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Modern notion of fitness VS. the struggle for existence

In Discussion on January 27, 2010 by Lynn Chiu

Here are my views of the two notions of natural selection (not the theory, but the “filtering” process).

Contrary to André, my opinion is that the modern Darwinism idea of “fitness” is just a broader idea of what Darwin had in mind. Both “fitness” and Darwin’s “struggle for existence” are provided to explain why traits spread through a population; the difference being while Darwin thought that only overpopulation can do the trick, modern theories also identify other mechanisms in which a fitter trait would be represented more in the following generations.

The upshot of my reasoning is that for traits to spread through a population and thus result in a different population composition, some but not all heritable variations must increase/decrease their frequencies in the population. For this to occur,  some individuals must pass on their traits to more/less offsprings than others. And thus there must be a reason that caused this difference in offspring quantity.

Both “fitness” and “the struggle of existence due to overpopulation (in Andre’s terms)”  explain how some but not all individuals get to have their traits passed on: because of the adaptiveness of these variations.

I take Darwin to be arguing that “when” the growth of populations exceed the supporting cap of the environment, those who survive more successfully (have better adaptive traits) have a chance to reproduce and/or reproduce more than other individuals. If individuals were merely struggling for existence but do not have more offspring in the long run, it is impossible for the trait to spread through the population because of their adaptiveness.

This is exactly what I take fitness to mean, too: the fitter traits have a better chance of being passed on to more individuals than less fit traits because of their adaptiveness, even though the adaptiveness of the traits caused the spread due to situations that do not necessarily involve population overgrowth.

Thus I maintain that Darwin provided (what he thought) the only reason why some but not individuals propagate, whereas for the modern Darwinists, there might be reasons other than population overgrowth that led fitter traits to be proliferated.

It is misleading to bring in random drift, etc. mechanisms to differentiate between modern Darwinism and Darwinism in their understanding of fitness. When these factors are involved, the resulting evolutionary pattern is no longer a result of “adaptation”. Instead, these are other ways a population can change without anything being fitter than the other. Thus when fitness is involved, we are talking about adaptationism. Exactly what Darwin had in mind.

Given my interpretation, statistics are useful in that they help us create models that illuminate whether an adaptive trait would spread (based on its initial frequency) and how long it takes for the trait to be fixed in the population. That is, it provides us the details of what happens “in the long run”. However, the forces driving evolution are the same (if one holds adaptationism): the adaptiveness of the traits cause them to be represented more in the long run.

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16 Responses to “Modern notion of fitness VS. the struggle for existence”

  1. Fitness in its various modern definitions all appear to be comparing the number of one thing to another. Thus if there is more of something it is fitter. For me the difficulty is comparative fitness. The reason I have a difficulty is that no one trait, organism, gene, or whatever one is measuring is necessarily more ‘fit’ with any consistency. While the comparative may be descriptive of something, it is not being more or less fit. I shall illuminate this below with examples.

    I do not think having contemporary notions of inheritance, genetic drift, mutation, recombination, or any of that does the least bit of damage to Darwin’s basic theory. He did not understand the origins of variation and his explanations were just wrong. He was wrong that excessive population growth was necessary. But every appearance is that the basic mechanism of evolution by natural selection is correct.

    Variance occurs. Existence is a struggle. Those variants that do not meet the challenges they face cease to exist. The only concept of ‘fitness’ here is that survivors are currently fit, and one cannot tell which is more fit than another, if one measures fitness by relative preponderance. Expected preponderance is even more absurd, as expectation will often not mach results.

    First example, the first anaerobic microscopic life forms were for a time the dominant species on the planet. They flourish for quite some time. Oxygen was toxic to them. They produced oxygen that came to oxygenate the atmosphere. Up until the tipping point where the oxygen levels became toxic to them, these anaerobic creatures were the dominant species, but other aerobic life had developed. Before the extinction even, anaerobic life forms are more fit by these statistical measures. But conditions have changed, and now they are gone. Well, there is yeast and some things out there, but they are not dominant. If conditions change again and yeast inherits the Earth, some of those gene will be more fit again. Yes, the idea measures something, but not fitness for continued existence.

    Second example, it is my understanding (and for the sake of argument the veracity of my understanding is irrelevant) that those with sickle cell anemia are immune to malaria, but tend to die in their mid twenties. By statistical modeling of propagation and preponderance of the genes or individuals, those with sickle cells are less fit. Now suppose malaria mutates into a hyper-aggressive air born variety that is universally fatal to those without sickle cells- only those with sickle cells are fit to inherit the Earth. But every dead human had genes that were “more fit” up until the environment changed. (In fact, the existence of sickle cell anemia even existing with any preponderance in the first place shows that for a time it was more fit in a certain geographic niche as a defense against malaria, but now it isn’t, unless I am just totally misremebering the discussion of sickle cell anemia from high school biology.)

