Genetics of closed community.

Hello! This I my first post here so my apologies if my thread is in a wrong place or breaks any rules.

I have been trying to put together a story, but I need to work out even the smallest details about the world it concerns. And for that I need to ask about genetics. Namely, how small can one population be to avoid genetic defects that eventually happen due to close blood marriages?

I am very grateful for all the help I could get.

edited 3rd Mar '11 1:08:44 PM by Hermit

By either being large enough to provide enough genetic variety of its self or by bringing in fresh blood. Or simply by not having the gene variants for serious afflictions.

You can see it in closed communities today: The Amish, for example, are mostly inbred to some degree. It's because of that that they are also the group with the highest probability of having more toes and/or fingers then the standard 5.

If variant genes are present in the first few colonists, then those genes will come to expression at some point in time. That being said, the variant must still be there in the first place. And even if the variant is already there, then evolution will claim it's sacrifice amongst those that are afflicted, eventually eliminating the variant almost completely.

Think for a second: Once upon a time, all humans around the world were little more then a few separate groups of inbred families. That didn't stop us from getting where we are now. while inbreeding is far from ideal, it probably won't lead to major problems for a population as a whole.

Assuming that no sort of disease crops up that targets individuals with a specific gene make-up. Or huge climate changes. Or heck, any sort of great change in environment.

edited 3rd Mar '11 1:23:02 PM by Kayeka

Thank you very much for the answer.

Technically there is some immigration, but it is sporadic. What I'm most interested in is the minimum number. Somewhere I heard that the minimum could be 150 to keep the trouble away. That is without diseases or natural catastrophes. I add their influence later after figuring out how large the numbers are.

"Think for a second: Once upon a time, all humans around the world were little more then a few separate groups of inbred families. That didn't stop us from getting where we are now. while inbreeding is far from ideal, it probably won't lead to major problems for a population as a whole."

Thank you. I can assume then that it shouldn't pose too much of a threat then.

By "variant genes" is you mean defective/weak/problematic genes? Sorry, English terminology is a bit of a weak point for me.

Thank you for the answer. (Loved the toe comment.)

One fifty is a generally accepted Upper limit for tribal organization but it isn't good enough for genetic diversity. You can create pretty severe inbreeding in a tribe that size in pretty short order. To create a perpetually randomizing population requires a much higher number. I can't recall it at the moment but some niggling part of my mind say four thousand will create a genetically stable pool.

150 to 4000... Oh my. I were quite off in my understanding. XD Thank you very much for clearing that up, Shrimpus-san.

It's because my English terminology is rather weak that I choose to go with 'variant' genes. The more correct term, in retrospect, would be 'mutant'. Keep in mind that 'mutant' means 'anything not standard'. This could be good, bad, or completely indifferent.

And yes, you need an enormous amount of people for a perfectly randomizing community. A functional community shouldn't need quite as much, though.

I saw plenty of perfectly functional primitive tribes on TV, who didn't have more then 100 members, and little contact outside the tribe.

edited 3rd Mar '11 2:34:52 PM by Kayeka

Thank very much for your help, Kayeka-san. I got my questions answered.

Methinks the number to keep genetic diversity is at least 5000, roughly the size of a small tribe.

edited 3rd Mar '11 2:37:14 PM by SavageHeathen

You exist because we allow it and you will end because we demand it.

150 is the number I've heard used for avoiding immediate (as in, within the next few generations) issues. If you have that number, you can generally survive long enough for an infusion of fresh blood to come into play without having any permanent problems with the bloodline.

For long-term survival (ie, no fresh blood will ever be injected into the group), the numbers are larger but less certain. I've seen anything from 1,000 to 5,000 individuals. Evidence suggests that early humanity went through a population bottleneck of about 15,000 individuals at one point, and we still (tens of thousands of years later) have unusually low genetic diversity for a species.

Really from Jupiter, but not an alien.

