Ok, following a conversation about Noah's Ark (we aren't religious though) we figure you couldn't just have 2 of each animal without creating problems in a species due to genetic defaults and the whole interbreeding issue.
So, if some people were stranded on a desert island, how many would it take to continue the population without interbreeding and incest?
If couple X have a child (X1) and so does couple Y (Y1) then in theory child X and child Y can reproduce. This creates child Z1. If couples X and Y have another child, this would be Z2. Great,the beginning of a new generation, except children Z1 and ZA1 would be beginning minor interbreeding as cousins.

X+X = X1 and X+X = X2
Y+Y=Y1 and Y+Y=Y2

X1+Y1=Z1 (and subsequently Z2, 3 and 4 etc)
X2+Y2=ZA1 and subsequently ZA2, 3 and 4 etc)

Are you with me so far?

So surely we need to introduce at least one more couple B. And assuming each first child is male and each second is female....
Gen 1
B+B = B1, B2 etc
X+X = X1, X2 etc
Y+Y=Y1, Y2 etc

Gen 2
B1 +X2 = BX1 and perhaps BX2
X1 +B2 = XB1 and perhaps XB2
B1+Y2 = BY1and perhaps BY2
Y1 + X2 = YX1and perhaps YX2
Y1 + B2 = YB1and perhaps YB2
X1+ Y2 = XY1and perhaps XY2

So now BX1 and BY2 cannot reproduce as they share the same father etc so this amounts to 4 remaining possible matche (I think) for them to reproduce with.
So, by my reckoning, you need 3 fertile couple to have at least 2 children each, one of each sex, in order to repopulate.
Am I right?

This is a lovely idea, but you have to factor in that all species began with only a couple of individuals, and the resulting inbreeding was what speeded up the mutations necessary for the creation of a new species.

Zoologists usually work on a much larger population than 3 unrelated breeding couples to avoid inbreeding, especially in species that have a dominant male system such as lions, where one male can father successive generations, but species have been kept alive in captivity with such small numbers.

So- you could have continuation with only 2 of each species, but what you come out with would in all likelihood be a different subspecies (or a totally different species) than you started off with. Of course, there would be genetic inbreeding problems, and this would mean that you would probably have a higher perinatal mortaility rate than in the background community, but the surviving progeny might have developed truly useful mutations.

By the way, are we assuming that the list of animals that went on to Noah's Ark are the same species that we know today? I'm also not religious, so I'm hoping that I'm not committing any form of blasphemy with my answer, but if it were truly to happen that there were only 2 of each species I would expect the appearance of the resulting animals to have varied enormously as a result of the spate of mutations following this.

Hmmm.
Why did all species begin with just a couple of individuals?
Surely this would depend on your belief in evolution? Possibly not, this area isn't my forte at all, just a train of thought I'm trying to puruse.
Can see your point and agree about the genetic mutations part and Noahs Ark. However, since I feel Noahs Ark is mythological I don't hold it as likely. (Sorry if that offends anyone) although the possible ramifications are interesting.
This really does provoke lots of interesting thoughts.
EDIT: maybe there's two questions here, some animals (gerbils in my experience) don't seem to suffer as a result of inbreeding. So perhaps one question is about mammal procreation and one is about human procreation.

I'm no authority on this but on QI the other day they said that the Bible stated that the animals are seperated into clean and dirty animals.

The clean animals such as a sheep went in groups of 7.
Whilst the dirty animals such as a pig went in as 2.

Don't shoot me down if I got my facts wrong this is what I heard.

One day I'm going to read it cover to back.

Interesting, I found this based on your reply Modesty:
http://www.answersingenesis.org/creation/v19/i2/animals.asp
and which is further defined in:
http://bible.gospelcom.net/passage/?search=lev%2011&version=50;

Originally posted by Zebra
Interesting, I found this based on your reply Modesty:
http://www.answersingenesis.org/creation/v19/i2/animals.asp
and which is further defined in:
http://bible.gospelcom.net/passage/?search=lev%2011&version=50;

Now my head is really spinning, think I'll go and watch some Uk Gold.

Of course we know the best things come in pairs, right Zebra:thumbsup:

Hahaha, too right!
Just wish they'd hurry up!

Here's a product of an MSN chat with a brainiac
http://www.thetech.org/genetics/ask.php?id=113

Originally posted by Zebra
Hmmm.
Why did all species begin with just a couple of individuals?

If you favour the evolutionist model (and I do) it's more correct to state that all species begin with one individual being born with a random set of mutations that differentiate it from others of its species (one fish evolves fins strong enough to support it to 'walk' out of the water, and we have the first of the land animals).

However, we have no idea how many times this has to happen as a genetic accident before something sufficiently similar happens to another fish,

and for them to occur close enough to each other that they can select each other and breed,

and for these mutations to carry forward to future progeny (odds of only 1 in 4 if the mutation is recessive),

and for these mutations to not also carry a lethal mutation that means that they are at a disadvantage in hunting or whatever and therefore die out,

and probably a million and one other factors that I haven't thought of.

Only after all of these things happen in the right order do we get a new species. Officially to be classified as a species there must be a means of reproducing a new generation, and that (with some exceptions) needs 2 individuals of opposing sexes.

Ok, having taken that on board, would you agree that:
6 people, selected for their genetic strengths and lack of weaknesses and without any external factors such as disease contamination and of reasonable longevity, should be able to reproduce A) the human race or B) a new strain of the human race?
This is assuming that incest is entirely undesirable, therefore 2 people are insufficient as, I think, would 4.
If the external environment is appropriate, there are no fertility issues and no genetically dominant issues to overcome then the equation of 6 people should work. Although, according to the link I highlighted earlier, 160 people is the ideal to escape the weaknesses.

I think that 6 suitably motivated and mutually attracted* humans could have a good go at it, yes. I couldn't guarantee the outcome though, and we don't yet know enough about the human genome and its intricacies to select for a lack of dangerous mutations faultlessly, or to know all of the combinations that could result from pairings. There are simply too many minute but desperately important deletion mutations that we have no idea of the effects. Current research has shown that over 60 mutations could be involved in cyctic fibrosis, but we don't know how or why.

If we give the hypothetical experiment with 6 humans a couple of hundred years, then I think there would be a very good chance that there would still be life at the end of it- but I don't gamble, so maybe that's just a cop out.

*but hopefully able to stay monogamous. Think of the confusion with trying to work out what constituted incest if the children shared parentage! A whole new can of worms.

That's good, the 6 people maths took me a while, glad there's some possibility behind it.
So, how come you know so much about all this?

My degree (in biochemistry) also included units in developmental biology, genetics and molecular biology (DNA study), but I don't get to use it often these days, except to answer the incoming 'so how does lack of vitamin c give you scurvy?' questions when people know what I studied.

These days I'm a volunteer teacher (of basic computer skills to adults) and I do mixed stuff at the Cats Shelter, so the chance for a stimulating discussion to keep you distracted while you're getting big and uncomfortable is a good one for me.