    My contention is that modern fitness says nothing useful about natural selection, except in predictable conditions that do not exist in the real world. As time is reduced and ecological stability maintained, this concept of ‘fitness’ may tell us something, but it tells us nothing about niche changing or gap filling.

    I would also suggest that more and less fit has no meaning in evolution by natural selection as that which is fit one day is unable to survive another day, and for long periods of time such a thing will have a significantly higher population and might be able to out reproduce that which will eventually dance on its grave. Thus any concept of fitness that tries to consistently assign the relative fitness is impossible.

    After rereading what you wrote, I am more seeing your point. If you are willing to admit that the ‘fitness’ of an organism will change, and change for the worse often, then I can agree that the struggle of existence (as I put it) could be implicit in the idea of fitness (thus population) variation. Interesting perspective.

    But, by changing the focus to a statistical analysis does one necessarily change the nature of the theory? (I am now thinking and typing at the same time, so I should probably stop before this gets too messy…) Darwin, to me, describes a mechanism in his theory. The statistical analysis measures the changes wrought by this mechanism, and assigns relative values of questionable utility. Interesting. I still think the idea of greater or less fitness is of next to no value, but I think I can now see how the statistical method could implicitly be including Darwin’s actual (or corrected- but not destroyed- for improved contemporary understanding) theory. Interesting…

  2. Hello all. I think there is merit to both of your post, but I have some things to add.

    FIrst, Lynn, I agree with just about everything you said concerning the relationship between these two concepts. This is precisely what I was trying to push yesterday in class concerning the general strucutre of Darwin’s form of explanation and his particular application of that explanation in the case of overpopulation (though you said it better). He simply focused on overpopulation due to geometric growth and limited resources (plus variation that leads to differences in the ability to survive and reproduce) as the sole cause of the struggle for existence that leads to natural selection and the changes in species. I understand him as focused on a particular problem (the mystery of mysteries) and he is in someways is offering a “how-possibly” story though one that is well supported by the evidence. Darwin’s theory answers how variance, inheritance, and the struggle for existence could lead to drastic changes in species over time, but modern Darwinism has brodened this idea to include several other ways that organisms’ abilities to survive and reproduce might be affected (since the modern explanandum is change in gene/trait/whatever frequency in general). Modern conceptions of fitness are, I think, merely an expansion of Darwin’s original understanding of the struggle for existence. Furthermore, modern evolutionary theory has identified several other factors – besides fitness- that can influence evolution. In short, the struggle for existence is sufficient for evolutionary change (and so answers the how possibly?), but not necessary (unless we are talking about only Darwin’s original type of evolutionary change), which explains the expansion of evolutionary theory to cover other influences as well.

    However, I think the statistical interpretations offered by modern evolutionary theory are useful for more than just long-term evolution. It has certainly opened up new ways of predicting and explaining the effects of natural selection and other evolutionary factors. Furthermore, I think it importantly reminds us that the view from the economy of nature that is in need of explanation (i.e. better adapted populations) is a (merely statistical?) RESULT of what is causally going on at the underlying levels. Either way, I don’t think these two types of explanations are necessarily in competition with each other. They are each fruitful in thier own way.

    On to Todd. You are certainly correct to point out that what is fit can change over time. In fact, that environments change with time is an important cause of evolutionary change since it will lead to changes in fitness for the existing variants. However, how much the environment changes is a matter of some debate. For one thing, if the environment changed so much that what is fit today would routinely be unfit tomorrow, it would be near impossible for natural selection to occur in a way that would lead to the current well adaptedness we observe. There must be a fair amount of consistency over long periods of time in order for natural seleciton to produce well adapted organisms. This is where the concept of fitness plays an important role since it is the measure of how well organisms are able to survive and reproduce in their current environments and those environments and the traits of those organisms get passed on through the generations in variant amounts correlated (though not perfectly) with their fitness (which is as you note a comparative notion). This is precisely what leads to the existence of adaptations. However, the environment and therefore fitness is by no means static. Various things change too fast for natural selection to select any traits concerning them, but not everything works this way. In short, the consistency of environments over geological time, though not perfect, is vital for natural selection to be able to explain the well adaptedness of organisms in nature.

