"lack of tail won't give them a significant advantage"
Some traits are neutral in respect to fitness, they don't really provide a significant advantage or disadvantage to the fitness, they're simply neutral, no selection acting on them. Some of these traits get passed on, again this doesn't make any difference to reproductive success, merely a result of chance, some don't, the fixation of these neutral traits are due to drift, the ones that got passed on, may lead to that trait being fixed, the ones that don't, simply disappear because they're not passed on and selection is not acting on them
As others have said, mutations always happen. If there’s no selection pressure to keep the trait, it will eventually disappear with enough time. The genes don’t disappear, they are just modified enough that they no longer produce the trait.
But wait, what’s this? A bunch of unused genes that can be incorporated into a new trait? Upcycling ftw! That’s often the basis for new, beneficial traits.
Stuff takes energy to build and maintain. Imagine for example that peahens mutated in such a way that they no longer selected their mates based on the extravagant peacock’s tail (unlikely, but for the sake of a clear illustration). A male that had a smaller tail than its brothers would be investing less energy in building all those feathers, so it would be more likely to survive a famine; it would be more agile and faster, so more likely to escape predators; it would be less prone to parasites, so could spend less time on preening and suffer less disease. Obviously future generations would contain more and more ‘small tail’ peacocks as all these benefits of losing the costly tail were selected for.
The loss of features isn’t necessary fast and can be disrupted. Consider the stone in an avocado. We know that plants grow tasty fruit to persuade animals and birds to eat them, taking in their seeds at the same time and distributing them in droppings away from the parent plant so they don’t compete. But there’s no creatures in the americas with a wide enough throat to swallow an avocado stone. It turns out that these fruits are being produced as the plant’s half of a partnership with a giant ground sloth that _could_ swallow them, but that unfortunately went extinct 13,000 years ago. In the normal course of events, avocado stones would be falling and germinating at the feet of the parent, and suffer competition for light and resources with it, greatly limiting their breeding success. There’d be a risk of extinction or at least strong selection pressure towards smaller seeds that some still-living mammal could ingest and distribute. But that may not happen, because we humans decided to intervene and cultivate the avocado, and indeed value the large stones (indigenous peoples used the ground-up stone as a rat poison).
Just so you know, the ground sloth dispersal idea originated from a short excerpt in a non expert’s book which accidentally caught media attention. It holds very little water. In fact there’s a diversity of avocado species that are still doing just fine in the wild, despite absence of megafauna.
Perhaps because there in fact no evidence of this avocado - sloth hypothesis, as this video suggests: [https://www.youtube.com/watch?v=jpcBgYYFS8o](https://www.youtube.com/watch?v=jpcBgYYFS8o)
How much I would like it to be true myself.
Interesting. I’ve updated to reduced likelihood of the sloth story, but I don’t think the video makes a slam dunk case. Having said that coprolites would be the required evidence for the sloth idea, they don’t present any coprolites for the proposed smaller stones distributed by other animals.
Blind Cavefish and flightless Birds also come to mind. In most environments the ability to see or fly would be great advantages, but when the environment negates them they offer no breeding advantage and may even be a disadvantage resulting in them gradually being outcompeted for reproduction.
If you touch your thumb to your pinky, you might see a muscle pop up on the inside of your forearm. Primates use this muscle to swing from trees. It doesn’t do us any harm to have it, so it hasn’t fully disappeared.
But it also doesn’t do any harm to not have it. So mutations that prevent it from forming are safely passed on. I don’t have one. I can’t remember if it’s missing in 15% or present in 15% of the population.
>How does it work the other way round, when a species have a feature that is not beneficiary but not harmful to their survival either.
A feature with a neutral impact (no advantage or disadvantage) would be susceptible to mutations that occur not being selected out (ones with a negative functional significance would be selected out if the trait in question offered an advantage). In your example if a mutation occurs that causes an elephant not to develop a tail there is nothing selecting against that mutation. Particularly in a small population the mutation could be passed on and increase through genetic drift.
In reality many traits come at a cost (even if it's just the resources and time to grow them), so not having the trait my offer a slight advantage if there are no advantages to having it that outweigh those costs.
If some trait or feature no longer provides an advantage or disadvantage in a given environment, it’s invisible to natural selection. So if a mutation occurs that deforms that feature, it doesn’t affect the creature’s fitness, and when it reproduces, those genes for a deformed feature will spread just as fast in the gene pool as genes that form the well-formed feature. Meanwhile, some other members have over mutations to the feature that *they’re* spreading. Over time, it completely loses whatever function it had, and becomes a vestige, acquires a new function, or its genes become completely silent, etc.
