Fitness: Hueristic or Fundamental to Biology?

And we arrive at the logical absuridity that all genotypes have 0 fitness. The last offspring in the lineage has (presumably) 0 fitness, and thus the product of all those fitnesses and fecundities becomes 0. I'd imagine that last of the genotypes has an 'average fecundity' of 0 too.So indeed - producing a large number of sterile offspring makes you exactly as fit as producing a large number of fertile offspring, if the end result is a single infertile descendant, or a generation of infertile descendants. If some gene came along, that gave genotypes a massive reproductive advantage, but rendered a significant number of the offspring of a certain genome infertile...we could quickly find ourselves with the dilemma of calling the original genotype unfit.Quetzal's orginal formulation doesn't give us this absurd result fortunately. Our genotype that gives birth to a 10 individuals has an average fecundity of 10 let's say. 5 of them survive to reproduce. Its fitness is thus 10 * .5 = 5. If its offspring's offspring are all infertile, then that reflects badly on their genome, not the grandfather's genotype's fitness.The issue you raised is a valid one though- a genotype that makes an army of infertile grandchildren is regarded as having a fitness of 5 which could be measured more fit than a genotype that makes only a small gang of very fertile grandchildren.These are the tough breaks of trying to calculate fitness. Personally I think a time period or generation level should be included in the calculation. The higher the generational level (or longer the time period), the lower the fitness is likely to be (since every lineage goes extinct eventually), naturally fitness will probably fluctuate - sometimes largely so, but it will come to a 0 in the end. The longer that is, perhaps, the fitter.In some species (such as succesful sexual species) any given genotype is very rare. So its average fecundity will generally be from a sample size of 1 - which means bad luck or good luck can drastically effect how we measure its fitness in any given environment. If it wasn't for some new predator being introduced next door to our family of snuggles by a sequence of happenstance situations resulting in the Henderson's pet dog tearing them to pieces, they might have become the biggest and longest standing history of snuggles in all time. Sure, we can conclude that the fitness of father snuggle's genotype to survive in the environment of the Henderson's was 0 - but how fit was his genotype to survive in its normal environment?In the above scenario of course - we know that bad luck tainted our calculation, but if we have to go on are the numbers, we'd conclude that the genotype was unfit for its environment (assuming mistakenly that the environment was average). By luck, we realize that father snuggle has an identical twin brother - with exactly the same genotype. Uncle snuggles has an absolute fitness of 8, and lives in an average environnment. Even with this amazing stroke of luck, we would deduce this genotype's fitness wildly incorrectly.If we pooled all the fitnesses of this generation together we could say that the genotypes with the highest (calculated) fitness were likely to be most actually fit individuals, with some exceptions.From here could deduce, approximately, how allele frequencies will change in the gene pool of the next generation.Edited by Modulous, : No reason given.

Naturally, organismal fitness is a useful tool, but it is not an accurate one. It relies on the principle that what is good for the individual (and some number of offspring) is good for the gene, which is usually correct. Certainly for genotypical or organismal fitness, yes. Gene fitness is tied much more closely to how frequencies of alleles shift, and is much more useful to us. It is practically difficult to assess the fitness of a gene simply because of their size - but if it weren't difficult it would provide us with a better picture of what is going on. Since most alleles have more than one copy, the 'average fecundity' of a gene can be much more accurately calculated. I think that gene fitness is an inextricable part in understanding natural seection, and the resulting allele frequency shift known as evolution.How? Those that are fit, will tend to increase in frequency. A fit allele generally gets naturally selected to increase in frequency.

Another issue with this measure of fitness can be discovered in a very simple population. Imagine a population where all individuals have the same number of offspring, there are no mutation events or any of the other evolutionary mechanisms. Yet still the allele frequencies could still shift.Imagine there are only three genotypes, but there are many copies of them. A, B and C. Since all genotypes have the same number of offspring, population size is unlimited.Genotype A takes one year to grow its phenotype into a reproductive agent. Genotype B takes 10 years and genotype C takes 10 days. Shortly after reproduction comes death.We can easily see from this extreme example that genotype C is going to increase in frequency compared with A and B. The frequency of that genotype (and thus its genes) is going to go sky high. This might be one of the nuances that Quetzal referred to since the simple calculation would render them all equally fit.An allelic fitness measurement could be defined as the rate of change of an allele's frequency. This then ties fitness directly and inextricably to the concept of evolution.

