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A member of a species becomes immortal, loss of senescence, incapable of dying from age-related issues. Otherwise it remains the same as other members of its species at the point at which it became immortal, including its current genome/dna and instinctual needs to find mates and reproduce.

Would such an individual stagnate the evolutionary march of its species?

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    $\begingroup$ Just one immortal? One immortal in a million isn't gonna do much. $\endgroup$
    – Nepene Nep
    Nov 14, 2021 at 11:55
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    $\begingroup$ With such a biological immortality, the importance of the low-probability, but anyways possible death scenarios grow. For example, you can expect death by a traffic accident in some thousands years. Now that we live typically lesser than a century, it is not a big problem, but only biological immortality is not enough to stop anything. $\endgroup$
    – Gray Sheep
    Nov 14, 2021 at 21:01
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    $\begingroup$ Is the immorality genetic? $\endgroup$
    – Ben
    Nov 15, 2021 at 10:05
  • $\begingroup$ @Ben Honestly I hadn't thought about the exact nature of the immortality, only that the individual is immortal. Though I have to admit that the answers containing scenarios where the immortality is inheritable have been more impactful and interesting so I'm leaning toward that the individual's genetics do in fact play a part like a case of it having a set of recessive genes whose combination would likely only reappear and make another immortal in the case of them or those closely related to them participating in some manner of inbreeding. $\endgroup$
    – Lemming
    Nov 15, 2021 at 10:22
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    $\begingroup$ If the immortality is genetic/heritable, it is by definition itself evolutionary. Therefore you would see an immediate effect to incrementing degrees over time. According to the patterns of evolution, I surmise that you'd eventually reach a point of divergent evolution (the immortals are ostracized from "normal" society), or constant convergent evolution (the immortals continuously breed into the normal evolutionary track). $\endgroup$
    – OhBeWise
    Nov 16, 2021 at 15:00

9 Answers 9

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An immortal could certainly have an impact on the gene pool if they lived long enough and had enough descendants.

Genghis Khan, who lived between the 1100s and 1200s, is believed to be the ancestor of roughly about 16 million men in the modern-day, and he only lived to be about 60 or 70. I'm certain that, with enough partners, a person could easily share their genes with the whole of a planet, even one with billions of people like Earth. It might take hundreds or even thousands of years, but it would happen.

It is exponential growth. The immortal has children and those children have children, who then have children in turn. Unless the descendants all have short life spans, this process would be faster than you would expect, especially if the immortal is purposefully trying to have as many as possible.

The biggest problem is what happens when there's overlap. If everyone is a descendant of this immortal, you run into the issue of inbreeding. It would be subtle at first, but damaging after enough time.

I suspect a world like this would eventually run into the problem many royal bloodlines faced by trying to breed only within their own group. If everyone has the genes of the original immortal, then every time the immortal contributes again they make things more complicated. I don't even want to imagine what a mess genes would be if the immortal repeatedly had children with its own descendants. As I said, it would probably be similar to old royal bloodlines which suffered from issues such as serious birth defects and other diseases. It might take 10000 years before the effects are noticeable, but it would certainly happen.

If immortality is a shared trait, then things are even worse. You have the same problem multiplied exponentially. Mortals would be crowded out almost instantaneously, unable to compete with their immortal counterparts. Then those immortals would have more children. Familial lines would crisscross and zigzag in a chaotic assortment until the descendant's DNA becomes a tangled mess. Then the messy genes of descendants would get contributed back to the gene pool to make things worse.

Honestly, I can't see any situations where this goes well. The immortal should try to have as few children as possible.

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    $\begingroup$ Inbreeding isn't really an issue once you're past first cousin levels of relatedness. Second cousin marriage is legal in most places, and is quite common in some regions. You're as closely related to your second cousin as you are to your great-great-grandparents, so as long as the immortal doesn't procreate with their immediate few generations of descendants, I don't expect inbreeding to be a big issue unless the initial population is very small. Genghis Khan is effectively no more closely related to his great (times 40) grandkids than any random person is, despite the direct lineage. $\endgroup$ Nov 15, 2021 at 19:07
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    $\begingroup$ The great Khan is slacking then, Charlemagne is considered to be an ancestor of every person of European descent. $\endgroup$
    – Separatrix
    Nov 16, 2021 at 8:33
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    $\begingroup$ I think you might mix up "having influenced" and "stagnating" evolution. While millions have Genghis Khan as an ancestor, that ancestry doesn't stagnate evolution any more than other grand-grand-...-parent. Some useless or harmful genes will be filtered out with time, others are recessive and will not carry on, or will simply dilute to 1:1024 after just 10 Generations. So, the evolutionary influence is quite limited to how many 1st-to-3rd-ish generation descendants he has an any time. $\endgroup$ Nov 16, 2021 at 9:55
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The answer to this question depends upon a number of factors:

  1. The population of the species.

With a small population, the immortal individual makes up a larger fraction of the gene pool than if the population is large, and therefore it has a greater influence upon the genetic makeup of future generations.