    Secondly, I don’t really understad what you mean by, “Expected preponderance is even more absurd, as expectation will often not mach results.” It seems to me that expected preponderance is vital to evolutionary theory because it is well established that the fittest traits do not always win out. Various other factors can overcome the influence of natural selection on a population. When this happens (i.e. when a trait is not an adaptation due to natuarl selection), then the expectations will not match the results, but that is precisely why expectated offspring – not acutal – is used. When the traits that are comparatively “more fit” than the alternative variations do not spread through the popultion, then natural selection is insufficient to explain the current trait frequencies in the population – that is, we need other evolutionary influences to complete the story. These additional influences are an imporant part of how biologists explain our observations since Darwin. Otherwise a single instance of the fittest trait not spreading would be enough to undermine the theory. How important thse other influences are, however, is precisely what is at issue in the debate over adaptationism. Either way, expected number of offspring is appropriate precisely because we know that the fittest traits will not always spread, only that if natural selection were acting alone then they should.

  3. My issue is with the concept of ‘fitness’ as the relative preponderance of genes. As what is more fit become less fit with environmental change (by the definition) I feel it invalidates the definition of ‘fitness’ not the theory of Darwinian evolution. I feel that relative fitness is absurd- everything living today is fit. Every gene about today plays an important part in being fit enough to survive.

    Underlying my problem, and heretofore not articulated, is the idea that genetic diversity is a positive thing in and of itself and beneficial for the ability of a species to survive changes in the environment. If one counts only genes in measuring relative fitness, then one is missing much that is important in the concept of evolution.

    Let us return to my sickle cell example. As a species humans have genetic diversity. The relative fitness method would state that sickle cell determining genes are less fit because of their relative preponderance in the ecosystem. In my hypothetical where mutant malaria wipes out all non-sickle cell humans, those with the ‘less fit’ genes (under the definition in the pre-mutant malaria conditions) are the survivors. Admittedly, under this absurd definition what was less fit has now become more fit (any positive number is higher than zero- the preponderance of non-sickle celled humans). So, my issue is that the definition is useless for predictions.

    More to the point, at a species level, humanity, as is any species, is in a better position with genetic and trait diversity, when confronted with natural selection. The idea of ‘fit’ genes does not take this into consideration. While natural selection reduces genetic diversity, greater genetic diversity before selection events increases chances of species survival. Thus, under a more dictionary definition of ‘fitness’ humanity is more fit with sickle celled people, while the technical definition of ‘fitness’ would say sickle cell genes are currently less fit.

    So, while Lynn is correct that natural selection may be considered implicit in the technical definition of fitness, when confronted with changes in fitness, this definition and perspective completely obfuscates what is going on, ignores the importance of actual traits that lead to improved species survivability over time (genetic diversity), and says very little that is useful about a gene (what is more fit today is less fit tomorrow).

    I would suggest that technically defined relative fitness (as it measures genes) is a trivial and nearly useless concept when considering the evolution and survival of species and hides the fact that genetic diversity is ‘desirable’ in and of itself as it provides better for the unknown exigencies of environmental change and the potential of moving into new niches.

    While ecosystems have the appearance of stability form the limitations of human lifespans, the systems routinely change. The first anaerobic life forms make the environment toxic for themselves. Global climate change caused mass extinctions of mega-reptiles. The introduction of European diseases wiped out more than half the indigenous human populations of the Americas. British industrialism provided better cover for soot colored moths. Human waste dumps on the edge of villages provided a new niche to inhabit. The Galapagos Islands appeared bereft of bird species, providing a new environment. Contemporary global climate change is altering the environment in significant ways. Virus generations are so short that at any time a new AIDS, SARS, or mutant malaria might appear. Significant changes in the environment and ecosystem are fairly routine.

    Increased genetic diversity, regardless of relative technical genetic ‘fitness’ will increase the potential a species will survive, in some form, in the long run when faced with such unpredictable changes. As the future is unpredictable, it is unknowable if sickle cell genes are better or worse for the continued existence of the human species, but the genetic counting method would say the genes are ‘less fit’. I am suggesting a more complete perspective would be to say everything alive is fit enough, but we don’t know about tomorrow (thus relative fitness cannot be measured).

    But, to be clear, if these technical definitions for gene counting are used for purely technical reasons they may have some valid limited utility in a limited set of highly technical circumstances. It is only when saying that changes in gene frequency ‘is evolution’ that I think I am having a problem. I am currently thinking that Lynn has shown how these views are compatible, but I am suggesting that the technical definitions obfuscate what is important about the theory and any real understanding of it.

    So, my position is evolving.

  4. I, like Collin, agree with much of what Lynn wrote (I’m still digesting what Todd wrote). Lynn is a hedgehog while I’m a fox. Hedgehogs like to see the world through the lens of unifying ideas. Lynn views evolutionary theory today as essentially the same as Darwin’s theory. Foxes like to make distinction between ideas. While I see the merit of the hedgehog point of view you should also see the merit of the foxes point of view. There are many points of differences between Darwin’s theory and our modern ones. And, as philosophers we should take note, especially if we’re interested in the nature of scientific explanation, its limits and scope, and its applicability to a variety of disciplines.

