# How is evolutionary fitness defined? **URL:** \ **Category:** Factual Questions\ **Created:** [May 31, 2017, 3:34pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777 "2017-05-31T15:34:40Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [May 31, 2017, 3:34pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/1 "2017-05-31T15:34:40Z")
See query: 1. “Mere” survival of a species over _x_ generations? Which I know begs the questions on speciation, but I’m expecting that the discussion doesn’t get tautological. The temporal aspect noted above: I’m assuming (correct me) that the concept of fitness is by definition time-limited: every species is on probation, as it were. So the sturgeon and crocodile and bacteria are not more “fit” than Homo sapiens. 1. My more loose question is prompted by the age-old “wouldn’t it be great to have wings.” I know there is no plan to fit an environment/niche. You get what’s dealt by the odds, the good and the bad. Me, I think it sucks that we lost the haw, and my stupid dog has one. But you gain and lose by chance: we might have lost the haw (except vestigially) for all sorts of reasons because it was detrimental to the organism, or, we might have lost it just 'cuz. But about that “it sucks” silliness: Is it possible to speak of “grades of evolutionary fitness?” Ie, are there species (perhaps even us non-flying bipeds) who got the short-end of the stick? NB: not determined by that (arguable) temporal criterion of generation length. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 31, 2017, 4:03pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/2 "2017-05-31T16:03:24Z")
What’s a haw? And it’s easy to find species that are less fit: They’re the ones that are currently approaching extinction, like the giant panda (seriously, it’s amazing that those things have survived as long as they have). ---
**Author:** ![Disheavel](https://avatars.discourse-cdn.com/v4/letter/d/e274bd/32.png) [@Disheavel](https://boards.straightdope.com/u/Disheavel)\ **Post date:** [May 31, 2017, 4:47pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/3 "2017-05-31T16:47:14Z")
There is another concept other than longevity which gets thrown into FITness as well- ability to adapt rapidly and spread to new environments. Taken to extremes ants and humans are some of the most fit biological solutions- but cheetahs are not fit even given their extreme speed as they have very little genetic diversity. But things like starlings and blackberries in the U.S. have rapidly spread due to their fitness. ---
**Author:** ![orcenio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/orcenio/32/1036_2.png) [@orcenio](https://boards.straightdope.com/u/orcenio)\ **Post date:** [May 31, 2017, 5:00pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/4 "2017-05-31T17:00:49Z")
Reproductive success. Higher fitness merely equals leaving a higher number of viable offspring for the next generation. If your genetically determined trait (wingspan/color/speed/size/hardiness/etc) adds to your genetic contribution to the genepool over other members of your species who don’t have it, you are more fit to your niche. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 31, 2017, 5:14pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/5 "2017-05-31T17:14:07Z")
Wikipedia to the rescue! > [@](#): > > Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection within evolutionary biology. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by individuals of the specified genotype or phenotype. The fitness of a genotype is manifested through its phenotype, which is also affected by the developmental environment. The fitness of a given phenotype can also be different in different selective environments. > > With asexual reproduction, it is sufficient to assign fitnesses to genotypes. With sexual reproduction, genotypes are scrambled every generation. In this case, fitness values can be assigned to alleles by averaging over possible genetic backgrounds. Natural selection tends to make alleles with higher fitness more common over time, resulting in Darwinian evolution. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 31, 2017, 5:26pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/6 "2017-05-31T17:26:30Z")
Actually, it’s a little more than just “the next generation”. A trait which leads to having many children but few or no grandchildren is not a very fit trait. But it’s hard to pin down just how long a term is appropriate to look at, because in the long term, all species eventually go extinct. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 31, 2017, 5:28pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/7 "2017-05-31T17:28:35Z")
> [@Chronos](#): > > Actually, it’s a little more than just “the next generation”. A trait which leads to having many children but few or no grandchildren is not a very fit trait. But it’s hard to pin down just how long a term is appropriate to look at, because in the long term, all species eventually go extinct. Haw! 😉 ---
