WS MoreOrLess: Kidney Donation
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This is the short edition of More or Less, first broadcast on the BBC World Service.
Hello, this is More or Less on the BBC World Service, and I'm Ruth Alexander. With the deaths of the actors Geoffrey Holder and Richard Keel in recent weeks, the world has lost two of the great villains from the James Bond films. We considered them irreplaceable. But then we had this email from a loyal listener.
I'm embarking on a career as a James Bond baddie, and I want to make sure everything is carefully planned. I'm under no illusions that Commander Bond will thwart my first efforts to take over the world, but I'm keen to become a recurring character, and that's where you come in. I intend to escape from Mr Bond at the last minute and populate an island or other planet, depending on budget. However, I'm not clear how many men and how many women I'll need to take with me to ensure we don't have any issues with inbreeding in the population that might cause genetic disorders and the like. So my question is, how many people do you need to create a new race of people? If you're able to help out, I'd be happy to offer you a job as either a henchman or a victim, depending on your inclination.
Thanks. Ben Moody. I know, I'm working on the name.
So who should we get to answer that? Well, we thought an evolutionary biologist we know. His name is Yan. Yan Wang.
Well, it's not that inbreeding creates genetic disorders. What it is, is that inbreeding exposes genetic disorders that are already present in the population. And in fact, if you do inbreeding, because it exposes it, that actually reduces the amount overall in the population eventually. But as for the actual numbers, you might have thought that it was quite large, say in the hundreds, because people who are interested in interstellar, intergalactic travel have done calculations and they come up with hundreds of people and ecologists have this... rule called the 50-500 rule, which says that for short-term viability of a population, you need about 50 individuals. In fact, that's the effective population size, so probably more like several hundred.
And for long-term maintenance, you need about 500 individuals. So the real number of individuals is probably in the thousands. I think that's slightly misguided, actually, because it's not the long-term population size you need to worry about. It's just the number of founders of your colony that you take with you on your little escape pod. There are examples from other animals where really quite small numbers of individuals have founded successful populations. There are deer in Finland, three females and one male, and there's even a single breeding pair of wild sheep here. that were introduced from a Paris zoo to Oort Island in the Curligan Archipelago. In terms of humans, there are two examples that people often come up with, and that's Pitcairn Island, which famously was founded by people from the Mutiny on the Bounty.
The breeding population there now springs from six Polynesian women and 11 Caucasian men. Nine of those were from the bounty and two arrived later. And if you look at the other example, Tristan da Cunha, it seems that you can trace everyone back to seven females and eight males.
Could a tiny group really found a new human population without inbreeding problems?
These aren't perfect experiments because there probably were the occasional people visiting and immigrating, but under 20 seems like a reasonable conclusion for founding a human population. The problems, from a genetic point of view, aren't really inbreeding. They're more to do with just having a small population size. And you get a thing called the founder effect, which means that if you have very few individuals, it might just be that you're dealt a bad hand at the start. For example, on Tristan da Cunha, there's a very high asthma rate. Two or possibly three of the original 15 founders happen to have relatively severe asthma and now the population is sort of dominated by this asthma problem. A second problem is if you keep the population small for long periods of time it's like repeatedly sampling and so now
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