What is fitness?

This just came up in a conversation - is fitness function for survival or only the ability to reproduce? I’m beginning to learn these terms and I’d like to know your definitions.

I’d like to know how or why @PDPrice and Carter are wrong here that many change their definitions mid-conversation.

Since you’ve tagged me here, I’m obligated to say that we are not wrong. :slight_smile:

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So @PDPrice why is fitness (the ability to survive and reproduce better than your competitors) going up in the long-term evolution experiment with E coli?

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This is from Biology Online. Biology Online: Relative Fitness

In biology, Darwinian fitness or simply fitness of a biological trait describes how successful an organism has been at passing on its genes. It is different from physical fitness wherein the latter is concerned about the physical wellbeing of an organism. In Darwinian fitness, it is about the suitability of an organism to reproduce offspring. The more likely that an individual is able to survive and live longer to reproduce, the higher is the fitness of that individual. There are two ways through which fitness can be measured: (1) absolute fitness and (2) relative fitness. Absolute fitness pertains to the fitness of an organism based on the number of offspring that a fit organism would reproduce in its lifetime and that its offspring would reach reproductive age. Relative fitness is a standardized absolute fitness. It is a measure of biological fitness wherein the reproductive rate (of a genotype or a phenotype) is relative to the maximum reproductive rate (of other genotypes or phenotypes) in a given population. It can be measured by absolute fitness divided by the average number of offspring in a particular population. It is expressed as wrel.

Relative fitness is a measure of biological fitness wherein the reproductive rate (of a genotype or a phenotype) is relative to the maximum reproductive rate (of other genotypes or phenotypes) in a given population.

The CMI mangling and twisting of the straightforward definition is just embarrassing.


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They don’t. It’s Price and Carter who do this. Every time. When the real world conflicts with the GE, they start talking about something other than fitness that they can’t measure or quantify. Something that has in some nebuluous way they can’t explain, to do with “integrity of information in the genome”, or the number or degree of functions, or complexity, or new capacities. Or innumerable similar but vague ideas.


I think it is best defined as the ability to pass on heritable traits compared to others in the same species. What we are interested in is how a species changes over time, and that will necessarily involve genetics. If an individual organism survives into old age but doesn’t reproduce then that organism has no direct impact on the gene pool of future generations. However, cooperation between close relatives can help to pass on genes even if a specific individual family member does not have offspring (i.e. kin selection). For example, a worker bee can support a hive, and in doing so that worker bee’s DNA is passed on by the queen bee, even if the worker bees don’t actually reproduce.


Ok, which definition of fitness that @Timothy_Horton shared does the experiment use?

Already your answers have got me extremely confused. It shouldn’t be this hard to define a term. IMO, probably because it doesn’t describe the real world very well.

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Words can have different meanings and different nuances depending on the context in which they are used. Creationists love to equivocate over these subtle differences to try and score cheap rhetorical points. This is just one more case.


Relative fitness. It’s there in the figure I posted. It is defined and it is measured like that. The reproductive success of an organism relative to the reproductive success of it’s ancestor. And as you can see on the figure, relative fitness goes up:

They become better at reproducing than the ancestor. They literally multiply faster as the generations go by.

There is some ancestral organism that was used to seed the experiment. Then as the generations went on and descendants evolved, they got better and better at reproducing in that environment. They now reproduce close to 80% faster than the ancestor. They measure fitness by direct competition, where they mix up a solution of descendant bacteria with ancestor bacteria and let them compete directly, and then they measure how the fraction of the population changes over time. They show that the descendants outcompete the ancestor, as it grows in numbers much faster than the ancestor.

That seems to be a problem with you, not with what is being said. It’s perfectly straightforward English. Perhaps you should try and try put aside whatever is preventing you from comprehending what is being said. Some times you have to be willing to try to understand, which might require that you suppose there is something to be understood. If you can get yourself to read with a willingness to consider that there is something to be understood, you really can understand it. I know, because I understand it and it makes perfect logical sense.

It isn’t. It’s been defined, and the definition is sensible and coherent. And it can be measured. Descendants outcompete the ancestor in the same environment.