  1. Whether the immortal individual can die from environmental hazards.

If the immortal individual may die from other causes, then its death from those causes would remove its influence from subsequent generations.

  1. If the immortal individual is seen as a fitter mate by the opposite sex due to age, experience and youthful appearance.

If the immortal individual is selected as a better mate than mortal individuals, then it will have a greater effect upon future generations.

However, it seems likely that unless the population is very low, while the immortal individual may have a relatively large influence upon the gene pool, it won't be able to contribute more than a single percentage point toward the next generation, which may slightly retard evolution, but wouldn't make it stagnate.

Eventually the point may come where the immortal individual is sufficiently different to the population that it will no longer be able to reproduce successfully, at which point it will cease to influence the evolution of the species.

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Try playing with a population genetics simulation

This simulation is pretty and very intuitive, working with red and white fish. Your immortal counts as a "migration rate" (except in his case, all the migrants are the same fish). If you set the simulation to a minimum migration rate (0.1) of fish of a different color, in a few hundred generations they take over the pond. If, however, they have a fitness penalty (set Rr to 0.9 and either rr or RR - whichever migrant color you picked - as 0.8) then about 1/3 of the original color will continue in the population. Now both those numbers are very, very high, but you are probably interested in time periods a thousand times longer, which (very crudely) implies numbers a thousand times less might work. Even so -- depending on the size of your population, your immortal may very well not be able to make a dent in it simply based on the numbers. And if there really has been favorable evolution going on in the rest of the population, his offspring will start to be selected out.

[assortative mating in this simulation seems to have a bug, sorry]

Of course, there is a wrinkle not present in the fish simulation, which are that the immortal's offspring really are all related to each other and could carry many different recessive genes. If they directly breed with one another, they could suffer immediate problems that would keep them from taking over. Or they may just not want to! In which case it's going to take more than a toy HTML5 simulation to figure out the problem. :)

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    $\begingroup$ I think there's probably a bug in that simulation when it comes to assortative mating: even with only one allele present in the population (in which case it should have no effect at all) setting any non-zero level of assortative mating seems to cause the population size to drop way below the carrying capacity. $\endgroup$ Nov 14, 2021 at 22:06
  • $\begingroup$ I think what they actually mean in that simulation is that hybrids are inviable. I sort of was thinking of that by speaking of discrimination against the mixed-ancestry offspring. A state-mandated discriminatory scheme meant to keep the immortal from taking over might end up resembling the simulation more than assortative mating as normally defined. $\endgroup$ Nov 15, 2021 at 1:24
  • $\begingroup$ Even if that's how they implement it (which would be kind of weird and IMO wrong: assortative mating is normally a pre-zygotic barrier, whereas hybrid inviability is post-zygotic), it still shouldn't have any effect on population size in populations with only one allele present (and thus no hybrids), but in that simulation it clearly does. So I think it's probably a bug. Anyway, I sent an e-mail reporting it to Dr. Jones; let's see if he gets back to me. $\endgroup$ Nov 15, 2021 at 10:11
  • $\begingroup$ After playing with it a little longer I see what you mean - no way to rationalize that one! $\endgroup$ Nov 15, 2021 at 13:23
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No.

The evolutionary process -- offspring adapting randomly and the ones with beneficial mutations surviving more often to pass on those mutations -- is not damaged by the existence of one immortal member.

What happens is the rest of the species breeds and evolves as normal, and the immortal stays the same through the generations. If the species evolves too much they no longer resemble the immortal member, and might not even be able to interbreed with them.

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  • $\begingroup$ Aditionally, consider an environmental prtessure that equires a specific adaptation - for example a new disease (easily transmissible, with high fatality rate) for which some have genetic immunity and others don't. If the immortal doesn't have that gene then they will survive, as they are immortal, but most of their offspring won't live, so the immortal's genetic code won't be passed on to the 'next' generation. $\endgroup$
    – Penguino
    Nov 14, 2021 at 21:03
  • $\begingroup$ @Penguino, the question does not specify the type of immortality, there is no Magic tag. Depending on the genes needed and the damage done by this virus.. when the "immortal" does not have any resistance genes to that new disease, it could die. Immortality cannot be regarded as a game rule that prohibits an individual to die from any cause. $\endgroup$
    – Goodies
    Nov 15, 2021 at 14:19
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Yes absolutely. IF the trait is passed down genetically.

