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How much genetic diversity would we need to found a colony on another planet?

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Whether it’s SpaceX or Lockheed Martin or NASA, eventually someone’s going to start sending people to Mars. And when that happens, we might decide to set up a colony there, too.

People could be born, raised, have their own families, and die without ever setting foot on Earth. Sending people across the void of space to a desolate planet is hard enough, but founding a remote colony comes with its own special challenge: low genetic diversity. If we’re going to create new societies on other planets, we need to know how many people to send to avoid the problems that come from inbreeding.

Whenever a subset of people goes off and starts their own society, they’re going to be less genetically diverse than the much bigger population they came from. That loss in diversity is known as the Founder Effect. The differences can be amplified over generations, especially if the new population is small.

The smaller the population, the harder it is to find someone to have babies with that you’re not related to, so you end up with a lot of inbreeding. It all depends on which founders produce how many offspring, and which alleles, or variations of a particular gene, they happen to pass on. Some traits can completely disappear from the gene pool, and rare diseases can become super common.

That’s happened before. The Amish in Pennsylvania, for example, are much more likely than other Americans to have Ellis-van Creveld syndrome, a type of dwarfism, because one of their founders carried the allele for it. And the South African Afrikaner community, which was started by a small group of predominantly Dutch settlers in the 17th century, has a higher frequency of Huntington’s disease.

To completely avoid the consequences of inbreeding, a founding group needs to have enough people so that, genetically speaking, it’s basically as diverse as the original group. And it has to be able maintain that diversity over time. In population genetics theory, that magic number is known as the effective population size.

It’s a really broad estimate that varies by species, and is based on things like how often random genetic mutations happen, which members of the population mate, and how many kids everyone has. For humans, the effective population size is thought to be at least several thousand, if not 10-20 thousand. But the more people we have to send to start our space colony, the harder and more expensive it’ll be.

So scientists have tried to calculate the bare minimum number of people we’d need, and they’ve come up with a few different estimates — along with a couple of other strategies that could help. In 2002, an anthropologist named John Moore tried to calculate the population you’d need for a 200-year interstellar mission to colonize a hypothetical Earth 2.0. Using a computer program based on the reproductive behavior of actual pre-industrial communities, he simulated different scenarios to see what would happen to the population over time.

He found that, if you choose pairings and make other reproductive decisions really carefully, you’d only need 75-90 young couples to start a colony. So, less than 200 people in total. But other anthropologists have argued that such a small number would only work if their descendants were able to make babies with new humans at the end of their journey.

Which could happen—I mean, assuming Earth hasn’t suffered some sort of armageddon, there’s no reason we couldn’t send another mission to Mars in a century or two, if not much sooner. Or the colonizers could pack a collection of frozen sperm and eggs from people who never actually leave Earth, allowing them to mix in new genes over time. Another issue with Moore’s estimate is that it’s only enough under the most ideal conditions.

For one thing, it doesn’t leave any wiggle room for disasters that kill off a significant percentage of the population. It also doesn’t take into account the fact that small pre-industrial groups occasionally interbred with neighboring communities. That means the actual number of people you’d need to maintain a healthy population might be much higher.

In a 2014 paper published in Acta Astronautica, one researcher estimated that you’d really need somewhere between 14,000 and 44,000 people. If we followed that advice, our Martian colony might be sort of like Iceland. Iceland was founded around 1100 years ago by about 10,000 people.

Thanks to the Founder Effect, the country’s modern population of about 300,000 is one of the least genetically diverse in all of Europe. So Icelanders keep meticulous genealogical records. There’s even an online genetic database that acts like one big family tree, with information on 95% of people born in the past 3 centuries.

If you’re in it, you can see how anyone else in the database is related to you. And it’s more than just an incest-prevention tool — the database has also helped identify genes associated with certain diseases, like Alzheimer’s and cancer. So, thanks, Iceland.

If we want to colonize another planet, a similar database to keep track of things would probably come in handy. And so might those sperm and egg vials. But before we worry about that, we should probably focus our efforts on sending any number of people to Mars safely.

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