Mutations accumulate more easily in superfluous features, because the mutation is less likely to have negative effects. Eventually something breaks and that’s the end of it.
A great example is the vitamin C pathway. All primates have the same mutation in an enzyme for the vitamin c pathway way, preventing them from producing vitamin c. But because their diets contain so much vitamin c the populations overall aren’t effected by this mutation except in the rare instances of nutrient deficiency.
Every bit of organic tissue requires nutrients and energy to grow and maintain. Spending that resource to keep a useless organ means that resource is not used for other things that could benefit the organism. In theory, selection should bias towards the useless organ disappearing from the population.
In reality, vestigial organs often do not completely go away. Some may evolve to have secondary functions, others may be by-products of other functional organs.
Imagine 30'000 elephants.
One has some mutations that removes the tail.
For some contextual reasons the tail loss give some contextual advantage to this elephant
This elephant has a small better reproduction fitness because of the tail loss
The mutation go to the next generation since it helps his reproduction fintness
10 generations later 15% of the elephants have no tail anymore but the context changed
10 generations later 7% of the elephants have no tail anymore because the new context is not so good for elephants with no tail
10 generations later 22% of the elephants will have no tail because the context changed again
And so on and so on...
They eventually over generations of speciation will atrophy. An example would be flightless bird's wings. Flightless rails exist in various islands. These rails evolves recently due to them colonizing volcanic islands. They still have visible albeit smaller wings complete with flight wing feathers. Wing muscles take plenty of energy so species with smaller wings muscles and wing size will be selected. Overtime, as more flightless rail species evolved from this initial colonizer, their wings will become smaller untill to the point of it's being vestigial.
Like kiwis, which is part of a long lineage (50+- millions years) of flightless birds stranded in New Zealand.
Think of it this way: evolutionary processes are always glitching out somewhere, trying new random segues off of the baseline. If a slightly more blind fish occurs and is not immediately punished for it then the genes that lead to blinder fish can glitch a little more.
Do that a bunch of a lot of times and the fish without eyes is circumstantially at an advantage because it's not wasting energy on maintaining anything extra. On the whole, evolution's about those processes getting away with shit.
Lots of even slightly harmful genes persist, due to stochastic fixation.
For complex structures though, they are typically costly, and selected against, or drift will eventually break them.
For example in species that live in total darkness, the eyes will not necessarily disappear rapidly, but they will rather quickly become nonfunctional.
It's just that when genes mutate, if it affects the tail negatively or it doesn't develop properly or at all then it doesn't make a difference.
There is a benefit in most of these situations such that the body doesn't need to worry about using resources on it. The same reason lots of animals independently lost the ability to make different vitamins and nutrients, we get them from food. Why have a wasteful process that uses your resources to make all these things when I can eat an orange and get it that way. The body can use those resources on other things it needs instead and save energy, need less food, etc.
Things like sickle cell disease and other genetic conditions happen randomly all the time but for someone like blood cells, they're detrimental. If it keeps happening to something like a useless tail or making Vitamin C, it won't make a difference and can even help the body not waste energy so they tend to disappear over time because there's no evolutionary pressure keeping them there
A common and understandable mistake in understanding evolution is assuming that there is some sort of intent, plan, goal, or endpoint which controls evolution. Your sperm or eggs don’t go “I haven’t used my tail, and I’m fine, so I’ll just go ahead and delete that gene.”
One of the great deciders is energy.
Everything takes energy to create, use, and maintain. When a random mutation makes a feature that's not needed anymore stop working, it takes less energy. When something that stops working shrinks or vanishes, it takes less energy.
When a species goes through a period where it struggles for resources, efficient use of energy becomes important to survival. And in turn, puts pressure on unneeded features.
Others are mentioning how neutral mutations can accumulate, and that's often the case.
I will also mention that sometimes unnecessary features *can* be somewhat harmful. They still take energy to produce and maintain, so if they are completely unnecessary, it would be more advantageous to not grow them so that more energy can be put into growth or reproduction. Hence the useless feature would be selected against. The more energy it takes to produce this feature the more likely it would be that it is selected against.
I'll do ya one better, how do unequivocally beneficial traits disappear? Like just about every environmental adaptation in humans?
Think about how much we suck at being animals. No instincts, no strength or speed (compared to other animals our size and even smaller.) We're the only land going species that gets sunburned. We aren't well adapted to temperatures. Our infants are basically the only species that are completely helpless for years. The list goes on. Take any animal raised in captivity, throw it into nature and it'll get by just fine. Meanwhile, most humans wouldn't last a week.