    While Lynn is right that there is a way of seeing the modern evolutionary theory as an extension of Darwin’s ideas I think it is important to identify the points of extension. For instance, Darwin’s theory operated under a specific, univocal ecological view (which is not in general true), he assumed false things like a struggle for existence is the inevitable consequence of overpopulations (it is not), and he assumed wrongly that the most adapted to local environmental conditions transmit these traits to their offspring (they do so only under certain conditions). To take into account Darwin’s problems, neo-Darwinians, I think, changed the nature of his theory.

    As a fox, I would like to identify those elements of change. I think it is important to do so when we are faced with the philosophical questions. For instance, I think some of my interlocutors in the literature have been arguing with me about the nature of evolutionary explanation while operating under a “hybrid” theory of natural selection (both Darwin’s and modern’s) which is true of neither. In the end they are telling us of the true nature of nothing.

  5. Todd said:

    “Fitness in its various modern definitions all appear to be comparing the number of one thing to another. Thus if there is more of something it is fitter. For me the difficulty is comparative fitness. The reason I have a difficulty is that no one trait, organism, gene, or whatever one is measuring is necessarily more ‘fit’ with any consistency. While the comparative may be descriptive of something, it is not being more or less fit. I shall illuminate this below with examples.”

    Slight quibble here. According to Wikipedia, at least, fitness is not the preponderance of a trait, but the *change* in the preponderance of that trait after selection. In calculus terms (if I remember calculus correctly), fitness is the first derivative of the preponderance curve.

    Todd said:

    “My contention is that modern fitness says nothing useful about natural selection, except in predictable conditions that do not exist in the real world. As time is reduced and ecological stability maintained, this concept of ‘fitness’ may tell us something, but it tells us nothing about niche changing or gap filling.”

    “My issue is with the concept of ‘fitness’ as the relative preponderance of genes. As what is more fit become less fit with environmental change (by the definition) I feel it invalidates the definition of ‘fitness’ not the theory of Darwinian evolution. I feel that relative fitness is absurd- everything living today is fit. Every gene about today plays an important part in being fit enough to survive.”

    My impression is that fitness makes sense *only* when you make it relative to environment.

    Collin said:

    “For one thing, if the environment changed so much that what is fit today would routinely be unfit tomorrow, it would be near impossible for natural selection to occur in a way that would lead to the current well adaptedness we observe. There must be a fair amount of consistency over long periods of time in order for natural seleciton to produce well adapted organisms.”

    This is true, but Todd has a point about diversity being a Good Thing. However, since natural selection is usually interpreted as working at the level of individuals (this is controversial), how could we say that diversity promotes fitness? Individuals aren’t diverse, populations are. I don’t have a solution here, just thinking out loud.

    To answer the original question, I would agree with Lynn that the modern concept of fitness is a more general form of Darwin’s struggle for existence. Fitness applies in situations where the struggle for existence doesn’t (just to pick low hanging fruit, fitness still works in environments where the carrying capacity hasn’t been reached yet), but it seems to apply in all situations where the struggle for existence does. Whether this makes modern evolutionary theory a completely different theory from Darwin’s is an interesting one. There is definitely variation, but there is also a family resemblance. Some traits have been inherited. Are the two variants still members of the same “species”? What criteria could we use to answer that question?

  6. I realized something on the way to campus. If fitness is defined as change in trait frequency over generational time, then fitness is like velocity (change in position over time). According to calculus, there is such a thing as instantaneous velocity. If you take a magic stopwatch and freeze the entire world, allowing you to investigate a single point in time, then objects still have velocity, even though they aren’t “moving.” I think this weird. Pete Abram and Angelo Graff don’t, but they do say that nobody takes calculus seriously on this issue. I suspect that mathematicians *do* take calculus seriously, which could explain why they’re so wonderfully crazy.

    If we do take calculus seriously, then this means that our instantaneous fitness can change over the course of the day. (On a related note, we can also say things like, “statistically speaking, I have .01 offspring right now.”) As we change environments, our instantaneous fitness also changes. Weird, huh?

  7. I forgot to login before I posted this. Right now, the original posts are still lodged in “pending.” I’m logged in now, so let’s try this again. (Hopefully I get this site figured out eventually).

    Todd said:

    “Fitness in its various modern definitions all appear to be comparing the number of one thing to another. Thus if there is more of something it is fitter. For me the difficulty is comparative fitness. The reason I have a difficulty is that no one trait, organism, gene, or whatever one is measuring is necessarily more ‘fit’ with any consistency. While the comparative may be descriptive of something, it is not being more or less fit. I shall illuminate this below with examples.”

