discussing doesn't happen, then stasis is what we would get. Edited by Dr Adequate, : No reason given.
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jar Member (Idle past 425 days) Posts: 34026 From: Texas!! Joined:
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Extinction arose when the selective pressures became extremely high, ie they went way over the limitation of the cost of selection and it drove the species to extinction. But certainly, you are not proposing that this is 'normal' in the existence of a species. A species that selective pressures are pushing towards extinction is certainly not evolving, it is at the end of the road. (Even if the selective pressures stop for whatever reason, there is still a chance that species will undergo genetic meltdown because of a lack of genetic diversity) This is why I am talking about the period in the existence of a species, where selective pressures where normal, and it's the whole point: during that period, can it even stay in stasis ? Yes, extinction does seem to be normal and almost universal.
slevesque writes: jar writes: Neo-Darwinian evolution is a century old term and guess what, it is no longer way back then. We have learned much, particularly in the last half century or so, and the Theory of Evolution itself has evolved since then. With all due respect to dr. Mayr, even if the term had a given definition back in 1895, does not mean that definition hasn't changed with time. The wiki quote you provided certainly shows this when it says that
quote: I understand that ''Neo-Darwinian evolution'' is globally understood within the scientific community to describe the modern synthesis. Note the qualifiers even there. "As practiced today". "current evolutionary theory". Return to the question I asked back in Message 4; "Further, what does the topic title even mean? What exactly is "NeoDarwinian evolution"? What was "Neo-Darwinian evolution" then years ago? How about a quarter century ago?" You are answering the very question YOU asked in the topic. Theories change as do critters; both evolve. The existing Lazurus species are NOT identical to examples from millions of years ago. There are examples of insects that gained wings, lost wings and then regained entirely different wings then in the earlier iteration. Modern Coelacanth are not the same as ancient ones. Modern Horseshoe Crabs are not the same as ancient ones. I think part of the issue can be seen in what I quoted above. You say "A species that selective pressures are pushing towards extinction is certainly not evolving, it is at the end of the road. " But that too is evolution. Evolution does not mean improving or even surviving. Critters can evolve into failures as well as evolve into successes. Anyone so limited that they can only spell a word one way is severely handicapped!
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PaulK Member Posts: 17828 Joined: Member Rating: 2.6
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I think that we need to clarify the basics of Punctuated Equilibria (again).
It is necessary to start with the fact that P.E. deals with evolution and Paleontology. The "species" of P.E. are paleontological species. Thus, changes which do not show in the fossil record or changes which cannot be distinguished from the normal variation within a species are all permitted within "stasis". Since we know of "cryptic species" - biological species which are morphologically identical - it is quite possible that some paleontological species would be considered two distinct biological species if we had living specimens - perhaps some would even be split into three or even more. The next important fact is that P.E. is a prediction of evolutionary theory - or to be more precise Mayr's theory of speciation (still felt to be the main mechanism by which new species form). Mayr's mechanism starts with a small population becoming isolated from the main body of a species. Large populations are slow to evolve, by drift or even selection - this smaller population can change far more rapidly. If this smaller population thrives and evolves to form a new species and if it is able to return it may overrun the territory of the ancestral population, giving the appearance of a change more sudden than it actually was. Since most of the evolution happens relatively rapidly (but still "gradually" by human standards) and in a small region it is not uncommon for it to be missed - either absent from the fossil record for one reason or another, or simply not found by us. Thus Eldredge and Gould's "stasis" is explained by gene flow acting as a stabilising force often restricting the change within successful species - while they remain successful ! - to change within the limits of paleontological species.
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slevesque Member (Idle past 4671 days) Posts: 1456 Joined: |
Yeah well, with conservative numbers and a species with a long generation time, I guess you could fall into the ballpark of acceptable.