From Ask Meta-filter:

"Assume for the sake of argument that whatever threat or change in the environment that has already reduced the species' population is reversed, so that the species has a chance to re-propagate itself. Also assume we're talking about a complex animal, not a paramecium. ...

"I did a bit of Googling and found a 1991 paper from the National Oceanic and Atmospheric that examines the population question specifically. It mentions a "50/500" rule:

The "50/500" rule of thumb initially advanced by Franklin (1980) and Soule (1980) comes the closest of any to attaining "magic number" status (Wilcox 1986). This rule prescribes a short-term effective population size (Ne) of 50 to prevent an unacceptable rate of inbreeding, and a long-term Ne of 500 to maintain overall genetic variability. The Ne=50 prescription (termed "the basic rule" by Soule 1980) corresponds to an inbreeding rate of 1% per generation, approximately half the maximum rate tolerated by domestic animal breeders (Franklin 1980). The Ne=500 prescription is an attempt to balance the rate of gain in genetic variation due to mutation with the rate of loss due to drift, and is based on a genetic study of bristles in Drosophila (Franklin 1980). ...

"HOWEVER... It's very important to understand that Effective population size and census population size aren't always the same. Variables such as breeding cycles, sex ratio, mating strategy, life span and historic population size all go in to calculating the effective population size. You can have a census size (number of actual individuals of the species) in the thousands and not have an effective population size > 500. ..."

Or, this thread, also on Ask Metafilter, which includes links to several papers about 3/4 of the way down.

In short — the numers that get tossed around vary hugely, but analysis of actual closed populations suggest that it could be much lower than expected — Pitcairn's Island was populated by a group of 15 men and 12 women of reproductive age (and a baby) and that population managed to establish a viable population and reach 3% population growth.

Or, Here's a breakdown of what happened on Pitcairn's Island:

Then there's Pitcairn's Island, destination of the mutineers of the Bounty, in 1789:

• 1789: It was originally fifteen men (nine mutineers and six kidnapped Tahitians), eleven women (kidnapped Tahitians), and a baby; through fighting, accidents, and illness, it pretty quickly became four men and nine women (and maybe the baby, and maybe some more children).

• By 1800 it was down to one man and nine women, plus children.

• By 1856, there were 194 people, which was enough that the island was too small, so they were all relocated to Norfolk Island.

• Over the next few years, 44 of them went back to Pitcairn.

• By 1937, the population of Pitcairn had reached 233.

edited 4th Mar '11 8:38:12 AM by Madrugada

...if you don’t love you’re dead, and if you do, they’ll kill you for it.

Keep in mind also that, not only would a very small population have problems with inbreeding, but it would also be extremely susceptible to genetic drift. A population of 150 or so might be enough to avoid immediate problems with inbreeding, but genetic diversity could decrease significantly simply by chance.

We're still not as bad as Snow Leopards; they're supposedly all connected to the same litter of cubs from around 10,000 years ago. I think Cheetahs are even more inbred than that.

That’s the epitome of privilege right there, not considering armed nazis a threat to your life. - Silasw

People could have gone through a severe bottleneck. We just don't know how severe or how long ago. mDNA (mitochondrial DNA, passed only from the mother) studies have indicted that virtually all people of European descent have one of 7 strains — that means that somewhere, way back, 7 women are the ultimate great-great grandmothers of virtually every European.

...if you don’t love you’re dead, and if you do, they’ll kill you for it.

The usual hypothesis is the Toba catastrophe, isn't it? Just a hypothesis, though.

[1] This facsimile operated in part by synAC.

All cheetahs can accept organ transplants from another cheetah without rejection — that's how genetically close they are.

A brighter future for a darker age.

: Or they have no mecanism for refusing "non-foreignlooking objects"

A guy called dvorak is tired. Tired of humanity not wanting to change to improve itself. Quite the sad tale.

Oh, it's worse than that. The Mitochondrial Eve concept ties everyone currently alive back to a single common (female) ancestor about 200,000 years ago. And on the male side, Y-chromosomal Adam only goes back around 60,000-90,000 years.

Really from Jupiter, but not an alien.