**Author:** ![orcenio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/orcenio/32/1036_2.png) [@orcenio](https://boards.straightdope.com/u/orcenio)\ **Post date:** [May 31, 2017, 5:59pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/8 "2017-05-31T17:59:32Z")
> [@Chronos](#): > > Actually, it’s a little more than just “the next generation”. A trait which leads to having many children but few or no grandchildren is not a very fit trait. Quite true. In fact, I committed a bit of an error as [Reproductive Success and Fitness are not the same thing](http://scienceblogs.com/gregladen/2013/07/19/reproductive-success-and-fitness-are-not-the-same-thing/). > [@](#): > > Reproductive Success (RS) is defined in many ways in different places by different people, but one of the most common definitions is simply the number of offspring an individual produces. This definition is further modified in most cases to mean only those individuals that will be fertile, i.e., capable of producing further offspring. (snip) > > Fitness is a property of an allele that refers to its relative likelihood of representation in future generations in a population.(snip) > > At first glance RS and Fitness are the same, or similar, but one might immediately notice that RS is a feature of an individual (that has offspring) while fitness is a feature of an allele. So, it is possible that a given individual will have a relatively high RS but contain a particular allele with low fitness. Presumably the higher RS of that individual is due to high-fitness alleles of other genes. ---
**Author:** ![blindboyard](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blindboyard/32/2833_2.png) [@blindboyard](https://boards.straightdope.com/u/blindboyard)\ **Post date:** [May 31, 2017, 6:36pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/9 "2017-05-31T18:36:10Z")
\*The fittest survive. What is meant by the fittest? Not the strongest; not the cleverest— Weakness and stupidity everywhere survive. There is no way of determining fitness except in that a thing does survive. “Fitness,” then, is only another name for “survival.” Darwinism: That survivors survive.\* – Charles Fort > [@Chronos](#): > > What’s a haw? Old English for “white”. The berry produced by the Hawthorn bush, also known as the Whitethorn or Mayflower. Edible, but unpalatable when raw. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 31, 2017, 6:55pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/10 "2017-05-31T18:55:12Z")
But **Leo Bloom** seems to be using it to refer to something that dogs have but humans lack, or have only vestigially. Dogs don’t have thornberries (or at least, they don’t if the vet does his job properly). ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 31, 2017, 7:04pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/11 "2017-05-31T19:04:44Z")
haw = nictitating membrane (extra eyelid) ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [May 31, 2017, 10:45pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/12 "2017-05-31T22:45:01Z")
It can seem circular, but it isn’t really… An antelope that sprints at 45 mph is “fitter” than one in the same environment that only sprints at 30 mph. There are objective traits that we can point to as elements of fitness. (Now, that extra sprinting speed comes at a cost, so it isn’t automatically “fitter to be faster.” There are gobs and gobs of feedback loops. That’s just how complicated the whole darn mess is.) ---
**Author:** ![igor\_frankensteen](https://avatars.discourse-cdn.com/v4/letter/i/839c29/32.png) [@igor\_frankensteen](https://boards.straightdope.com/u/igor_frankensteen)\ **Post date:** [May 31, 2017, 11:30pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/13 "2017-05-31T23:30:06Z")
from John Mace, post #5: > [@](#): > > Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection within evolutionary biology. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by individuals of the specified genotype or phenotype. The fitness of a genotype is manifested through its phenotype, which is also affected by the developmental environment. _ **The fitness of a given phenotype can also be different in different selective environments.** _ > > With asexual reproduction, it is sufficient to assign fitnesses to genotypes. With sexual reproduction, genotypes are scrambled every generation. In this case, fitness values can be assigned to alleles by averaging over possible genetic backgrounds. Natural selection tends to make alleles with higher fitness more common over time, resulting in Darwinian evolution. I added emphasis to what I think is most critical to this discussion. Fitness is RELATIVE. In fact, I think it’s an amusing coincidence that “fitness” is an extension of the word “fit,” which has more than one definition. In the case of evolutionary concerns, a critter can be the most “fit” they can be, as far as being healthy, energetic, sexually attractive and so on, but no matter how “fit” they are in that sense, they might well not “fit” the limits or