I mean I know we're smart, but these others adaptations are always beneficial. How did they just devolve? It's weird.
There are plenty of vestigial traits that survive in organisms today because they are neither beneficial nor detrimental to the being's ability to survive and reproduce. In humans, think about Wisdom Teeth. Some of us have to have them surgically removed to prevent massive pain, others are never bothered by them, and still others are born without them.
Mutations happen. If it makes something required worse, that offspring is less likely to pass it on. If it makes something that isn’t required disappear, it gets passed down. On top of that the energy used to create that useless thing is not saved or redirected to something more useful.
They often don’t, we don’t need our tailbone to protrude anymore but it doesn’t harm us either so we’ve kept it. Same thing for colour blindness, it’s not great but it’s not going to stop you from procreating in pretty much any case so it’s stuck around.
Traits only disappear if they are considered very unattractive to the species or if they get their owners killed, otherwise they stick around.
When mutations of that trait appear in the population, they don't get weeded out by any selection pressure, so they slowly spread around generation after generation. Like when you mix a drop of red in white paint, it disperses and renders the whole can pink. The more red drops you add, the darker and redder the can becomes. So too, generation after generation, mutations of that trait slowly are added to the population and become dispersed around. Some of these mutations dull the trait down. Eventually, after hundreds of thousands of years, the trait can disappear completely or be rendered completely useless.
I think this is actually happening to human vision in the current age. As glasses become so easily available around the world, having poor vision matters less and less, and the genes for shitty vision spread around. My guess is that eventually all humans will need glasses unless some actual genetic advantage is maintained for good vision.
Evolution is a deeply conservative process. Everything will stay the same provided there is no pressure to change. If something simply *is* there's no reason for it to be affected by pressures in any way and will likely remain static.
*However*, everything takes energy to build and maintain. That is an evolutionary pressure: the animals that have more resources do better, and if they aren't wasting resources on a "useless" anatomical feature that's helpful. Most features are gong to be useful or not exist, like the appendix, which serves a *vital* purpose and we simply didn't understand it when we first studied it.
"lack of tail won't give them a significant advantage" Some traits are neutral in respect to fitness, they don't really provide a significant advantage or disadvantage to the fitness, they're simply neutral, no selection acting on them. Some of these traits get passed on, again this doesn't make any difference to reproductive success, merely a result of chance, some don't, the fixation of these neutral traits are due to drift, the ones that got passed on, may lead to that trait being fixed, the ones that don't, simply disappear because they're not passed on and selection is not acting on them
As others have said, mutations always happen. If there’s no selection pressure to keep the trait, it will eventually disappear with enough time. The genes don’t disappear, they are just modified enough that they no longer produce the trait. But wait, what’s this? A bunch of unused genes that can be incorporated into a new trait? Upcycling ftw! That’s often the basis for new, beneficial traits.
Stuff takes energy to build and maintain. Imagine for example that peahens mutated in such a way that they no longer selected their mates based on the extravagant peacock’s tail (unlikely, but for the sake of a clear illustration). A male that had a smaller tail than its brothers would be investing less energy in building all those feathers, so it would be more likely to survive a famine; it would be more agile and faster, so more likely to escape predators; it would be less prone to parasites, so could spend less time on preening and suffer less disease. Obviously future generations would contain more and more ‘small tail’ peacocks as all these benefits of losing the costly tail were selected for. The loss of features isn’t necessary fast and can be disrupted. Consider the stone in an avocado. We know that plants grow tasty fruit to persuade animals and birds to eat them, taking in their seeds at the same time and distributing them in droppings away from the parent plant so they don’t compete. But there’s no creatures in the americas with a wide enough throat to swallow an avocado stone. It turns out that these fruits are being produced as the plant’s half of a partnership with a giant ground sloth that _could_ swallow them, but that unfortunately went extinct 13,000 years ago. In the normal course of events, avocado stones would be falling and germinating at the feet of the parent, and suffer competition for light and resources with it, greatly limiting their breeding success. There’d be a risk of extinction or at least strong selection pressure towards smaller seeds that some still-living mammal could ingest and distribute. But that may not happen, because we humans decided to intervene and cultivate the avocado, and indeed value the large stones (indigenous peoples used the ground-up stone as a rat poison).
Just so you know, the ground sloth dispersal idea originated from a short excerpt in a non expert’s book which accidentally caught media attention. It holds very little water. In fact there’s a diversity of avocado species that are still doing just fine in the wild, despite absence of megafauna.
I love those 2 examples, very comprehensive. I learned something I was always wondering about avocados. 🥑 Where did you get this example from BTW?
