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Member (Idle past 4844 days) Posts: 624 From: Pittsburgh, PA, USA Joined: |
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Author | Topic: Tautology and Natural Selection | |||||||||||||||||||||||||||||||
Quetzal Member (Idle past 5872 days) Posts: 3228 Joined: |
[Added by edit:: I'm not sure but I think I might be mixing 'levels' in my view of fitness] Ya know, I think you're probably right. Not only are there several biological meanings of "fitness" depending on whether you're talking gene or organism or population (or whatever), but several different uses of the term. You can talk in terms of how well adapted ("mean fitness") a population or species is for its particular niche/habitat. You can talk in terms of the fitness of a particular individual (or trait or suite of traits) compared to others in the same population or in relation to the mean fitness of its species or even in relation to other competitive species. Etc. So the problem is quite likely one of both level and useage. Which is why, when we get back to the original discussion of "survival of the fittest" as a tautology, it's extremely easy to conflate or confuse several uses/meanings/levels because of a fixation on the word "fitness". It's such a bloody useful term tho', when used properly and in context, that I can't think of another way of saying it. It's like the people who want to replace "species" - because it's so squishy and hard to define - with "evolutionarily stable unit" or some such awkward construction. Still, it makes for interesting discussions...
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Peter Member (Idle past 1479 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
quote: Was ... was that a subtle way of saying we're off topic here I have been unable to see where 'survival of the fittest' isa tautology, though. It was a phrase coined to summarise what Darwin originally said about natural selection so maybe it's just out-dated wrt modern definitions of fitness et al. Darwin's idea of fitness seems to me to have been an individualquality that aided survival, and that enhanced survival probability was considered to increase the chances of high reproductive output. Saying 'the ones that are fittest stand the best chance ofsurviving' is kind of defining 'fitness' for that context. Personally I'd be interested to see the definition of reproductivesuccess -- that seems even more vague to me. Syamsu would say it means to breed or not to breed
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Quetzal Member (Idle past 5872 days) Posts: 3228 Joined: |
Saying 'the ones that are fittest stand the best chance of surviving' is kind of defining 'fitness' for that context. Yep. And therein lies the rub, as it were. The way you used fitness here is only valid in terms of comparing two (or more, I suppose) organisms in a particular population at a specific moment in time. Survival in that case IS the "win" for the individual. However, it is misleading (at the least) for discussion of relative fitness in an evolutionary context. You can use fitness-as-survival as a measure of where a particular critter sits on its fitness landscape, I suppose, in comparison to its conspecifics. However, no matter how long-lived and happily scampering about it is - watching its friends and relations bite the dust around it - if it never reproduces it is evolutionarily unfit. Literally a dead end. Even if it DOES reproduce, it may not necessarily be "fitter" if its traits get swamped - homogenized - by the traits in the rest of the crowd down through the generations. It's all context.
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Peter Member (Idle past 1479 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
Comes back to theory vs. description though.
Are there organisms that are long-lived wrt peers thatleave less offspring? --apart from humans that is Just reproducing a lot doesn't make you evolutionarly fit ifall your offpsring die within a week of birth. Fitness must be a function of both reproductive output and survival.
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
When you say with respect to peers do you mean 'Are there variations of r and K strategy within species?'?
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JustinC Member (Idle past 4844 days) Posts: 624 From: Pittsburgh, PA, USA Joined: |
quote:This is what I think of when I say reproductive success, i.e producing offspring that have a good chance of producing offspring of their own. So individual survival would be a part of it, and reproductive output would be apart of it. In 'survival of the fittest', I would say if you are speaking of individual organisms you would have to interpret survival as reproductive success since their is no selection for just surviving without reproductive output and no selection for reproductive output without survival. Do you know see how the tautology issue arises? And I'm sure some organisms in the wild are born sterile and survive along time, although I never observed it.
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Peter Member (Idle past 1479 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
Yes.
Within a species do you get some idividuals that producea lot of offspring and die young and others that produce one or few and die old?
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Mammuthus Member (Idle past 6475 days) Posts: 3085 From: Munich, Germany Joined: |
Hi Justin,
Sorry for the late response..I have been writing a paper for publication...and it is really boring ..so I had to do something to spice it up like frying my modem so I had no internet connection for the last 3 days. quote: Yes, the new introduced variation would be in competition...it is a new allele or trait that can spread in the population and will either be more or less relatively good at spreading itself.
quote: If a mudslide kills the member of the population before it reproduces it has a fitness of zero. Chance plays a huge role in all of this. Drift is important, freak accidents, all of this must be factored into determination of relative fitness. If the cause is an accident you don't look for traits invovled in the fitness differential. If it is traits you don't look for design..as it could be as simple as the deletion of a cell surface receptor i.e. in principle a detrimental mutation that causes the fitness advantage like with individuals naturally resistant to HIV infection. It is also complicated since it is often quantitative i.e. lots of small effects accumulated bestow the fitness advantage and it can be a real pain to tease out the individual contribution of each trait..similar problem with quantitative genetics.