But 50 Mpipg was very conservative. First off, you have assumed all these mutations were point mutation, affecting only one base pair in the genome. This is not necessarily true. Also, 50 is a very small number, Sanford in his ''genetic entropy'' book cited a recent study which had point mutations only to have a lower estimate of 300 (max 600. in humans). Add unto that insertions, deletions, inversions and you could probably get two ten-fold the number we used here. (actually, I think he got the 300 lower estimate from personnal correspondance) PS And yeah, 2M not 2,5M. That what happens when you calculate at 4h in the morning
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slevesque Member (Idle past 4671 days) Posts: 1456 Joined: |
This is what we would expect. Once a species has evolved to fit its environment, there's no selective pressure any more, and if the sort of visible morphological neutral drift I've been discussing doesn't happen, then stasis is what we would get. The I guess I could formulate it this way: Given the high mutation rates, how can morphological drift NOT happen ? If every single offspring will have 1 mutation or more, and usually much much more, how can the species even stay at that local optimum ? Won't it be 'forced' to drift away from it by the high mutation rate ? I guess natural selection would slow down this drift (meaning the drift isn't morphologically neutral), but I can't see how it could ever bring it to a stop.
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slevesque Member (Idle past 4671 days) Posts: 1456 Joined: |
I know the basics of Ponctuated equilibrium, and I don't see how all you said about it affects what I'm talking about. Except maybe this piece:
It is necessary to start with the fact that P.E. deals with evolution and Paleontology. The "species" of P.E. are paleontological species. Thus, changes which do not show in the fossil record or changes which cannot be distinguished from the normal variation within a species are all permitted within "stasis". But unless you are saying that Gould gives a definition to ''stasis'' that is completely foreign to me, in which said definition allows for significant morphological changes to still be described as stasis, then even this part is irrelevant.
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slevesque Member (Idle past 4671 days) Posts: 1456 Joined: |
I think part of the issue can be seen in what I quoted above. You say "A species that selective pressures are pushing towards extinction is certainly not evolving, it is at the end of the road. " But that too is evolution. Evolution does not mean improving or even surviving. Critters can evolve into failures as well as evolve into successes. I know it seems tempting to ascribe the term evolution to every single step in a species life, but I think ultimately it is fudging the factors. A species being driven to extinction is experiencing extinction. You can't be saying that it is gaining any novel features, you can't say it in way of becoming another species, you can't even say it is experiencing a change in the frequency of an allele in the population. When selective pressures are at such a high level, all this breaks down and I can't see how any definition of evolution could encompass such a situation.
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PaulK Member Posts: 17828 Joined: Member Rating: 2.6 |
I'd say that you missed the point that gene flow reduces the rate of change. And I thought the point that stasis definitely does not mean "no change" was worth mentioning as well as the fact that biological species can be morphologically identical.
I'd also say that it looks as if you are assuming that most mutations have visible morphological effects, which I rather doubt even of those mutations which do have effects - and your "high" mutation rate must be counting many more which have no effects whatsoever (by a large majority).
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jar Member (Idle past 425 days) Posts: 34026 From: Texas!! Joined: |
You can't be saying that it is gaining any novel features, you can't say it in way of becoming another species, you can't even say it is experiencing a change in the frequency of an allele in the population. When selective pressures are at such a high level, all this breaks down and I can't see how any definition of evolution could encompass such a situation. HUH? The species did not pass through the filter of Natural Selection. It became extinct. The Theory of Evolution not only helps us understand the diversity of life we see today, it helps us understand the life that is not here today. Evolution is not about any one specific species, it is simply the result of all the processes in the past. Remember there are two parts. There is the side where there is change and the filter that determines what continues. Anyone so limited that they can only spell a word one way is severely handicapped!
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RAZD Member (Idle past 1436 days) Posts: 20714 From: the other end of the sidewalk Joined:
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Hi slevesque, interesting question, and I think Dr A has the jist of it.
