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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?
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.
The answer to this question depends upon a number of factors:
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.
If the immortal individual may die from other causes, then its death from those causes would remove its influence from subsequent generations.
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.
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. :)
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.
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.
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).
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.
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.
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.
The initial impact will depend on a few factors
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.
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.
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.
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.
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).