    Slight quibble here. According to Wikipedia, at least, fitness is not the preponderance of a trait, but the *change* in the preponderance of that trait after selection. In calculus terms (if I remember calculus correctly), fitness is the first derivative of the preponderance curve.

    Later:

    I realized something on the way to campus. If fitness is defined as change in trait frequency over generational time, then fitness is like velocity (change in position over time). According to calculus, there is such a thing as instantaneous velocity. If you take a magic stopwatch and freeze the entire world, allowing you to investigate a single point in time, then objects still have velocity, even though they aren’t “moving.” I think this weird. Pete Abram and Angelo Graff don’t, but they do say that nobody takes calculus seriously on this issue. I suspect that mathematicians *do* take calculus seriously, which could explain why they’re so wonderfully crazy.

    If we do take calculus seriously, then this means that our instantaneous fitness can change over the course of the day. (On a related note, we can also say things like, “statistically speaking, I have .01 offspring right now.”) As we change environments, our instantaneous fitness also changes. Weird, huh?

    Todd said:

    “My contention is that modern fitness says nothing useful about natural selection, except in predictable conditions that do not exist in the real world. As time is reduced and ecological stability maintained, this concept of ‘fitness’ may tell us something, but it tells us nothing about niche changing or gap filling.”

    “My issue is with the concept of ‘fitness’ as the relative preponderance of genes. As what is more fit become less fit with environmental change (by the definition) I feel it invalidates the definition of ‘fitness’ not the theory of Darwinian evolution. I feel that relative fitness is absurd- everything living today is fit. Every gene about today plays an important part in being fit enough to survive.”

    My impression is that fitness makes sense *only* when you make it relative to environment.

    Collin said:

    “For one thing, if the environment changed so much that what is fit today would routinely be unfit tomorrow, it would be near impossible for natural selection to occur in a way that would lead to the current well adaptedness we observe. There must be a fair amount of consistency over long periods of time in order for natural seleciton to produce well adapted organisms.”

    This is true, but Todd has a point about diversity being a Good Thing. However, since natural selection is usually interpreted as working at the level of individuals (this is controversial), how could we say that diversity promotes fitness? Individuals aren’t diverse, populations are. I don’t have a solution here, just thinking out loud.

    To answer the original question, I would agree with Lynn that the modern concept of fitness is a more general form of Darwin’s struggle for existence. Fitness applies in situations where the struggle for existence doesn’t (just to pick low hanging fruit, fitness still works in environments where the carrying capacity hasn’t been reached yet), but it seems to apply in all situations where the struggle for existence does. Whether this makes modern evolutionary theory a completely different theory from Darwin’s is an interesting one. There is definitely variation, but there is also a family resemblance. Some traits have been inherited. Are the two variants still members of the same “species”? What criteria could we use to answer that question?

  8. I have (mostly) deliberately conflated ‘absolute fitness’ and ‘relative fitness’ to simply ‘relative fitness.’ My understanding of the wikipedia definitions is that ‘absolute fitness’ is comparison of before and after of the same population with reference to an evolutionary event. ‘Relative fitness’ is the comparison of preponderance of genes to each other, either by comparison to a gene set as the standard for occurrence or to the entire population of genes being examined. While I have more problems with each, the relative fitness is what I write of when comparing fitness.

    Absolute fitness, is even less interesting as a characteristic and relates to any sensible notion of fitness even less. To say gene populations change in number is trivially true. To assign a value to this as ‘fitness’ is to suggest that a trait that has lost population is less fit. Maybe, maybe not. Perhaps the meteor killed lots of them, but the new environment is one they will thrive in.

    I think something is either fit or not in a particular historical moment, and there can be no relative comparison, outside abstractions that do not relate to any idea of fit with relation to natural selection. At this moment humans, worms, cats, and whales are all fit. In ten minutes, fifty years, or a million, those particular species may or may not be fit and may or may not have evolved into new species (in which case many of there genes would likely still be in existence). My problem is saying any one species or cluster of linked genes is more or less fit, either in the same generation or (more problematically) across generations.

    I strongly disagree with our good professor. While Darwin was wrong about several things, origin of variation, mechanism of inheritance, the nature of the struggle of existence- he was close enough that only the most minor of tweaks are necessary to bring the theory into accord with our greater understanding of phenomena. In fact, had he understood mutation and inheritance, his argument would have been stronger as he would not have confused people with ‘use/disuse.’

    Darwin set out to explain why the ecosystem is such an amazing (apparently) harmonious thing with (nearly) every imaginable niche filled up. He got diversity of traits wrong, but he was right that variation exists. He is correct that existence is a struggle. Again, he just had the particular mechanism wrong. I suggest that Darwin was essentially right.