quote: The problem with such a model is that single cell organisms typically reproduce asexually and can transmit traits horizontally by conjugation. Thus a bacteria that has not photoreceptor system could get the entire pathway with one conjugation even in theory. But the ultimate effect in such a population is selection pressure was high tfor have such a system would be to drive it to fixation in the population as the individuals without it would not produce viable offspring...or those with it would consume all the resources available or find the resources more efficiently leaving nothing behind for those without the trait...or the population with the photoreceptor gets to close to a thermal vent and is completely eliminated and only the non-photoreceptor variatns remain..and evolution has to start all over again
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Peter Member (Idle past 1479 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
If 'survival' is reproductive success and 'fitness' si reproductive
success I think I can see a tautology. One of the suggestions I have made before is that it is tautlologousonly with modern conception, and that when phrased it was simply stating a definition or summary of Darwin's natural selection observations. Darwin focussed his attention on survival to reproduce. Before anyone starts moaning about harping on about Darwin whenToE has moved on .... 'Survival of the fittest' was coined in Darwin's time, and used by Darwin as a sub-title in chapters on natural selection. Natural selection is still more about survival of the fittestthough. Extreme PoV example: Suppose you have a population with 400 variants, all of whomhave indefinite life-span and none of whom breed. Environmental factors operate such that some individuals havea reduced survival capacity relative to others and so find it harder or impossible to survive. Result, some of the original 400 variants disappear completely,and the representation of others diminishes significantly. Allelic frequency in the population has changed over time. Isn't that a definition of evolution? In the above the absence of reproduction means there is noadaptability, so many changes of environment would eventually wipe out the whole population. It does not prevent natural selection changing the allelicfrequency of the population.
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
You really are turning into Syamsu now Peter, you have just started an argument based on a fictional hypothetical population specifically tailored to suit your point but completely divorced from anything which actually exists in the real world.
How can you show that the fitness in your population is related to the genetic variation rather than anything else, given their incapacity to breed?
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Quetzal Member (Idle past 5872 days) Posts: 3228 Joined: |
Are there organisms that are long-lived wrt peers that leave less offspring? I've been wracking my brain to see if there were any examples of this in nature to no avail. The closest I came was a vague memory of differential reproductive success between two populations of neotropical songbirds (and I can't remember what the genus was to save my life) based on acceptance/rejection of cowbird nest parasitism. One population solved the problem of nest parasitism by the simple expedient of booting the cowbird eggs out of the nests as soon as they were laid. The other population tolerated the cowbirds (or at least didn't get rid of them), consequently suffering at least 50% reduction in nest survivorship (i.e., reproductive success). It turns out that both populations suffer from infestations of parasitic flies which caused staggering infant mortality if not controlled. In the non-tolerant population, the birds had developed a comensual relationship with aggressive wasps which controlled the flies. The tolerant population accepted the cowbirds because they would have lost even MORE infants to the flies. Apparently with a cowbird in the nest pecking away at the flies (which are evidently a cowbird delicacy), fewer infants were lost to cowbird parasites than were lost to fly parasites - making the best of a bad situation and actually increasing reproductive success for the population as a whole. I don't know whether this relates to your point about survival or not. Hmm, just realized that this might be speciation in action: a significant behavioral isolating mechanism, maybe. It would be an interesting experiment to pull a switcheroo on the populations and see what happens. Nature is soooo cool. [Edited for clarity. I had my flies and cowbirds mixed up.] [This message has been edited by Quetzal, 08-04-2003]
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
There are some interesting studies on the links between longevity and fecundity in drosophila, of course, and some other animals. There was an interesting review of work on guppies in high and low predator environments by a fellow called Reznick and others. Sadly my computer refuses to cut and paste in Mozilla anymore so I cant give you all the details of the reference, but a pubmed search for 'Reznick' and 'guppy' should bring it up.
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Peter Member (Idle past 1479 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
quote: LOL!! Humour me for a couple of posts and we'll see whetherlooking at hypothetical limit cases is worth the bother quote: The same way you would with a breeding population -- you lookat trait frequencies as a snapshot of the population at some point. One assumes (as is the case for real populations) that thetraits which dominate are those which confer a 'fitness' advantage. You can apply exactly the same reasoning and analysis vianatural selection to changes in trait frequency in this limit case, even though there is no reproduction going on. Removing survival issues and just looking at reproduction we couldhave 400 variants who each produce a different number of offspring during their maximal lifespans. If all live the same amount of time the one which reproducesthe most dominates traits set. If they do not -- a survival factor -- this changes. Survival must be a part of what fitness means. That's all I'm saying.
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Mammuthus Member (Idle past 6475 days) Posts: 3085 From: Munich, Germany Joined: |
quote: Why do they have to live the same amount of time? If one variant produces 50 offspring in one year and then dies and another produces 3 offspring but lives 100 years the first one that died after one year has a much higher relative fitness though it was relatively lousy at long term survival....produce more offspring..you win
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Peter Member (Idle past 1479 days) Posts: 2161 From: Cambridgeshire, UK. Joined: |
But then you haven't elliminated survival from the
equation -- which is what what I was trying to do in the same way that I attempted to elliminate reproduction from the other case. Assuming the offspring have a similar life-span the sepciespersistence matters, doesn't it? Case A: lifespan = 1 year reproductive output = 50 represents 50 organism-years ( in management terms) Case B: lifespan 100 years reproductive output 3 represents 300 organism years. Reproductive output alone cannot be used as a fitness measure. Even if you just look at generation years, A above has to persistfor 100 generations to match B in persistence terms.
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