I am watching the new Battlestar Galactica, and it got me to
wondering: what is the absolute smallest human population that has
sufficient genetic diversity to be viable in the long term? And how
much larger does it have to be before there's no need to explicitly
manage for diversity? I mean right now, in our countries of millions,
no-one really needs to keep track to ensure that inbreeding doesn't
threaten our long term survival, it just happens automagically, as
enough people move away from where they were born to faraway cities
and find partners there.

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Request for Question Clarification by pafalafa-ga on 17 Jul 2005 20:31 PDT

I can't think of any biological reason why a population of two -- one
man, one woman -- couldn't produce a viable population.

You're right in supposing that a lack of genetic diversity can be
costly in an inbreeding population.  However, that cost doesn't
necessarily doom the population, and the spontaneous diversity that
arises from natural mutations can offset the lack of original
diversity.

Heck, it worked for Eve and Adam.

Of course, a two person community can easily die out very quickly. 
But there's no biological imperative that insists this must happen. 
The two person community could also grow and thrive over time.

Do you have any reason to suppose that a lack of genetic diversity
would be deadly to a human population?

Let me know what you think.

pafalafa-ga

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Clarification of Question by ghammond-ga on 16 Aug 2005 02:19 PDT

Hi pafalafa-ga,

I don't have a specific reason, but it just seems intuitively that it
wouldn't work with a population smaller than a certain size unless a)
everyone was in great genetic shape to begin with and b) something was
done to deliberately manage diversity, to track who was related to who
and how and to bias reproduction towards the most distantly related
couples. Starting with just two, if one had any sort of genetic
defect, wouldn't it become amplified throughout generations until it
became catastrophic (either because the individuals were unable to
function, or because the healthy individuals had to spend so much time
caring for the disabled that they were unable to do other tasks needed
to maintain a colony).

Say NASA or ESA or whoever decided to colonize a planet. It was to be
a one-shot mission, no further colonists could be sent. Obviously the
crew would need to be large enough to have all the specialist skills
and enough redundancy that essential skills would not be lost if a few
died but ignoring that for the moment, and assuming the crew was
entirely made up of couples who wanted children, what is the smallest
crew that could do it and have a population no less genetically
healthy that a typical present day Western population say a hundred
years later? Or a thousand?

Thanks,

G

ghammond,
Yours is a very interesting question.  I immediately thought of the
Pitcairn Islanders, originally only 6 British sailers, 2 Tahitian men
and 8 women.
(Numbers from one of the sites.  I thought they were more.)

The last of the sites below may be a good starting point.  The others
discuss the subject from different points of view.
There is one  - with links to related articles - that mentions cases
of sexual abuse among the small population.
I included it to propose totally inappropriate question:  if it could
be demonstrated that the choice of young girls who were misused agreed
with a genetic analysis of the population that indicated that just
that man and that girl were a couple with the least inbreeding.

I am sure this comment is not an answer to your question, but hope
that you will find the sites to be useful.

Myoarin

http://jom-emit.cfpm.org/2004/vol8/gatherer_d.html

http://www.answersingenesis.org/docs/4055.asp?vPrint=1

http://www.pbs.org/wgbh/nova/sciencenow/3209/01-diamond.html

http://millennium-debate.org/ind3july4.htm

http://news.bbc.co.uk/2/hi/programmes/from_our_own_correspondent/2990726.stm

http://www.100megsfree4.com/farshores/tp_east.htm

http://www.tamu.edu/classes/plan/magill/gene310/popgen/H-W_equil.html

http://www.tamu.edu/classes/plan/magill/gene310/PDF%20files/popgen.pdf

This last site discusses the genetic problems in a way that a
layperson may be able to understand.

Some of the info here may be of interest to you:

http://answers.google.com/answers/threadview?id=378894

In pre-internet days I remember reading something about Indian
uncle-niece procreation being both (historically) common and with no
measureable problems.

Provided the small population implemented eugenics (or more accurately
culling unsatisfactory sports) it seems likely that one could maintain
and increase a population from very few members - subject to a
plentiful supply of food and minimal external risks.

As an aside I've often wondered about the sacrifice of the 'first
born', Molloch, Herod et al.

I've got a theory that it might be an ancient survival mechanism.

It is very likely that the 'inbred cretin' syndrome found in some
areas is because the more successful specimems walked away to pastures
new, leaving the failed 'sports' to carry on with each other -
de-selective breeding.

Personally I have noticed a distinct preference for genetic diversity,
both in myself and in my friends.

in inbred population more than the lack of genetic diversity by it
self, the real problem is the occurence of recessive diseases.
a patient with a recessive disease receive a defective gene version
from both his parents in contrary to dominant disease.
"autosomal recessive defect - a disease caused by the presence of two
recessive mutant genes on an autosome"
the typical example is in pakistan where inbreeding is high and where
the sudies on large scale of recessive disease is possible