parameters required for survival or success. Most of the serious writings I’ve stumbled across which talked about “evolutionary fitness,” essentially awarded a given entity a higher “score,” if they had lots of possible “versions.” Essentially, they figured that if there’s only five kinds of critter X, but there’s ten gazillion kinds of critter Y, that come the bad times, it’s mathematically more likely that SOME of critter Y will survive the culling. But even if we go for that, it’s still relative to what the evolutionary “challenge” is. Math wont help you a bit, if whatever befalls critters X and Y, is such that it kills every single one of the ten gazillion versions of Y, but isn’t lethal to the X critters. So bottom line, I’d suggest going with the general explanation about fitness shown above, but be sure to stick an asterisk on the part about how it still depends on what happens. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 31, 2017, 11:41pm UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/14 "2017-05-31T23:41:18Z")
> [@Chronos](#): > > And it’s easy to find species that are less fit: They’re the ones that are currently approaching extinction, like the giant panda (seriously, it’s amazing that those things have survived as long as they have). But that’s just under current conditions, where they have lost most of their original habitat and are under pressure from humans. They evolved to have the characteristics they do because in previous environments the pandas that had those characteristics left more offspring than those that didn’t. Pandas were perfectly fit for the environment they evolved in. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 1, 2017, 1:45am UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/15 "2017-06-01T01:45:54Z")
Eh, I have a very hard time imagining any habitat where it’s advantageous for a creature to be unable to fight off a cat half its size and native to the same habitat, or for it to eat a diet so nutrient-poor that it has to spend every waking hour eating, or where the females are only fertile for a couple of unpredictable days every couple of years, with territories so large (due to the poor diet) that they’re unlikely to meet any males at all over those days, and which they’re likely to reject even if they do find any, or where the young are so helpless that they can’t even poop without help, or that routinely gives birth to twins despite only producing enough milk for one, or… There must be some environment that pandas are suited for, or they wouldn’t be what they are, but that environment is so specific and unstable that their long-term prospects were never very good. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [June 1, 2017, 3:01am UTC](https://boards.straightdope.com/t/how-is-evolutionary-fitness-defined/787777/16 "2017-06-01T03:01:51Z")
An organism “fits” into an ecological niche. It is fit if it deals well the circumstances of that niche - suitable diet, predators, ecology and climate, success in competition with its own species or other species for food, etc. It becomes less fit when either the niche changes or it changes - and it is less able to compete with others in that circumstance. You can list assorted challenges that may result in the species or specimen being less fit - climate change, food sources change or overpopulation destroying food sources, diseases or new predators (on it or in competition); even something simple like invasive plant species replacing its food source… Then the circumstances - the niche a species occupies - change, and fitness is judged by different criteria. What was once a disadvantage may become a “plus”. An important point to make in any discussion of evolution is that every attribute also has a cost. Birds have bird-brains because if you want to fly, you minimize the weight of everything that does not help you get just enough food to get by. The same could be said for cheetahs - to be super-fast, they forego fat reserves, big brains, etc - to maximize speed. Our brain, IIRC, uses one third of the calories we consume. We better get some advantage from it that helps us feed ourselves. Muscles, even at rest, consume calories, so Conan the Barbarian would be one of the first to starve to death. Bigger animals need more food than smaller ones, so in an environment where scarcity happens, smaller animals are “more fit”. In an environment where males fight each other for mating rights, bigger/stronger is better. Where an mammal gestates their young for a long time, reliable food source - or significant body fat - is an advantage unless they evolve a “abort on demand” feature -versus birds who lay eggs and then can abandon the nest, or marsupials who can discard the barley formed baby any time after a short and less demanding gestation. But a mammal that gestates for a long time, like many herd animals, can give birth to a youngster already able to stand and walk. …Plus vs… minus. Cost vs. benefit. Fitness depends on the evaluation criteria of the moment.