I’m very pleased have another query to answer from a listener to More or Less. And what a great question:
I am embarking on a Career as a James Bond baddie, and I want to make sure everything is very carefully planned. I am under no illusions that Commander Bond will thwart my first efforts to take over the world, however I am 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 I intend to populate an island or other planet, depending on budget. However I am not clear how many men and how many women I will need to take with me to ensure we do not have issues with inbreeding in the population creating genetic disorders and the like. How many people do you need to create a new race of people?
The first thing to say is that genetic disorders aren’t created by inbreeding. Inbreeding doesn’t even increase the frequency of genes responsible for genetic disorders (quite the opposite, in fact). What inbreeding does do is expose disorders that may have been previously masked from view.
In the typical case, otherwise healthy “carriers” inherit a defective piece of DNA from one parent but a working copy from the other parent. Close relatives are more likely than average to be carriers of the same disorder, and if two carriers breed, then their child has a half chance of getting a defective copy from one parent, multiplied by a half chance of also getting it from the other parent. Hence there is a quarter chance that both copies will be defective, potentially revealing the disease. But there is also a quarter chance that the child will be completely disease-free, not even a carrier. Recent genetic research using a fascinating inbreeding sect called the Hutterites reveals that an average human inherits just 0.29 seriously defective ‘recessive’ genes per parentaHence there is a 1 in 2 chance that a random person will have no serious recessive mutants, since (1-0.29)2 ≈ 0.5.. Multiplied by a quarter, this corresponds to only a small proportion of defective henchmen: I imagine this as a minimal and temporaryb“temporary”, because in the long term, inbreeding should actually purge the population of deleterious recessives, by exposing them to natural selection inconvenience to your colony.
In fact, I would go so far as to say that inbreeding itself isn’t much of an issue. That’s borne out by two “natural experiments”, often discussed by aspiring Bond villains on the internetcPleasingly, internet forums provide some reasonable answers and some fairly informed discussions too.. These are successful colonies founded on remote islands by small numbers of people. The breeding population on Pitcairn Island springs from 6 Polynesian women and 11 Caucasian men (9 from the mutiny on the Bounty, plus two later immigrants). The population on the island of Tristan da Cunha supposedly traces back to 7 females and 8 males who arrived gradually during the 19th century. Neither are perfect experiments, since their reproductive isolation probably wasn’t absolutedFor example the study by found at least one Y chromosome that must have come from off-island. Even so, mating with visitors to the islands was extremely rare. In neither population did inbreeding expose genetic disorders serious enough to jeopardise the colony, and so it seems reasonable to conclude that under 20 people can form a viable human population.
While I would argue that inbreeding per se isn’t an issue, what might well be a problem are “founder effects”, where small populations are dealt a bad hand, purely by chance. The classic examples of this are Tay-Sachs disease in Ashkenazi Jews, varigate porphyria in Boers, and hereditary deafness in Martha’s vineyard. Indeed, it is founder effects that are the source of a few low-level genetic problems on Tristan da Cunha, in particular asthma and glaucoma. In the case of asthma, simply by chance at least two or three of the 15 founding colonists happened to be asthma sufferers.
Today, 23% of people on Tristan da Cunha are asthmatic, with over 50% showing partial evidence of asthma. This illustrates another problem of small populations. The original “founder effects” can be exacerbated if the population stays small over extended periods of time. Asthma sufferers were in a minority at the start, but by chance they happen to leave more offspring than average (this is an example of what is called “genetic drift”eIn small populations, it can be said that genetic drift overwhelms natural selection, the strength of drift being proportional to 1 over the population size (1/Ne)). At worst, where populations stay small, even mildly deleterious genes can “drift to fixation”, with the normal, beneficial gene going extinct. After fixation, the situation is almost irreversible.
So what you really want to avoid is having a small breeding population over many generations. That should be less of a problem for a villain intent on another encounter with Commander Bond, but it does worry those planning centuries-long interstellar trips. “Research” into this problem tends to recommend minimum viable populations of hundreds of people . It is also of concern to conservationists who worry about trying to maintain healthy populations of wild organisms. They have a often criticised generalisation called the 50/500 rule: you need an effective population size of about 50 individuals for short term survival and 500 for long term survivalfNote that the effective population size (Ne) is a smaller number than the census population size. In fact, it is common for Ne to be one tenth of the actual number of countable individuals in a population. But this is a very approximate rule of thumb, originally based on laboratory populations of fruit flies, which may not reflect the details and circumstances of other populations and species.
In fact, for populations undergoing only a temporary bottleneck in population size, there are a many cases of successful animal (and plant) populations founded from fewer than 10 individuals. Five little spotted kiwis were used to establish a successful breeding population on the New Zealand island of Kapiti. Three female and one male American white-tailed deer managed to establish a large population in Finland, and a single breeding pair of wild sheep from a Paris zoo gave rise to a colony of hundreds on Haute Island in the Kerguelen archipelago, in the remote south of the Indian Ocean. All of these managed to preserve a reasonable amount of genetic variability (though most of the Haute Island sheep have now apparently been culled)
Humans can take advantage of advanced medical treatments, and the ability to modify their environment, so I would expect them to be much better at coping with mild genetic disorders than a pair of wild sheep. In fact, I strongly suspect you could found a successful colony with a single breeding pair of humans. Moreover, if you are really limited for space (and you happen to be a young female villain) then I would suggest escaping in a one-person escape pod simply taking sperm samples from a number of ethnically variable, youngg80-90% of the de novo mutations in humans are from sperm, and the older the male, the more mutations healthy males (although to avoid accidents such as dying in childbirth, you might also want to take a spare young female companion too)hYour effective population size is 4×Nm×Nf/(Nm+Nf), so if you have one female and sperm from ten males, your effective population size is Ne=3.6, whereas if you have two females and twenty vials it is Ne=7.3. Once you arrive, you should put all of your efforts into increasing your population size: perhaps aim for over 10 children per female? Of course, all that childcare might not leave much time to implement plans for world domination.
I should also point out a much greater problem, exemplified by the unfortunate Pitcairn islanders. They had serious long-term social issues: a high murder rate among the initial founding colonists, and an infamous bevy of convictions for sexual child abuse as recently as 2004. Your main problems are unlikely to be genetic, but cultural and psychological.
Notes [ + ]
|a.||↑||Hence there is a 1 in 2 chance that a random person will have no serious recessive mutants, since (1-0.29)2 ≈ 0.5.|
|b.||↑||“temporary”, because in the long term, inbreeding should actually purge the population of deleterious recessives, by exposing them to natural selection|
|c.||↑||Pleasingly, internet forums provide some reasonable answers and some fairly informed discussions too.|
|d.||↑||For example the study by found at least one Y chromosome that must have come from off-island|
|e.||↑||In small populations, it can be said that genetic drift overwhelms natural selection, the strength of drift being proportional to 1 over the population size (1/Ne)|
|f.||↑||Note that the effective population size (Ne) is a smaller number than the census population size. In fact, it is common for Ne to be one tenth of the actual number of countable individuals in a population|
|g.||↑||80-90% of the de novo mutations in humans are from sperm, and the older the male, the more mutations|
|h.||↑||Your effective population size is 4×Nm×Nf/(Nm+Nf), so if you have one female and sperm from ten males, your effective population size is Ne=3.6, whereas if you have two females and twenty vials it is Ne=7.3|