    At the same time, the gene perspective only implicitly considers this perspective. It appears to be asking a different set of questions that are not nearly as interesting. But I perhaps should wait to pass judgment on that until I see what exactly modern number crunchers are doing with this.

    Perhaps the Tick, in the second season episode ‘Grandpa Wore Tights’ summed up the difference in perspectives best:

    ‘Arthur, you have no historical perspective. Science in those days worked in broad strokes! They got right to the point. Nowadays it is just “molecule, molecule, molecule!” Nothing ever happens BIG!’

    Everything is just about molecules (genes) now. Darwin got right to the point in broad strokes. But because of Darwin we might have the ability to defeat the Kremlin dome missiles or the atomic robot zombie men.

  9. Todd said:

    “To say gene populations change in number is trivially true. To assign a value to this as ‘fitness’ is to suggest that a trait that has lost population is less fit. Maybe, maybe not. Perhaps the meteor killed lots of them, but the new environment is one they will thrive in.”

    What gets me is that wikipedia definition seems to reverse the causal arrow. Given this definition, it’s not the case that a trait increases in frequency because it’s fit, it’s that it’s fit because it increases in frequency after selection. So the question becomes why a trait is being selected for or against. It seems to me that absolute fitness wouldn’t rule out orthogenetic causes of trait frequency change, such as genetic drift, which is probably why relative fitness is used more.

    As far as the meteor thing goes, you’re right about that. Very few species (or members of species, or whatever) are fit in the environment of a meteor impact, but there is a fitness differential in that environment. That’s why the dinosaurs died out and our rat-like ancestors survived. After the ash settles, then the new environment causes a different set of traits to be selected for. Fitness is always a thing of the moment, relative to the current environment. It’s not about how the trait will work in the future, or under “normal” circumstances, it’s all about how the trait works in the current environment. Evolution has absolutely no foresight.

    Todd said:

    “At this moment humans, worms, cats, and whales are all fit.”

    It probably doesn’t make sense to refer to the fitness of a species as a whole. For that matter, the fitness of an individual, but it easy to fall into that (I think it did that above). It is true that the fitness of a trait is relative to a particular environment/historical moment in time. However, it is not the case that all traits are fit (having a fitness above replacement, >1) just by virtue of currently existing. We can retrospectively measure the change in the frequency of a trait over a span of time. As we decrease the span of time over which we measure the frequency change, our measure of fitness approaches instantaneous fitness. So, we can speak of the instantaneous fitness of a trait at the *present* moment of time. Predicting what the instantaneous fitness of that trait will be in the future is a bit more difficult, since we don’t know exactly what the future will be like, but we could probably make an educated guess. (Especially if we assume that the near future will resemble the present.)

  10. “At this moment humans, worms, cats, and whales are all fit.”

    Oh, when I speak of ‘fit’ I am explicitly rejecting any of the relativistic variants for my more general idea of simply ‘in existence at the moment.’ I also feel it makes more sense to speak of a species being fit as they tend to be the package for large numbers of genes and traits as well as the method for these genes and traits to be carried into future generations.

    I am pushing back towards the Darwinian ecosystem approach and rejecting the view that these statistical counting ideas make much sense. Thus making our good professor’s point that they are incompatible, though I have also agreed with Lynn that the one approach is implicit within the ideas of the other.

    Actually, my problem is simply using the word ‘fitness’ in the statistical model. Change it to ‘occurrence’ or something, or acknowledge that it has deeply limited uses that say nothing about general dictionary notion of ‘fit’, and my problem disappears.

  11. By the time I really had a chance to sit down and look at this thread there was enough here that it is difficult to know where to begin or what to address. Here are a few thoughts though.

    Re: Todd, et. al’s discussion of fitness
    I’m worried that there might be some confusion between the statistic ‘fitness’ and the ordinary judgment ‘fit.’ Fitness within evolutionary biology names a statistic that is used to evaluate, predict, explain, etc.It may not coincide exactly with our everyday use of the term, but it is a definition, therefore its correctness isn’t really up for debate. Put another way, ‘fit’ does not equal ‘adaptive’.

    This statistical nature of fitness is why the causal direction seemed funny to Dan. It is *exactly* that a trait increases in frequency that we call it fit. Thinking about it this way definitely takes some getting used to, since its so different from our ordinary use.

    Another quick note, @dcmzb8 (who is this, by the way?). You (and Todd) are right–sometimes diversity is very important. A specific, well known example, is frequency dependent selection (I’ll leave it to those interested to look up what this is). I mention this merely to say that there has been significant work on including the benefits of diversity to fitness in evolutionary models.