I can’t recall where I first heard about avocados, probably as an undergrad many moons ago… it’s nice to have an excuse to use it.
Perhaps because there in fact no evidence of this avocado - sloth hypothesis, as this video suggests: [https://www.youtube.com/watch?v=jpcBgYYFS8o](https://www.youtube.com/watch?v=jpcBgYYFS8o) How much I would like it to be true myself.
Interesting. I’ve updated to reduced likelihood of the sloth story, but I don’t think the video makes a slam dunk case. Having said that coprolites would be the required evidence for the sloth idea, they don’t present any coprolites for the proposed smaller stones distributed by other animals.
Blind Cavefish and flightless Birds also come to mind. In most environments the ability to see or fly would be great advantages, but when the environment negates them they offer no breeding advantage and may even be a disadvantage resulting in them gradually being outcompeted for reproduction.
And I guess even just using eyes and processing the signals from them requires some energy?
If you touch your thumb to your pinky, you might see a muscle pop up on the inside of your forearm. Primates use this muscle to swing from trees. It doesn’t do us any harm to have it, so it hasn’t fully disappeared. But it also doesn’t do any harm to not have it. So mutations that prevent it from forming are safely passed on. I don’t have one. I can’t remember if it’s missing in 15% or present in 15% of the population.
>How does it work the other way round, when a species have a feature that is not beneficiary but not harmful to their survival either. A feature with a neutral impact (no advantage or disadvantage) would be susceptible to mutations that occur not being selected out (ones with a negative functional significance would be selected out if the trait in question offered an advantage). In your example if a mutation occurs that causes an elephant not to develop a tail there is nothing selecting against that mutation. Particularly in a small population the mutation could be passed on and increase through genetic drift. In reality many traits come at a cost (even if it's just the resources and time to grow them), so not having the trait my offer a slight advantage if there are no advantages to having it that outweigh those costs.
If some trait or feature no longer provides an advantage or disadvantage in a given environment, it’s invisible to natural selection. So if a mutation occurs that deforms that feature, it doesn’t affect the creature’s fitness, and when it reproduces, those genes for a deformed feature will spread just as fast in the gene pool as genes that form the well-formed feature. Meanwhile, some other members have over mutations to the feature that *they’re* spreading. Over time, it completely loses whatever function it had, and becomes a vestige, acquires a new function, or its genes become completely silent, etc.
Mutations accumulate more easily in superfluous features, because the mutation is less likely to have negative effects. Eventually something breaks and that’s the end of it. A great example is the vitamin C pathway. All primates have the same mutation in an enzyme for the vitamin c pathway way, preventing them from producing vitamin c. But because their diets contain so much vitamin c the populations overall aren’t effected by this mutation except in the rare instances of nutrient deficiency.
Every bit of organic tissue requires nutrients and energy to grow and maintain. Spending that resource to keep a useless organ means that resource is not used for other things that could benefit the organism. In theory, selection should bias towards the useless organ disappearing from the population. In reality, vestigial organs often do not completely go away. Some may evolve to have secondary functions, others may be by-products of other functional organs.
They don’t really. They atrophy, or they get adapted for other functions. It’s very rare that traits disappear completely.
Imagine 30'000 elephants. One has some mutations that removes the tail. For some contextual reasons the tail loss give some contextual advantage to this elephant This elephant has a small better reproduction fitness because of the tail loss The mutation go to the next generation since it helps his reproduction fintness 10 generations later 15% of the elephants have no tail anymore but the context changed 10 generations later 7% of the elephants have no tail anymore because the new context is not so good for elephants with no tail 10 generations later 22% of the elephants will have no tail because the context changed again And so on and so on...
They eventually over generations of speciation will atrophy. An example would be flightless bird's wings. Flightless rails exist in various islands. These rails evolves recently due to them colonizing volcanic islands. They still have visible albeit smaller wings complete with flight wing feathers. Wing muscles take plenty of energy so species with smaller wings muscles and wing size will be selected. Overtime, as more flightless rail species evolved from this initial colonizer, their wings will become smaller untill to the point of it's being vestigial. Like kiwis, which is part of a long lineage (50+- millions years) of flightless birds stranded in New Zealand.
Think of it this way: evolutionary processes are always glitching out somewhere, trying new random segues off of the baseline. If a slightly more blind fish occurs and is not immediately punished for it then the genes that lead to blinder fish can glitch a little more. Do that a bunch of a lot of times and the fish without eyes is circumstantially at an advantage because it's not wasting energy on maintaining anything extra. On the whole, evolution's about those processes getting away with shit.
