# Minimum number of individuals for a starter colonyDecember 26, 2009 12:48 AM   Subscribe

PopulationGeneticsFilter: You are designing a space-ship to bring X number of humans to a new planet to populate it. No more ships will be arriving. What is the minimum number for X to avoid excessive genetic abnormalities resulting from inbreeding? Bonus points for optimum male/female ratio!
posted by zachawry to Science & Nature (12 answers total) 7 users marked this as a favorite

You can do some digging around among the google results for minimum viable population size and effective population size. Most of the studies are related to animals and ecological concerns, but there's some human stuff in the mix.
posted by amyms at 1:03 AM on December 26, 2009

If you add "for humans" to "minimum viable population size" you get even better results, including this previous AskMetafilter question.
posted by amyms at 1:06 AM on December 26, 2009

This question (or a closely related question) has been tackled before by both Ask MetaFilter and Google Answers. More generally, you might research Minimum Viable Population (MVP). A nice discussion of MVP can be found at the Encyclopedia of Earth Minimum viable population size article.
posted by RichardP at 1:11 AM on December 26, 2009

For the sex ratio you want to maximise effective population size, to do this you maximise the equation 4*males*females/(males +females). This is almost always 50:50. To intuit this think what would happen if you had only 1 male - first generation would be ok, 2nd generation, would all have the same grandfather, same goes for 2,3 etc until you hit 50:50.

For number of individuals, the more the better. The case is abit more complex - You have founder effects (which a pre-migration screening could overcome) and various genetic load and stochastic effects like drift. You need to think about minimum viable population sizes, but these are notoriously wooly. As a complete guess, I would say a properly managed (ie you don't get to choose who you mate with) population would be stable at 100-1000 individuals.

DNA evidence suggests that Maori populated New Zealand with about 50-100 females (and unknown numbers of males) and expanded from there with very very rare new immigrants until European contact 1000 years later.
posted by scodger at 1:16 AM on December 26, 2009 [1 favorite]

Response by poster: Following the many interesting links presented here, I see that the answer is apparently between 20 and 100,000.
posted by zachawry at 2:18 AM on December 26, 2009 [2 favorites]

Avoiding the actual question but providing a practical solution:
Bring frozen sperm and eggs.
posted by sciencegeek at 4:31 AM on December 26, 2009

Optimum sex ratio?
Easy. Almost all females, and stock the space ship with donor sperm, and possibly eggs.

Spin the sperm so that it's primarily female for the initial generations on the ship.
posted by Elysum at 4:40 AM on December 26, 2009 [1 favorite]

The answer is going to be very imprecise. It depends on the effective population size (we have that worked out, mutation rate (more is worse), and what the distribution of fitness effects for new mutations is (if new mutations were very bad you would need more people to have enough normal ones at each generation).

Current estimates of the distribution of fitness effects in people are imprecise, and may be context dependent. That is, during our recent expansion people may not have been very picky, so fitness was kinda flat.
posted by a robot made out of meat at 7:17 AM on December 26, 2009

Without defining what you mean by "excessive" genetic abnormalities, the answer is two. Or, with the idea that Elysum points out, one plus some sperm.

But ignoring that:

According to an census done in the 1880s, there were 155 Rapanui left on Easter Island, including 68 men, 43 women, 17 boys under 15 years old, and 27 girls under 15 years old. There were also eleven foreigners on the island. Not all of the men and women were of breeding age, I'm sure. There are currently thousands of Rapanui.

So there's an upper bound for you, unless you define "excessive genetic abnormalities" to include the typical Rapanui.

Then there's , destination of the mutineers of the Bounty, in 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.

Since that time, there's been a lot of emigration, and the population is now about 50, most of whom are 100% descendants of the original population (the mutineers and kidnapped Tahitians).
posted by Flunkie at 9:18 AM on December 26, 2009

Ugggh, should have previewed. Sorry.
Then there's , destination of the mutineers of the Bounty, in 1789:
That's "Then there's Pitcairn Island, destination of...".
posted by Flunkie at 9:20 AM on December 26, 2009

One mitigating factor to consider is that genuine variety would be nearly as important as random quantity. So many genetic predispositions exist in so many mixed populations. Think Noah's Ark. You wouldn't want one of "everything you can think of" because what if unforeseen problems arise with, ah, samples 1-30? Plan for redundancy, but also for variety. Also plan for a combination of purity and hybridization.

I've already mentioned Joe Haldeman's Forever War once today, my first day of posting, but I'll second it as a read here. In the vast far-flung future, the intermingled human species may find use for "pure" "ancient" DNA in case something unforeseen goes wrong with its current DNA.

Anyway, I've read fiction citing anywhere from a few hundred to tens of thousands.
posted by carlh at 1:56 PM on December 27, 2009

*shudders*
Ok, can we not suggest the Pitcairn Islands as a healthy model?
Don't know how it's doing genetically, except that they have had new people over time, but culturally?
Any place that has had possibly most of the men raping and sexually abusing the girls and women on the island from a young age is... not doing fantastically.
posted by Elysum at 2:48 AM on December 28, 2009

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