How can I make a net beneficial genetic trait occur only in a small fraction of the population, sustained?
For a species I am working on, there is a specific genetic trait that I want to introduce into the population. The exact nature of either is not important for the purposes of this question, but the trait is of such a nature that it is visible in the individual's phenotype to others of the species and it provides the individual with a pretty good advantage in certain situations without coming at too great a cost.
Normally, such a trait would spread readily throughout the population as it provides a net advantage and it is clear to potential mates whether a particular individual has it or not, leading to most individuals having it after some amount of time. However, in my world I want this trait to:
- pop up (at least seemingly) randomly; even if both parents show this trait their offspring may not, and even if neither parent shows the trait their offspring might, so it cannot be a simple dominant or recessive inheritance model (maybe polygenic?)
- occur in something like 0.01% to 0.1% of the population sustained over time (this is the hard part...)
- while showing some variance in the degree through which it shows in the phenotype in the individuals that do have it, be a marked difference between those individuals that have it and those that do not
- exist, and have similar effect, in both males and females
Without invoking magic or phlebotinum, can I make that work? How?
Assume Earth-like biology and evolutionary processes.
4 answers
Random and rare: you want it to be an advantage to the species only at those low numbers.
Once you have that, evolution will have made the connection to the phenomena that trigger/regulate the process, no matter how ridiculous the connection may seem to us.
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The gene is present in most of the population, but manifests rarely, AND causes problems if it manifests too densely (preventing selection for increased manifestation chance):
- social problems - possibly instinct-based, or just because of tradition. It doesn't do to have too many powerful leaders, and there might be a reason (like long-distance telepathy) not to let defeated rivals live.
- renewable resource usage that is not a problem otherwise. Maybe even affects 'normal' beings - with harm/threat magnitude growing exponentially with the number of uplifted on the continent. No problem if regulated, though.
- variant of resource usage: costly to attempt producing such an individual (stimuli are rare, but renewable, e.g. peculiar astronomical/atmospheric/geological phenomena, unpredictable and expensive to reproduce - sounds like magic, but happens in nature)
- the presence of one uplifted individual suppresses the growth of any who are weaker (resource hogging, pheromones, state of mind).
- manifesting the trait makes it less likely to be active in offspring
- too great a density would bring malady, harming the targets and anyone nearby (descendants). Maybe affected individuals are more likely to attract the attention of something undesirable.
- the trait lies dormant in many people, but most choose not to activate it (religious reasons? expectation of a different lifestyle? Suppression of mating instinct?), or don't get a chance to (traumatic experience + downtime needed?)
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Have the trait sustained in another species that have a symbiotic relationship with the Jed... ugh. The capability to uplift a host is useful to have, but the beneficent's evolutionary goals may prefer infrequent usage.
- example: gut bacteria for whom hosts displaying the phenotype in even the least degree are less... hospitable. The bacteria can function, but outside of an already-changed host, the necessary adaptation is a burden, so they are inferior to the rest of their species. This keeps them defaulting to 'normal' mode, but the host species, overall, benefits from that occasional quirk, and indirectly so does the symbiote.
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"without coming at too great a cost" to the adult individual, I assume. The parents are a different matter, as is the infant.
- sterility/sickness afterwards
- high risk of failure (no effect, death, or opposite effect)
- the child is indistinguishable from a sickly runt until the alternative growth starts
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If it is caused by a combination of several otherwise unrelated dominant traits each of which in separation also causes a slight decrease in fertility or increase in mortality.
March Ho had an interesting idea about heterozygous phenotypes. This means that the gene needs two different alleles to have the trait. The benefit is that only half the children will have the phenotype, which prevents creation of pure-bred lineages, which using just dominant traits would allow. Downside is that half the children would have the trait, which would be noticeable by simple observation, after which new children with the trait could be produced by simply sacrificing the other half.
However, by combining this with my idea of requiring multiple genes in different chromosomes and making "incomplete versions" slightly negative... You'll end up with something that can't really be bred for.
Also your compound trait is rare enough that it is unlikely to make its component parts that much more common as long as there is enough of them and some of them are negative.
Something like that would probably be sensitive to mental state and health.
Human generations are fairly long, so as long as the balance of probabilities and selection pressures is roughly correct, nobody would notice slight variations.
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We can look to sickle-cell anaemia for an example.
Three quarters of sickle-cell cases occur in Africa. A recent WHO report estimated that around 2% of newborns in Nigeria were affected by sickle cell anaemia, giving a total of 150,000 affected children born every year in Nigeria alone. The carrier frequency ranges between 10% and 40% across equatorial Africa, decreasing to 1"“2% on the north African coast and <1% in South Africa. There have been studies in Africa that show a significant decrease in infant mortality rate, ages 2"“16 months, because of the sickle-cell trait. This happened in areas that were known to be predominant areas of malarial cases
The heterozygous phenotype can be the "beneficial" phenotype, and the homozygous recessive phenotype can be the deleterious phenotype. As long as there is a fitness balance between the two, the selection pressure on the allele as a whole should be balanced, and allow there to be a constant amount of people with the beneficial phenotype. As long as the population is sufficiently large, the gene is unlikely to become extinct.
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- Your right, it is probably a multi-gene combo
2-3 to keep it to a small % the visible variance could be a large piece of the puzzle. The visible items could be very unappealing for sexual partners, or even more simple that it also is paired with a reduced ability to procreate.
4b. I don't see why there would have to be any difference between in effect for male vs. female. The procreation can be affected on both sides. There could also be something that makes the pregnancy more likely to fail. Maybe they also always are born early, often too early.
4a. Or the pregnancy could cause some other issue that might often result in the mother dying. This would keep them with the visible characteristics more of a pariah, if the mothers often die then the women will be less likely to want children and the men will be less desirable as a partner if their kids will kill their mothers.
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