Tautology and Natural Selection

Hola Mr. H:Would it help for the purposes of this conversation to simply forego discussion of reproductive success as it relates to fitness? Substituting the slightly more reductionist concept of "marginal fitness", which merely describes the tendency for the frequency of a specific allele or suite of alleles to vary in a population over time based on environmental interaction without discussing the "how it increases", would seem to eliminate the tautology problem. (Damn, that was a long sentence.) In addition, it removes the emphasis on reproductive success that seems to have everybody in knots. If the allele increases, it may be caused by anything from drift and other stochastic processes to deterministic processes. Finally, it's a fairly easy one-step to get from there to discussions of mean population fitness, when you want to take it to a different level.Hope that made sense.

Hey there my hairy friend,I don't disagree with you - and I don't understand either what the problem is with the observation that the critters that have alleles that provide some kind of advantage are more likely to leave offspring. However, it looked like the discussion had completely bogged down, so I was trying to be creative in getting it going again."Reductionist" the way I meant it was to say marginal fitness takes it down to the genetic level and ignores the individual organism - a frequency of alleles in a population. Could be a plus or a minus in terms of the individual's chance to survive to reproduce. Oh well, so much for trying to get the conversation going again.

Which, of course, is why I prefer talking in terms of populations or communities rather than organisms. The only "purpose" for a given population in a particular habitat (or a given community in a particular ecosystem), is to persist. It really doesn't matter whether or not one or any number of its members lives or dies, as long as the population continues. In a hypothetically pure equilibrium situation, the number of individuals that bite the dust or emigrate is precisely balanced by the number of individuals that are born or immigrate. From the population's standpoint, it really doesn't matter HOW that occurs. Reproduction is what individuals do - persistence is what populations do. Obviously the actual dynamics in a wild population are significantly more complex, with a myriad of stochastic and deterministic factors which will effect it. However, speaking of purpose beyond persistence at equilibrium is waaaay too anthropomorphic for me.I'd damn Spencer to the outermost hells for the miserable "survival of the fittest" phrase he invented - if I believed in hells, that is. As it is, I just think that conceptualization has done more to skew people's understanding of natural selection than anything else I can imagine. Shows you shouldn't use soundbites if you want to understand something.Hey, everyone has a pet peeve...

You were okay up to here, I think. As Mammuthus pointed out, the engineering analogy falls flat as the only way we can call an organism "fit" is by the post hoc analysis: if it survived, it was fit enough to do so. Merely an observation, and somewhat trivial to boot. In addition, the idea of "best engineered" doesn't work when you take into consideration evolutionary anachronisms like Persea (avocado), Gleditsia triacanthos (honey locust), etc. They reproduce in the wild, more or less, but do so veeerry inefficiently, since their reproductive strategies rely on seed dispersal by critters that have been extinct for 10,000 years. They are "fit" in the sense that they have persisted as a lineage even in the absence of a well-engineered reproductive strategy. They are "good enough" (barely), but not necessarily fitter than other plants in their various habitats.

I don't disagree at all inre fitness landscapes.You raise an interesting question, however. I wonder if it is really necessary to take mass extinction events into consideration when discussing population or species-level adaptive fitness? I don't mean source-sink dynamics (the "christmas lights" phenomena), or Vrba's turnover pulse species exchange (which is regional). These more-or-less local events would/could have differential survival based on phenotype etc. I mean the really BIG die-offs like the K/T extinction or the P/T extinction event which literally rewrite the evolutionary script for a substantial fraction of the planet. Heck, the P/T extinction nearly cancelled the entire experiment. I'm not clear that any fitness measure at all would be relevant to this level of event. Meaning I'm not sure it would be necessary to take them into consideration for a discussion of relative or absolute fitness or adaptation.Mammuthus is our resident extinction expert (although specializing in Late Pleistocene). Maybe he's got an idea or two?BTW: You're right, paying off the margin call for the damn dinosaurs was the pits...

