Any ideas about the smallest possible human gene pool that is
I'm probably using the wrong terms, so somebody please correct me.
Here's what I mean:
Imagine that a very small group of humans is genetically isolated from
the rest of humanity for many generations. (European royalty, the
people on Pitcairn Island, and the Amish have provided real examples;
and science fiction writers have given us plenty more scenarios.)
What is the smallest possible size of the starting group which can
reasonably be expected to have genetically healthy descendants after
Of course, we could start with one pair. After repeated inbreeding,
the offspring would likely survive and continue reproducing
indefinitely, but certain health limitations would soon become
apparent. How many individuals would we require in the starting
group, in order to make it possible for this never to happen? (Let's
assume that the future generations involved will agree to reproduce
according to whatever "safe genes" scheme we come up with.)
(No, this isn't a homework assignment.)
If the population can go flush (i.e., grow catastrophically),
then the gene pool of two would be sufficient. If there was enough
room and food for a rapidly growing population, the recessive genes
can be selected out of the population without destroying it. What
would happen would be in a few generation health problems would become
VERY apparent. At some point, most of the population would die.
However, since the population is growing rapidly anyway, the newborns
would soon take up the slack.
Mutations would eventually occur in the population, and most of
these would also be selected out. However, a few mutations would be
beneficial. The population may end up diversifying.
However, the real question that you are probably asking is what
is the minimum gene pool necessary for the survival of a zero growth
population. If the population was both zero growth and narrow in gene
variety, the population could die. In a zero growth population which
is also highly inbred, a small statistical fluctuation could wipe
everyone with a good gene out. Then the population would die. A small
change in environment will kill off a zero growth population without
genetic diversity and without mutations.
I conjecture that the minimum size gene pool necessary for the
survival of a zero growth population depends on both the size of the
zero growth population, and the rate of small mutations. The larger
the zero growth population, the smaller the initial gene pool has to
be. The larger the mutation rate, the smaller the initial gene pool
has to be. The mutations will provide the necessary gene diversity.
The downside is that most mutations die each generation.
A large population with a large rate of mutation can survive no
matter how inbred the original population was. No matter how many get
wiped out, a small number of mutants will survive. The zero growth
population then becomes a catastrophically growing population and
comes back (see first paragraph).
At some point of size, mutations would help the population
survive. A large zero point population would always have a small
number of mutants who can adjust to the change in conditions. Most
mutants would die each generation, but a change in environment will
make some of them viable.
So when you ask the question again, please distinguish between a
zero growth population and a growing population.
I have heard this called "The Gilligan's Island Problem". I doubt
that would be a good search term, though.
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With increasing advances in modern genomics and technology you only
need one human to form a viable population.
Single base-pair editing of the genomic sequence is now possible.
Pluripotent cells are beginning to be produced. Couple the two
technologies and you can fix just about any mutation that you can
identify, produce as many individuals with the fix as you want, and
cull the mutations that you can't fix.
The individual to keep would likely be female. You might need human
eggs from time to time, but you no longer need a sperm donor.
If you didn't want to use biotech, my first post would apply, but with
humans the effective population could be less if artificial selection
were applied. Eugenics has bad connotations, but if you launched a
small population on a colony ship it would have to be practiced. It
might take generations to reach their destination. My guess is that
they would probably carry frozen embryos and germ plasm, but if you
only have a small population and you can't breed like rabbits to
achieve a rapid population expansion, your options are limited.
There were several papers on it in the late 1990's. I believe that
they called it genomic meltdown if mutation and drift of detrimental
polymorphisms outpaced the populations ability to reach mutation
selection balance. They were coming up with numbers like an effective
population size of 10,000 for an endangered species to avoid genomic
meltdown. The population was probably smaller if you just wanted a
50:50 chance of survival. The failure rate for populations is pretty
high, and this is only one factor in that failure rate. We only see
the survivors in extant species, over 99% aren't around anymore.
> So when you ask the question again, please distinguish between a
> zero growth population and a growing population.
Okay, ask again and clarify the problem. . . . . .
For the sake of clarity, I'll posit a science fiction scenario. This
is only for the sake of the discussion; it is not the main point.
Suppose that a small group of people, with N unrelated fertile members
in the group, both sexes, finds themselves stranded on . . . an
island, a postnuclear world, a space ship . . . whatever. They might
be rescued soon; their descendants might be found after many years,
they might die out, or they might just as well decide to start their
own civilization. (You can call this the "Noah's Ark" problem, I
The population can grow for a while, until it reaches the limits of
the environment; then the people need to attain some kind of
population equilibrium with that environment. That seems (to me)
neither to be flush growth nor ZPG, but rather one after the other.
Further, let's assume that the favorable mutation rate is negligible.
(Although, in many scenarios, we might find that the *total* mutation
rate could be catastrophic -- never mind, that's another story.)
Also, let's assume that the given population can completely identify
each individual's genetic makeup (but cannot induce genetic
modifications), and can prescribe appropriate pairings for the sake of
I've probably over-detailed the situation, but I'm trying to avoid
So, how many people should we send on our expedition? (I.e., how big
It seems to me that there are a number of interrelated factors
which must be considered in answering the question.
First and foremost are questions like:
1. How friendly or hostile is the environment?
2. How rapidly is the environment changing?
As Lorentz pointed out:
3. What is the reproductive rate do we allow?
As Ron Okimoto pointed out, with humans, there is the additional
4. How much knowledge and technology is available to them?
Still others are:
5. Is the environment open or closed?
6. What sort of culture do the people have?
7. Which selection of genes is found in the population?
8. What social and political culture do the people have?
9. Is there a dangerous homocidal maniac among them?
In the worst case, for example, all of the people are infected with
a severe and rapidly mutating pathogen, or there is one in their
environment. The weather is hostile and changes rapidly. The
surrounding ecology is changing rapidly. New invasive species
are coming in all the time. The people in the group are divided
into blocks that are at war with each other. Homocidal maniacs
have taken control of each block. Even a large population might
die pretty quickly.
In the best case perhaps, the people are on a totally self-supporting
space station with zero occurrence of pathogens or predators
attacking them, their food supply, or any of their life support systems.
The station itself is completely self-repairing, and there are no
collisions with asteroids or space debris. The people are well
educated and psychologically stable. A quite small population
might live for many, many generations.
If we have nailed down answers to all of the questions about
interrelated factors, we could come up with some hypotheses
about minimum population size under those specific conditions.
I don't know how we could test with humans, but we probably
could test very well with simpler, rapidly growing organisms like
yeast or bacteria or, better for higher organisms, fruit flies or
This has been explored in science fiction:
Rimmer has discovered that he can make a woman out of his DNA strands.
In a voice over, he debates if it would be right for them to have sex,
as technically, it would be his sister...
Rimmer: After much soul searching, I reluctantly decided: "What the hell,
I just wouldn't tell her." He smiles to himself, and shrugs.