We run into a slight hitch when calculating the fitness of a sterile insect genotype though. Since it has no offspring - it has a fitness of zero. In the other thread I attempted to include this in there with a fitness definition of 'a genotype that works towards increasing the frequency of its alleles is a fit genotype'. This fitness metric can be applied to the whole hierarchy from gene all the way up to kingdom, and beyond depending on your chosen hierarchy.

Indeed - so how do we calculate the fitness of the genotype? We can't! And therein lies the rub. We have to define fitness from a reproductive unit point of view, which assumingly is a genome that is capable of directly reproducing. A reproductive genome doesn't make exact copies of itself though, only the root reproductive unit - the gene - does that.As I said, thinking of things at this level is usually good enough -don't think me an extremist. Its just that nature is a funny thing, and there end up being exceptions to rules all over the place if you aren't careful. Odd - that unfit entities can spread so well Agreed - it all depends on how much resolution you require. Sometimes, to find out what is happening we need to zoom all the way into the alllele, but oftentimes we can look at how traits change to deduce indirectly how the alleles change.

Hi there, thanks for the input. I don't know if you followed the related thread at all - but this idea was bandied around there too, and it is worthy of discussion. I was asked to define fitness in a way could explain the positive selection of these insects - and it was Hamilton's views I put forward. They were also alluded to earlier in this thread.And there is a good reason for it to come up - I believe Hamilton et al were right on the money when they tackled this issue.

What exactly is natural selection and precisely where does it occur?

Rats are small omnivorous scavengers that thrive alongside man, by eating his scraps. They can afford to have fourteen kids, because there is plenty of food to feed them. Lions are large carnivorous hunters, with scavenging on the side. If a lioness was carrying a litter of fourteen, she'd be much much much larger (she and her cubs are bigger than rats, so her surface area would go through the roof). Rats gestate for three weeks, and the rats become completely self sufficient after a very short period of time.Lions have pushed the limits of childbirth to their max. They only gestate for 108 days or so, their cubs are not full sighted and are reliant on their parents for up to two years (by which time, rat pups have possibly had a couple of litters of their own!).Any lioness that pushed the limits any more (increasing litter size, reducing gestation etc) would probably be less fit than other lionesses, less cubs would survive (or the lioness wouldn't survive past childbirth) and so the genes to push the limits would be lost (they'd be selected out).As such, given their evolutionary background, lions are as fit as they can be now in their environment.If you have any doubt over which is more successful, the lion or the rat - you need help! Lions are stronger and more ferocious than rats. Medieval kings had no concept of evolutionary fitness. Evolutionary fitness is irrelevant to medieval symbology/heraldry. Symbolic meaning is more relevant to heraldry.A springbok is fit for running away from lions. A lion is fit for killing springboks. It should be obvious which one a medieval autocrat (not just kinds, but any nobles) would want to be associated with. Eagles are fit for finding hard to find prey and swooping on them and destroying them. Rabbits are fit for running and hiding in dark holes. You seem to think that 'fitness' means 'healthiness' combined with 'strength' combined with 'freetime'. That is certainly one definition of fitness but the key to evolution is hereditary, and things only get inherited through progeny. The more progeny you have, the more you pass on. The more you pass on compared with other members of your population, the more evolutionary fit you are. Fit as in 'fit to task', as in 'gets the job done'. Once again, irrelevent to evolutionary fitness. How does 'free time' have an affect on natural selection? Sure - more efficient feeding can be selected for, because those that spend less time feeding spend more time replicating the genes for efficient feeding. But how quality of life and freedom come into it you don't explain. The reason why the peppered moth has more discussion than a swallowtail should be readily apparent. The peppered moth is the easiest example of natural selection to describe to anyone - so it comes up a lot. It has absolutely nothing to do with the fecundity of peppered moths vs swallowtails.

Quetzal - you brought up fitness as some kind of acid test for genecentrism, now you are saying that it is a essentially an arbitrary metric. I'm sure I'm missing something here, what is it?

I'm not misrepresenting your position here Quetzal, I misunderstood it and asked for clarification. Hence why I said 'I'm sure I'm missing something here'. I see. It seems our differences lie in the question in the OP. If fitness is tied directly to natural selection, then in my view fitness has to be of a gene. We can talk of the fitness of an individual, but that wouldn't so directly tied with natural selection - it would just be a 'fuzzy' guide. Naturally, if you define fitness as something only an individual possesses - then genecentrism makes no comment - why would it? Genecentrism isn't 'the gene is the only interesting thing to biology', its just an argument about where evolutionary selection occurs.