The extent of the stagnation would depend on how many of the offspring of the immortal themselves become immortal.

If the answer is zero, then the stagnation will be virtually undetectable , even under deep investigation.

Using human reproduction rates, and relatively normal sexual behavior from the immortal, we can expect less than one offspring per year if it is male, and less than one per 3 years if female.

After 10000 years, well under 1 percent of the human race will be genetically related to the immortal. Under 1/10th of a percent if the immortal is female. And this is for any relation, even 9999th cousin 9999 removed.

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Of course, if even 1 in 10 of the immortal's offspring is also immortal, then after 10000 years the species will be 90%+ immortal, and 99.many-nines% related to the original.

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Very little

One immortal, permanently fertile individual is genetically equivalent to a long line of heavily inbred relatives, without personally experiencing signs of inbreeding. Families that breed repeatedly with the immortal across generations may experience decreased genetic variability which in turn would probably be selected against due to recessive traits being exposed. If the immortal is somehow super-desirable, or the population very small, the overall effect would be similar to a genetic bottleneck, but it would have to be a pretty unusual situation requiring other restrictions to keep it in place (e.g. lack of influx of new genetic material from the outside of the group).

Important caveats

  • Immortal and permanently fertile are not the same. The human ovary, for example, performs all meiosis during embryonic development, and can make no more eggs after birth. So if your immortal is a human or humanoid woman (or other ovary-bearing gender applicable to your setting), she will have as many eggs as she was born with, and the immortalisation process won't change this. So she could have, at best, a relatively large number of children but not a continuous output across the ages.

  • Loss of senescence does not mean loss of genetic variation. Gametes work quite differently from somatic cells in the way they generate and, in controlled ways, encourage genetic variation. So, although the somatic cells of the immortal will be unchanged over time, each new meiosis will still produce a unique combination of alleles through recombination, and new mutations could appear through all sorts of mechanisms like defective transcription/recombination and DNA damage (if these mechanisms are not also modified by the immortalisation event). Quite a few genetic defects that we know of appear de novo, meaning neither parent had it.

  • Psychological aspects: if the immortality trait is not inherited, the immortal will have to watch all their children age and die. I can definitely see this putting you off having children in the long run, or at least space them out quite widely. If immortality is inherited, this is an entirely different can of worms, see PcMan's answer

  • Social aspects: is the immortal open about their status, or do they have to hide their immortality? If the former, then people would know that this surprisingly youthful looking person is in fact their grandfather and you really shouldn't have kids with your close relatives, no matter how incongruously hot and young they look. If they hide their immortality, chances are that they would not be able to keep up the charade in the small and isolated community where their genetic impact would be meaningful.

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  • $\begingroup$ I'd also have accepted this answer along with Haylen's if I could accept more than one answer. $\endgroup$
    – Lemming
    Nov 16, 2021 at 10:23
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First of all, the phrasing "evolutionary march" implies that evolution is a goal-oriented process, which it is not. Evolution does involve random drift, and removing old individuals does reduce that. But to the extent that there is a consistent "fitness", removing an individual just because it's old doesn't help that. Why would it? Why would an old individual dying make for better fitness than a young one dying? Evolution works by death being based on genes. Individuals dying from something other than their genes, such as age, doesn't help fitness. There is no "planned obsolescence" in evolution. If the individual has genes that are "more fit", then it staying around improves the average fitness of the species. If it has less fitness, then it will be outcompeted, and the fact that it's not being killed by old age won't stop that.

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  • $\begingroup$ A good example of this is the shark. Sharks have only marginally changed in the past 450 million years. They got a good set of traits that have worked very well through every epoch; so, there has never been a huge demand for them to genetically drift much. For them, the "evolutionary march" has been one generation after another selecting against change. $\endgroup$
    – Nosajimiki
    Nov 16, 2021 at 15:43
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TL;DR: There will be some, but it will depend a lot on the initial situation.

MWoT Answer:

Your ageless immortal is merely ageless. While that might eliminate some diseases that are caused by old age, it may not eliminate diseases by outside factors, nor will it prevent accidents, illness, and predation from taking their life. And make no mistake, something will kill them eventually.

