How many people are needed to avoid inbreeding in a population?

11 August 2015

Question

How many people are needed to avoid inbreeding in a population?

Answer

Connie - Well, Louise. That question all sounds a little Adam and Eve. And it's also something the naked scientists have been wondering. You see, we recently bought a desert island in the Pacific, and we want to avoid any sticky situations in the future. So, how many people do we need to start with to keep our island healthy? When I asked you on Facebook and Twitter, Glenn Fisher thought that only one was a safe bet, whilst Jay Michael Antovics II thought that it might depend on what definition we use. Maybe Professor Mike Weale, a statistical geneticist from King's College London can help us.

Mike - So, inbreeding means different things to different people. So, there no one single answer to this question. Everyone is related to their partner somehow, it's just a question of how far one needs to go back in time before a common ancestor is found.

Connie - Oh wow! So wait a minute! Does that mean I'm technically related to my boyfriend?

Mike - Technically, yes! I mean, to stop all relatedness between all mating partners, you would need, in fact, an infinite number of people.

Connie - Okay. I see. But our island isn't going to be infinitely big, and more importantly, I'm not sure I can stomach the idea of being related to my boyfriend. And that must also mean that absolutely everybody is inbred, which just doesn't feel quite right. Surely, there's another way?

Mike - Well. Yes. There is. To a population geneticist, the definition of inbreeding is simply a situation where mating partners are more closely related than what's expected by chance. So, using this definition all one needs to do to avoid inbreeding is to select mating partners purely by chance, as though you were in the lottery. And then, the population can be as small as you want. Well, you need to have at least two. But in a small population, even one that was enjoying some hedonistic version of the national lottery, mating partners will unavoidably tend to be more closely related to each other.

Connie - And I suppose that can't be good for the future?

Mike - Yep. In the short term, this increases the chances of people suffering from certain types of genetic diseases, diseases such as cystic fibrosis or Tay-Sachs disease, for example. This is because these diseases are caused by inheriting a bad genetic variant both from one's mother and from one's father. And the chances of them both having the same bad genetic variant are increased if they are closely related to each other.

Connie - Okay. So, where does that leave us then?

Mike - Ultimately, there's no magic population threshold that will make this problem go away. But a study in 2002 suggests that a population of 160 onboard a so called generation spaceship travelling to the stars should be able to keep itself genetically healthy. So, this would be a reasonable guideline for your desert island. In fact, real human populations on islands in the Pacific have survived population crashes down to as few as 20 people, but I wouldn't recommend this as a way to keep your desert island either healthy or happy.

Connie - Well. There we have it. It all depends on your definition. I think to be on the safe side, I'm going with at least a few hundred. Who wants to come?

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