    Re: Lynn and original topic
    I understand the appeal on both sides here, but I want to push back from the ‘fox’ side, as André would put it. Darwin’s line is that new species evolve when overpopulation leads to limited resources and the variant individuals that are fittest to the circumstances of obtaining those resources survive while the ones that are less fit do not. The continued selection against certain traits of the species leads to the formation of a new species. This is an elegant, if narrow idea.

    Now consider that synthetic theory (modern fitness-based theory) doesn’t even say anything about the formation of new species! It is an attempt to model changes in gene frequencies by identifying the processes which change them and their interaction. Note that if we woke up tomorrow and suddenly realized that the creationists have it right, synthetic theory would survive without a scratch; could we say the same of Darwinian theory?

    The appeal of the ‘hedgehog’ position is that part of what gets modeled in synthetic theory is natural selection, and we can even model n.s. occurring such that we get what we judge to be a new species (the elements Darwin was going for). But there is a different explandum and vastly broader (if not entirely different) explanans. So synthetic evolutionary theory is not just broadening Darwinian theory. Synthetic theory is doing its own thing and it happens to be the case that it explains what Darwin tried to explain in a manner very similar to Darwin’s attempt at explanation.

    To reiterate what André said, there is indeed a bit of a fox/hedgehog thing going on here. Looking back at Colin’s response, he said much of what I just said while emphasizing unity of the theories. Part of judging the merits of these vantage points is going to come from historical work that I simply have not done. To what extent is Darwin applying a new way of thinking to a specific problem, and to what extent does he have the narrow focus I described above? But even if Darwin sees himself as developing some more broadly applicable mode of explanation, I am not convinced that it’s enough to get us (essential) sameness of theory. Rather, I think the better way to understand it would be that Darwin gave an early attempt at describing a phenomenon whose underlying structure included several other phenomena, all of which were later described by synthetic theory.

  12. Oops! Now that I look back over the Griffiths paper a little more closely, it probably just says exactly what we all agree on anyway: genetic informatoin should not be separated from other types of biological information. However, it still lays some interesting terrain for the information metaphor (e.g. see Maynard Smith’s view). I still wonder about the definition of information and if using it and a more generic inheritance requirement will really be able to avoid any counterexamples (I’d have to think about it some more). For one thing, I’m not sure that the other causes (sorry Andre) of evolution necessarily require inheritance; e.g. suppose a mutation or recombination occurs. Would we then say that no evolution had occured until the change had been inherited? It seems that the change in gene frequency is sufficient in this case to say that the population has evolved, yet no inheritance has occured (though it likely will later on). Also, my worries about a quantitative test above still trouble me. Anyhow, for those of you (like me) who don’t have time to read the whole Griffiths piece, but are interested, I’ve pasted a short summary I wrote on the paper for Zac’s seminar below (I’ve left off the critique, although it is surely devestating):

    In “Genetic Information: A Metaphor In Search of a Theory,” Paul E. Griffiths argues that the proposal that development is the expression of genetic information is misleading about explanation in molecular biology (Griffiths, 2001). To critique the proposal, Griffiths looks at various definitions of information and argues that all the defensible notions do not imply a difference between genetic and non-genetic developmental causes.

    According to teleosemantic theories of information, things refer to whatever evolution designed them to represent (Millikan, 1984). That is, things derive their meaning from their biological functions that are provided by natural selection. John Maynard Smith proposes to use teleosemantic theories of information to defend the information talk used to distinguish between genetic and non-genetic causes in developmental biology (Maynard Smith, 2000). The basic idea is that genes are intentionally directed onto their effects. Genes’ biological function is to produce the effects for which they were selected. Thus, according to teleosemantic theories of information, genes provide intentional information about their effects. This is supposed to set genes apart from other, non-genetic, causal factors in development. Non-genetic factors, supposedly, do not provide biological information – they are merely material causes. In short, Maynard Smith’s proposal is that teleosemantic theories of intentional information entail that genetic causes provide intentional information and non-genetic causes do not. Therefore, because they are the sole sources of intentional information, the characterization of genes as special types of developmental causes is justified.

    In opposition to Maynard Smith’s proposal, Griffiths defends what he calls the parity thesis: ‘Any defensible definition of information in developmental biology is equally applicable to genetic and non-genetic causal factors in development’ (Griffiths, 2001). In response to the teleosemantic approach, Griffiths argues that it is perfectly consistent with the parity thesis since non-genetic developmental causes will likewise carry intentional information under that approach. This is because several non-genetic causes also have naturally selected for biological functions, which in turn implies that, under teleosemantic theory, they too have intentional information. More precisely, Griffiths’ argument runs as follows. According to teleosemantic theory, things refer to their naturally selected biological functions. Thus, any mechanism that has a naturally selected biological function will have intentional information. Moreover, there are several non-genetic mechanisms in development that have naturally selected biological functions. Therefore, there are several non-genetic mechanisms in development that have intentional information – which implies that having intentional information cannot be what distinguishes genetic from non-genetic causes in development.