This is where Sanford and Price and all the GE types proceed to change the definition of fitness. Notice that it is they who do this. They want the bacteria to become more complex by adding new proteins and molecular machines, evolve more new functions in their genes, increase information in the genome (by measures they can’t define), speciate and turn into a giraffe, and fly out of the petri dish while shooting their human captors with X-men lazors. Otherwise it’s not the right kind of fitness in their mind. But then it’s THEM who are changing the definition.


The fitness of a genotype is its expected reproductive success, i.e. the number of its offspring, on average for that genotype in that environment, that survive to reproduce. The number itself is absolute fitness. The number compared to that of other genotypes in the population is relative fitness. It’s not a difficult concept.


Ok. I will take a look at the experiment again and try to figure out the definitions and explanations as I have time.

Note that on their simplistic definition of ‘fitness’ as ‘speed of reproduction’, we can see a whole host of damaging changes get ignored. This is the sleight of hand in action. The overall organism may be much less robust, and in fact unable to survive at all outside the artificial environment. But if it simply reproduces faster than the wild type, they’ll chart this as an “increase” in “fitness”. Note also that the precise definition of fitness, and means of measurement, can be altered at will in each experimental environment. Using this metric, they would consider it an increase in human fitness if we all started reproducing by age 10 and we started having an average of 10 children, but we lost half of our average IQ and our average lifespan declined by 40%. Statistics can lie.

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Fitness is always and everywhere in the context of environment.


And there we have it, Price has now delivered his own sleight of hand. He has ignored the term fitness because we can show it goes up. So now he starts complaining that it’s not the right measure.

He wants organisms that become better adapted to all circumstances simultaneously. For inexplicable reasons he thinks IQ must always increase, while also being able to survive better in both environment A and environment B and environment C simultaneously even without being exposed to them. Which just doesn’t make sense.

But that’s just not the theory of evolution, and it never was. The ancestors of whales lost the ability to run around on land, but became able to live their entire lives and reproduce in water. Adaptations were lost and others were gained.

Evolution is not, and never was a theory that says everything gets “better” at all things simultaneously. It just isn’t.

So how can it then make sense to complain that there’s something wrong with the definition of fitness? It’s measuring exactly what matters to evolution. Being able to survive and reproduce. That’s it. Evolution is not a theory of continuous and indefinite improvement in total number of abilities and their degrees.

But that wouldn’t be a lie. That really would constitute an increase in relative fitness.


I read through that steaming pile by Price and Carter again. it’s amazing just how bad and misinformed it is. At the end they suggest evolutionary biology use their definition of “fitness” to mean only physiological traits like intelligence or strength or longevity. This completely ignores the fact “fitness” only has meaning with respect to the local environment. The biggest strongest gorilla in the forest is going to be much less “fit” than a minnow if they are both placed in the middle of the ocean. These underhanded attempts to define evolution away and squeeze in their anti-science Creationist claims like “Genetic Entropy” are just pathetic.


We have always acknowledged that fitness “can go up”. This is no new revelation. In fact it’s stated right in the article that was linked here in the OP.

Note also that your graph doesn’t show any kind of linear increase in fitness. It appears to show a fitness level that approaches an asymptote. That’s not really Darwinian at all. Darwin saw no reason that slight incremental changes couldn’t continue to improve organisms ad infinitum. Approaching an asymptote looks more like there are limits to what might happen under a mutation/natural selection scenario. Reality doesn’t match Darwin’s fantasy world.

Fitness levels in an unchanging environment do asymptotically approach local maxima. Darwin correctly saw the evolutionary process of small improvements could continue indefinitely in a changing environment where there were new fitness maxima constantly appearing. Your understanding of basic evolutionary theory is just abysmal.


@PDPrice A quibble:

But in cases like sickle cell anemia, where the corruption of an important gene just happens to allow people to better survive malaria, children who carry the disease are more likely to live to adulthood.

I think you mean to say, "children who carry a single copy of the gene.

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Who defined fitness as ‘speed of reproduction’? @John_Harshman provided the definition: expected reproductive success. One way to achieve greater fitness is faster reproduction, which was the case in the example @Rumraket was describing. Are you sure you understand fitness?

No, if it consistently produces more offspring, we’ll count it as an increase in fitness, not an “increase” in “fitness”, because as fitness is defined, it’s an increase in fitness.

Right. Are you making this statement because you genuinely don’t understand what fitness is, or because you want to confuse others about the concept?

Note that you just said something untrue.

Still true, and still pointless.