We living things might be survival of the fittest in an individual sense, but evolution is all about survival of the good enough. It's about random mutations showing up and seeing if they breed true and help the species.

As a side note, I am making no assumptions as to species -- this should hold for anything from aardvarks to zebras. We, of course, are filed under H for human.

Initial Impact

The initial impact will depend on a few factors

  • Gender -- A female can only have one set of children at a time while a male can impregnate multiple females at a time
  • Initial population -- Them being 1 out of 100 individuals will have more of an impact than being 1 in 1,000,000.
  • Heritability -- How easy is it to inherit all or part of this individual's immortality
  • Environment -- How likely is this individual to die of non-aging related disease/illness, accident, or predation?

The inheritance mechanics will be the largest factor in this set. As stated, if everyone descended from this one individual gains the full immortality, then it will eventually become the dominant trait I would think. What effect that has will certainly be based on what the species in. Immortal bunnies will have a different impact than immortal humans or wolves. Remember, they can still starve to death should their numbers exceed their environment's capacity to support them and they cannot relocate to an area that can support them.

Inversely, if they don't inherit anything outwardly, then they aren't getting anything interesting unless another random mutation or atavism down the progenitor's line makes another individual immortal. In which case, the whole process starts again.

Another possibility is that inheriting part of the immortal's genes gives them part of the benefit, but not the whole package. Some might be immune to age-related diseases, while other gain an increased theoretical lifespan though little defense against age-related diseases. Breeding between the lineages might wind up with another fully immortal member of the species to again propagate the gifts to.

Future Impacts

As time goes on, your immortal will still have the urge to reproduce (as per the question) and will likely do so, either continuously siring children or having them. This means their presence in the gene pool will always be topped up, so to speak. This could definitely impact the diversity of the species. The impact will depend on how much the immortal's species manages to avoid inbreeding, especially close inbreeding.

But another thing your immortal has is experience. If they can teach their descendants, immediate or otherwise, a portion of their experience and knowledge, that will potentially give those youngsters an advantage in their life.

  • For a prey species, that could be ways to avoid predators better.
  • A predator species could hone its hunting tactics over the decades/centuries and pass those skills down.
  • For humans or other similarly intelligent species, that could be more in life hacks, connections or money.
  • Your immortal will also know the best ways to get a mate so should have no trouble reproducing should they want to -- another thing they can pass on if they desire their bloodline to continue.

What this could lead to is others trying to get the immortal to accept their children, even if they aren't the immortal's themselves. Depending on the intelligence of the species, this could lead to the immortal procreating less by being a surrogate parent for other's children, not like how aunts/uncles or grandparents can fulfill the role for us.

Intelligence Factors

These immortal's cells do not age out so in theory, they could be cultivated forever. With an appropriate sample kept, cell samples could be continuously gown and experimented on over the years, allowing for a constant medical reference. As the cell shouldn't be susceptible to cancers from bad divisions, the only issues with the cells should be introduced ones provided the initial sample was healthy. This could prove valuable for medical research.

While this does not affect evolutionary pressures in so many words, medical treatments will allow for some to reproduce that might not have been able to due to illness or disease.

Likewise, if the immortal's status is known, they might become a sought after commodity for unscrupulous humans to serve as genetic fodder in hopes for immortal children. Their drive to reproduce might be hampered by persistent people that will not take no for an answer.

As has been brought up, they might also be driven from a smaller settlement once their status is known as they will be "the other" at that point and the (possibly jealous) mortals may force them out, meaning the immortal will have to find a new place to live. Then again, the opposite might be true if the society reveres its elders.

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TL;DR: Evolution would eventually solve any potential problem by your population evolving not to reproduce with the immortal.

Let’s assume for the sake of the argument that the answer to your question is yes, i.e., the immortal would stagnate its species population.

Then, on the long run, the tendency to mate and reproduce with the immortal (and possibly its close descendants) would be a negative trait itself as the subpopulation having that trait cannot evolve. Thus a subpopulation would evolve that does not reproduce with the immortal and is immune to whatever negative population-genetic effects the presence of the immortal has – which contradicts our initial assumption. Whether this subpopulation forms a new species or takes over the entire population depends on the details.

Mind that there are several instances where traits evolved that do not have an direct positive effect, but that primarily effect genetic diversity and evolvability, e.g., the Westermarck effect which avoids incest, the evolution of sex, or the evolution of mutators in the E. coli long term evolution experiment (Lenski experiment).

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