    Griffiths provides several examples in support of his claim that non-genetic mechanisms have biological functions. First, the DNA methylation inheritance system attaches additional chemical groups to future generations’ DNA. These additional chemical groups block the transcription of the genes to which they are attached and thereby influence the development of organisms. They are, however, inherited via an inheritance system that runs parallel to DNA inheritance mechanisms (what Griffiths refers to as an epigenetic inheritance system). A second set of examples comes from developmental systems thinking. Various microorganisms, bee colony cultural structures, and parasite-host associations are passed on to future generations via non-genetic inheritance mechanisms. All of these examples cite non-genetic developmental causes, and yet, these mechanisms are likely to have adaptive explanations. In other words, because these non-genetic factors are inherited and persist long enough for natural selection to act on them, they are likely to have naturally selected biological functions. Consequently, under the teleosemantic theory they will have intentional information. Therefore, teleosemantic theories of information obey Griffiths’ parity thesis since both genetic and non-genetic causes will have intentional information.

  13. Josh said:

    “This statistical nature of fitness is why the causal direction seemed funny to Dan. It is *exactly* that a trait increases in frequency that we call it fit. Thinking about it this way definitely takes some getting used to, since its so different from our ordinary use.”

    Yep, that’s right. (I’m dcmzb8, btw – I forgot to login or something.) We normally think of fitness in terms of how well an organism is adapted to their environment. Fitness causes selection. Fitness according to the technical definition is the effect of selection, not its cause. This might be one reason Andre thinks evolution is a statistical theory, not a causal one. There is correlation between fitness and selection, but the causal relation is the reverse of the way we usually think about it. On the positive side, fitness(technical) makes an excellent proxy measure of fitness(well-adaptedness).

    Josh said:

    “Now consider that synthetic theory (modern fitness-based theory) doesn’t even say anything about the formation of new species! It is an attempt to model changes in gene frequencies by identifying the processes which change them and their interaction. Note that if we woke up tomorrow and suddenly realized that the creationists have it right, synthetic theory would survive without a scratch; could we say the same of Darwinian theory?”

    Are you sure that’s the case? Modern fitness-based theory focuses a lot on gene frequency changes, but evolutionary theory as a whole still accepts the tree-of-life, right? Perhaps we should specify what we mean by “modern fitness-based theory”?

  14. As most (or all?)of you, I agree that Darwin’s idea of struggle for existence can be extended to be the modern idea of fitness. What interests me most is where Lynn said the following:

    “It is misleading to bring in random drift, etc. mechanisms to differentiate between modern Darwinism and Darwinism in their understanding of fitness. When these factors are involved, the resulting evolutionary pattern is no longer a result of “adaptation”. Instead, these are other ways a population can change without anything being fitter than the other. Thus when fitness is involved, we are talking about adaptationism. Exactly what Darwin had in mind.”

    Fitness is supposed to explain why some species survive while others become extinct. Random drift is one of the reasons for a species being fit. Why can’t we incorporate it? It seems to me that Lynn wanted to only consider the properties of species, for “adaptation” seems to imply the efforts made by species given the environment. If I am understanding Lynn correctly, then Lynn’s conception of adaption is too narrow. Adaptation can be any process that’s relevant to a species’ survival, including both properties of the species and the environmental conditions. I don’t see any problem to incorporate random drift.

    • Wenwen, In the contemporary literature “random drift” is not “one of the reasons for a species being fit”. It is supposed to be another reason, outside of the fitness of traits that a population evolves. My view is slightly different: drift is the difference between expected reproductive success and actual reproductive success when a population is finite. Take coins for example. A coin is slightly weighted towards heads. Now, flip the coin ten times. Say you get 5/5 H/T. That result is “due” to the weight or the “drift”? I say that question doesn’t make any sense. The right answer is that built into the concept of fitness is the law of large numbers. In a finite population, like the 10 coin flips, the slight weight towards heads is still the cause of the outcome even if it wasn’t manifested.

  15. Andre, Thank you for your explanation. I went back to read Sober’s chapter 1 to see why Sober talked about random drift. He explained that factors other than natural selection can also cause evolution, random drift being one of the factors.Undrstood in this way, Lynn is right that random drift is not part of natural selection, strictly speaking. Thus, natural selection is the internal reason for evolution while other causes such as random drift, migration and mutation are external reasons for evolution. This helps distinguish natural selection from the evolutionary theory as a whole.

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