Hi Justin. This is a neat discussion, btw. I agree in these cases. However, the critical distinction here is that with experiments such as you reference, you know every aspect of the fitness landscape - in fact, you control it - in advance so it's fairly easy to make a reasonable prediction as to potential outcomes. When examining a population in locus, however, the only way you can determined fitness is by a detailed examination of the fitness landscape in which the population resides. An a posteriori analysis. Sometimes you can get enough of the natural history of the organism to state that a given characteristic or suite of characteristics provides "better" fitness. Most of the time, however, you have to extrapolate from questions such as why this organism rather than that one occupies a given niche. IOW, what traits are possessed that makes the organism "fitter".What the E. coli experiments and others of the same type provide is unequivocal (well, to anyone but a creationist) evidence of the creative power of natural selection acting on statistical variation within a population. As to the wind tunnel, of course you can occasionally take a given design and determine whether it is "better engineered" than another. However, translating that better wing shape into population mean fitness - or even relative fitness - is misleading without, as Mr. Hambre pointed out, taking into consideration the rest of the fitness landscape.There are documented cases where certain obvious traits in natural populations have been isolated to the point where we can say "this trait increases the statistical probability of survival and/or reproduction in this organism". I'm thinking primarily of beak size in the Grants' Geospiza studies where .5 mm difference spelled a statistical difference in survival (!!) and the one with the guppies (I'll have to look up the specifics, I don't have my books here at the office) where variation in spot patterns, color, and size spelled the difference in predator-selected survival. There were fairly significant and key (read easily identifiable) selective pressures on both of these populations that would tend to swamp or overwhelm potentially countervailing or reinforcing selective pressures. However, these are relatively rare occurances and it has been extraordinarily difficult to document many other examples.Then again, evolutionary ecology is still a very young science. I have high hopes that much refinement and many more examples will be forthcoming - to the point where we may be able to get an accurate picture of what's going on. Hypothetically speaking, that would be true. However, I mentioned specific organisms which are "holding on" with significantly reduced ranges (in the wild), which are not occupying many potential habitats, and/or which rely on secondary or even tertiary dispersers for bare replacement. There's a whole flipping list of plants - especially in the neotropics - which aren't "better than all their ancestors". These guys are evolutionary relicts - fading memories of the distant past (forgive the lyricism). If I might speculate for a moment, I would say that these organisms are living examples of species exchange - caught in the act, as it were. Further, I'd speculate that - absent your hypothetical mutation that allows adaptation - these species would be doomed in the wild if left to their own devices. Or they'll end up like Wollemia nobilis - a microscopic remnant of a once widely spread species hidden away in some remote, inaccessible habitat where they cling to a precarious existence in the absence of competition from other, hardier and more adaptable organisms.These organisms aren't "fittest" by any stretch. They're sitting way down the slopes of their particular adaptive peak. Or rather, they've passed the peak and are way down the other side. Again, taking the wing out of its context, it really does make it a tautology. Most definitions are. However, just because a particular wing shape gives it better lift than some other similar shape doesn't translate into fitness for the organism possessing that wing shape. Better lift might be counterproductive in a case where flying increases the chances of predation or being blown out to see. I understand what you're trying to say. I'm merely attempting to point out that the "best engineered" concept is misleading without taking into consideration the remainder of the adaptive pressures operating on the organism/population.

It's entirely possible I'm not understanding what you're trying to get at (yeah, that's a dangling participle - so sue me.) My principal (and thus far only) objection to what you've written revolves around your attempts to apply engineering terminology to biology. The concept may be useful as an analogy in specific, very limited cases, but overall it can be highly misleading. Especially if you use the analogy with non-biologists who don't understand the distinction. Perhaps I'm just an old fart who has a knee-jerk reaction because of all the creationists who misuse the analogy to push their agendas. Maybe you could take a moment to explain exactly what you're getting at (another dangle) and why you find the engineering analogy so compelling. THEN we can disagree on something substantive that doesn't rest on mutual misunderstanding. Well, let see. Mammuthus and Taz are molecular biologists (IIRC), SLPx is a biologist/primatologist, Wounded King is a biologist of some ilk or other. There are a couple of other people that I suspect are biologists of one stripe or another but who are "undeclared". Andya is studying to be a zoologist, and my background is ecology (could'a guessed, right?) with most of my field work in conservation biology. I'd say go for evolutionary ecology, but I'm biased. Screw paleoanthropology, there's too many "personalities" dominating the field. And, Nyet! And this is where we part company, I think. None of those organisms are better than their immediate ancestors. In every case they are worse off, if anything. In one case at least (Wollemia), they ARE their "immediate ancestors" because of the asexual reproduction strategy it uses (coppicing). Simply because an organism reproduces doesn't make it better than its forebears. Mutation-induced variation might make organisms different, not necessarily better. In fact, mutation probably has an even greater chance of making things worse, especially for an organism near its fitness peak. Yes and no. Species exchange is a term used to describe the replacement of one species in a given niche or habitat by another. They may or may not be closely related. It can be seen in modern species when exotics are introduced into an ecosystem and wipe out the indigenous species (competitive exclusion) for instance. Another possibility is where a local extinction (background extinction) event or ecosystem degradation occurs that leaves open a number of niches allowing for ecological release of unrelated organisms. In any case, the net result is that one species replaces another - which may have significant impact on the community - to the point where an entire community changes membership.The term can also be used to describe what is occasionally observed in the fossil record when one species is replaced by a similar one or even a daughter species. In the record, this looks like one species appearing for the first time in a region at the same time another disappears. Elizabeth Vrba coined the term "turnover pulse" for the phenomenon - "turnover" is the species exchange, "pulse" is the ripple effect down through the ecosystem that changes bunches of other species at the same time in the same community. However, she thinks the proximate cause is climatological. IMO the record isn't fine-grained enough to determine causes at anything less than wide geographical or continental space and millions of years of time. (Q ducks while all the geologists jump on that statement.) I think you've lost me. Could you clarify your point? Sorry - it's probably just a senior moment...