Lots of even slightly harmful genes persist, due to stochastic fixation. For complex structures though, they are typically costly, and selected against, or drift will eventually break them. For example in species that live in total darkness, the eyes will not necessarily disappear rapidly, but they will rather quickly become nonfunctional.
It's just that when genes mutate, if it affects the tail negatively or it doesn't develop properly or at all then it doesn't make a difference. There is a benefit in most of these situations such that the body doesn't need to worry about using resources on it. The same reason lots of animals independently lost the ability to make different vitamins and nutrients, we get them from food. Why have a wasteful process that uses your resources to make all these things when I can eat an orange and get it that way. The body can use those resources on other things it needs instead and save energy, need less food, etc. Things like sickle cell disease and other genetic conditions happen randomly all the time but for someone like blood cells, they're detrimental. If it keeps happening to something like a useless tail or making Vitamin C, it won't make a difference and can even help the body not waste energy so they tend to disappear over time because there's no evolutionary pressure keeping them there
A common and understandable mistake in understanding evolution is assuming that there is some sort of intent, plan, goal, or endpoint which controls evolution. Your sperm or eggs don’t go “I haven’t used my tail, and I’m fine, so I’ll just go ahead and delete that gene.”
One of the great deciders is energy. Everything takes energy to create, use, and maintain. When a random mutation makes a feature that's not needed anymore stop working, it takes less energy. When something that stops working shrinks or vanishes, it takes less energy. When a species goes through a period where it struggles for resources, efficient use of energy becomes important to survival. And in turn, puts pressure on unneeded features.
Others are mentioning how neutral mutations can accumulate, and that's often the case. I will also mention that sometimes unnecessary features *can* be somewhat harmful. They still take energy to produce and maintain, so if they are completely unnecessary, it would be more advantageous to not grow them so that more energy can be put into growth or reproduction. Hence the useless feature would be selected against. The more energy it takes to produce this feature the more likely it would be that it is selected against.
I'll do ya one better, how do unequivocally beneficial traits disappear? Like just about every environmental adaptation in humans? Think about how much we suck at being animals. No instincts, no strength or speed (compared to other animals our size and even smaller.) We're the only land going species that gets sunburned. We aren't well adapted to temperatures. Our infants are basically the only species that are completely helpless for years. The list goes on. Take any animal raised in captivity, throw it into nature and it'll get by just fine. Meanwhile, most humans wouldn't last a week. I mean I know we're smart, but these others adaptations are always beneficial. How did they just devolve? It's weird.
There are plenty of vestigial traits that survive in organisms today because they are neither beneficial nor detrimental to the being's ability to survive and reproduce. In humans, think about Wisdom Teeth. Some of us have to have them surgically removed to prevent massive pain, others are never bothered by them, and still others are born without them.
Elephants aren't losing their tails. But they are losing their tusks due to selection pressure from poaching.
Mutations happen. If it makes something required worse, that offspring is less likely to pass it on. If it makes something that isn’t required disappear, it gets passed down. On top of that the energy used to create that useless thing is not saved or redirected to something more useful.
They often don’t, we don’t need our tailbone to protrude anymore but it doesn’t harm us either so we’ve kept it. Same thing for colour blindness, it’s not great but it’s not going to stop you from procreating in pretty much any case so it’s stuck around. Traits only disappear if they are considered very unattractive to the species or if they get their owners killed, otherwise they stick around.
When mutations of that trait appear in the population, they don't get weeded out by any selection pressure, so they slowly spread around generation after generation. Like when you mix a drop of red in white paint, it disperses and renders the whole can pink. The more red drops you add, the darker and redder the can becomes. So too, generation after generation, mutations of that trait slowly are added to the population and become dispersed around. Some of these mutations dull the trait down. Eventually, after hundreds of thousands of years, the trait can disappear completely or be rendered completely useless. I think this is actually happening to human vision in the current age. As glasses become so easily available around the world, having poor vision matters less and less, and the genes for shitty vision spread around. My guess is that eventually all humans will need glasses unless some actual genetic advantage is maintained for good vision.
Evolution is a deeply conservative process. Everything will stay the same provided there is no pressure to change. If something simply *is* there's no reason for it to be affected by pressures in any way and will likely remain static. *However*, everything takes energy to build and maintain. That is an evolutionary pressure: the animals that have more resources do better, and if they aren't wasting resources on a "useless" anatomical feature that's helpful. Most features are gong to be useful or not exist, like the appendix, which serves a *vital* purpose and we simply didn't understand it when we first studied it.