But unless you are saying that Gould gives a definition to ''stasis'' that is completely foreign to me, in which said definition allows for significant morphological changes to still be described as stasis, then even this part is irrelevant. If the ecology is static then the opportunities for change are limited, what selection there is, survival and reproduction, will be towards the mean of the population, that portion which has already become adapted to the ecology for that species, and thus enjoys the most success at survival and reproduction. You can model adaptive fitness as a topology with a peak at optimum adaptation for the species. Change away from that peak means selection for less fitness, which would be selected against. This will not rule out non-adaptive change as long as it is selection neutral for reproduction as well as survival. Thus you could have coloration changes or "spandrels" develop (as on the nautilus shell, perhaps). This is another term from Goulde, taken from architecture.Spandrel (biology) - Wikipedia And coloration, mating behavior and cryptic speciation (mating at different times of the day?) would not show in the fossil record, as has been pointed out. Over time you would likely see drift around an average stasis morphotype with the same kind of differences observed in the Asian Warbler ring species:Greenish warblers Thus you would likely observe reproductive isolation if you could have some way to test mating of individuals across generations, but not be able to observe it in the fossil record. Enjoy. by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)
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slevesque Member (Idle past 4671 days) Posts: 1456 Joined: |
You can model adaptive fitness as a topology with a peak at optimum adaptation for the species. Change away from that peak means selection for less fitness, which would be selected against. I understand all these, but I don't see how it answers what I'm asking. Which is that given the high mutation rates, how can it stay at that optimal peak when every single offspring will have inherited so many mutation (the majority deleterious, most only very slightly). Whichever one natural selection ''chooses'', it will still be less fit then it's parents were. As I said, any mutation rate over 1pipg seems to be forcing the population from the optimal peak, with natural selection slowing down the drift but unable to stop it. In fact I have difficulty imagining how any species could sustain the mutational burdain of such high rates.
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Coyote Member (Idle past 2137 days) Posts: 6117 Joined: |
I understand all these, but I don't see how it answers what I'm asking. Which is that given the high mutation rates, how can it stay at that optimal peak when every single offspring will have inherited so many mutation (the majority deleterious, most only very slightly). Whichever one natural selection ''chooses'', it will still be less fit then it's parents were. As I said, any mutation rate over 1pipg seems to be forcing the population from the optimal peak, with natural selection slowing down the drift but unable to stop it. In fact I have difficulty imagining how any species could sustain the mutational burdain of such high rates. Your scenario results in all species going extinct, so obviously it is incorrect as total extinction is not supported by the evidence (several million extant species). Related point: are you thinking of this scenario with old or young earth in mind? Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.
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Dr Adequate Member (Idle past 315 days) Posts: 16113 Joined: |
The I guess I could formulate it this way: Given the high mutation rates, how can morphological drift NOT happen ? Well, perhaps sometimes it does. But in the first place it would have to be neutral to avoid selection, and in the second place, how many morphological mutations do you see?
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Dr Adequate Member (Idle past 315 days) Posts: 16113 Joined: |
I understand all these, but I don't see how it answers what I'm asking. Which is that given the high mutation rates, how can it stay at that optimal peak when every single offspring will have inherited so many mutation (the majority deleterious, most only very slightly). Whichever one natural selection ''chooses'', it will still be less fit then it's parents were. As I said, any mutation rate over 1pipg seems to be forcing the population from the optimal peak, with natural selection slowing down the drift but unable to stop it. In fact I have difficulty imagining how any species could sustain the mutational burdain of such high rates. Since this is contrary to observation, the sensible conclusion is that you are overestimating the frequency of deleterious mutations.
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Dr Adequate Member (Idle past 315 days) Posts: 16113 Joined: |
Cost of selection obviously limits natural selection from ever keeping up with all these new mutations. Hold on a minute. Cost of selection usually refers to the wastage involved in getting a beneficial mutation from one in the population to fixation --- that is, the cost of adaptive selection. The cost of getting a deleterious mutation from one in the population to zero is obviously much lower. Conservative selection is easy. Cost of selection as the phrase is usually used has nothing to do how hard it is to maintain stasis. Now if you want to use the phrase in a broader sense, and will stipulate as much, then that's fine --- but in that case it is no longer "obvious" that cost of selection places any stringent limit on what natural selection can do. Edited by Dr Adequate, : No reason given. Edited by Dr Adequate, : No reason given.
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