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Author | Topic: Does Neo-Darwinian evolution require change ? | |||||||||||||||||||||||||||||||||||||||||||
RAZD Member (Idle past 1405 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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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined:
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Hi slevesque, you certainly stirred up an hornet nest this time eh?
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. Two things: First, not all the population will be at the peak. If you look at the bell curves mentioned by AZPaul3 in Message 36, you will see that in each case the vast majority of individuals will still not be at the optimum. This is partly due to shifting optimum points and partly due to the non-direction of mutations. Some will be towards the peak, some will be away, some will be neutral, the further they are away, the stronger will be the selection against their continued success. Second, one generation of offspring may be forced away from the peak for the optimumest individuals, but that leaves room for the next generation to move back towards the peak, while selection acts against any further movement away from the peak: result is oscillation around the mean optimum values. The stasis involves the whole population, with all the various hereditary lineages involved in these oscillations for all of their relevant hereditary traits, they average out, and thus the population as a whole will still form a bell curve around the optimum adaptations. 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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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined:
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Hi slevesque.
But this is statistically very unrealistic. If a generation moved away from the peak by 50 mutations, in a genome of 3 billion, it is extrememly improbable that the next generation will move back towards the peak on not simply farther away. Why? Draw a circle around the original individual and then draw the same size circles around a point on the circumference of that circle. The first circle represents the range of possible mutations away from the position of the parent individual, the second is the possible range of mutations away from one of the outermost the offspring -- how much of that outer offspring circle is inside the parent circle? I get 39.1%, which I do not count as extremely improbable, and that is the worst case. You could have second generation individuals slightly away from center such that >90% of their offspring would be within the parent circle. With neutral drift there could be a slight tendency to move subsequent offspring gradually away from the parent center, but when selection is included, the portions inside are differentially selected from the portions outside, thus weighting the proportion within the parent circle higher than the proportion outside the circle. If sexual selection is involved there would be a strong selection for individuals at the center to reproduce more, to the disadvantage of outer individuals, where some may be left entirely out of the picture for the next generation. If we disregard sexual selection, there can still be significant selection pressure. Just because the population appears to be in stasis, this does not mean that there is no selection pressure in the population: if the population has expanded to consume the limits of the ecology (which is normal in a stable ecology), and reproduction produces many more offspring than needed to replace the losses, then competition for limited resources can produce high survival selection, such that any second generation individuals within the circle could reproduce significantly more than those outside, thus moving the offspring back towards the center. You can also envisage fitness topologies around the peak, and the further away an individual is, the more of their offspring circle is inside the topology line, and the stronger would be the selection pressure to benefit those inside versus those outside the topological boundary. It becomes very easy, imho, to see that most of the population would always be near the peak, that the trend for the offspring of the outer individuals would be towards the center, overcoming any slow trend of the offspring of central individuals away from the center, thus generating oscillations in the lineages where movement away in one generation is balanced by movement back towards the center in other generations. Enjoy. (2/3-(3)0.5/2π) = 0.391 Edited by RAZD, : clrty 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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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Hi sfs, thanks,
I haven't been following the argument here, but this analogy is not a good one. If you haven't followed the argument, then how do you know the analogy is not a good one?
Mutations are, to a first approximation, orthogonal to one another, so you should be working in a much higher dimensional space. Imagine the 50 mutations to be fifty unit steps in 50 of perhaps a million dimensions. The next 50 mutations will be in another 50 random dimensions. The probability of heading back toward the peak (neglecting selection) is tiny. What was being discussed was the relation of the individual to a fitness map, with a peak of fitness for the population in a static ecology. You can correlate all your 50 dimensional mutations into direction to or away or orthogonal to fitness and from that derive a radius for each individual relative to the parent.
(neglecting selection) And we definitely are not neglecting selection. 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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RAZD Member (Idle past 1405 days) Posts: 20714 From: the other end of the sidewalk Joined: |
Hi sfs, becoming pedantic to maintain your original position?
No, you can't. That's why your analogy is a bad one. My point was that there are many, many orthogonal directions to take away from fitness. Or towards fitness. Seeing as it is virtually impossible to be 100% fit, there is always room to become more fit, the more so for second generation offspring when the first generation is moved away from their parents locus to a lower fitness topology.
In biological terms, there are an enormous variety of ways that you can screw up an organism which are nearly independent of one another; even if you already have mutations that cause some of those defects, the next set of mutations is still overwhelmingly likely to cause new defects, not to fix the old ones. But it isn't necessary to fix old ones, it is just a matter of becoming more fit than the first generation.
I know. But your analogy was about the selective landscape, not about how populations respond to selective pressures. Curiously, I thought it was about visualizing the fitness landscape, and whether third generation offspring can move towards, or away, from fitness, according to their survival and reproductive success relative to their parent/s. 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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