Hey Moose! Yep. Not only an approximation, but a misleading soundbite coined by a journalist to popularize and conceptualize natural selection for the masses. One of several things to take Darwin to task for is that he allowed himself to be convinced to include it in his later writings and even in later editions of Origin. Yep again. Mammuthus and someone else mentioned it. Thanks for bringing it to the fore. In fact, unless there's some realy drastic change in selection pressures that just happens to coincide with a pre-existing trait, significantly changing the frequency of alleles in a population is a lengthy, multi-generation process. Most times, new variants simply disappear over time. Sometimes they can persist as long as the newcomer can live polymorphically with the other variants. Sometimes, especially in smallish populations, drift causes the frequency to increase or decrease more or less randomly. I've also read that. I'm not sure how "radical" the change was, but there is certainly indications that dinos were on the decline for at least 10 million years before the Big One. Another topic, tho'. I'd still be curious as to anyone's ideas on whether mass extinction events like that asteroid figure into the fitness landscape for a species.

Okay, I think I'm beginning to pierce through the fog, here. It comes down to the way you're using the terminology, I guess. You appear to be using "function" and "purpose" in the sense of "role played by" when discussing engineering and natural selection. This is where I got confused. It's a somewhat idiosyncratic way of looking at it, but if that's what makes you comfortable with conceptualizing the idea, then I really don't have a problem with it. I would caution you that function and purpose are not interchangeable. In biology, a strict definition of purpose is meaningless (nature has no purpose), and function is context-dependent. When you use either one in discussions of biology or biological systems or populations or whatever, you may be opening yourself to misunderstanding. This is apparently where my difficulty with your idea of "best engineered for reproductive success" comes from. Or your phrase "the function of natural selection is reproductive success." I guess you're not wrong conceptually the way you've described it - it's just a very odd way of describing what occurs.Now I'm gonna go argue with Peter .

I don't think reproductive success IS a definition of fitness. It's how we quantitatively measure relative fitness at the organismal level. At the level of a population, however, you have a couple of choices, depending on what you're trying to express. F'rinstance, from a pop gen standpoint as Mammuthus pointed out above, you can talk about how an allele or suite of alleles changes in frequency over multiple generations (increase to fixation of novel or rare alleles, for example) as a measure of the fitness of those alleles. Or you can talk about the persistence of a population over the generations in the face of variable environmental factors or changing selective pressures as a measure of the mean fitness (read adaptability) of the population. The "how" of the frequency change and the "how" of persistence over generations is through reproduction. Reproduction only indirectly bears on fitness in and of itself.I don't know whether that made any sense.

I completely agree with everything here, except that I’d rephrase almost always to always. I can’t think of any case where predators have a higher population density than prey. However, I think a discussion of trophic levels, energy webs, carrying capacity, ESS, arms races etc, will get way off topic.The key point for Justin to take away here is that relative fitness is a valid tool to describe both inter and intra population/species/etc dynamics.

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...

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.

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]