MadSci Network: Genetics

Re: How many people would it take to form a viable colony?

Date: Sun Jun 10 21:34:44 2001
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Children's Hospital Oakland Research Institute
Area of science: Genetics
ID: 991680830.Ge

Thanks for the interesting question Denise.

For me, the answer to your question really boils down to what you mean by the term "viable". People will argue the meaning of this term, but lets say that a viable population is one which is capable of expanding to the limitations of its environment, whatever those limitations might be. So, in the case of your deep space colony, the viability of the colony would be defined by the environs of the new site. If the new site is the controlled environment of a synthetic habitat, the measure of viability will be very different from what it would be if the site were a new planet. We will return to this issue at the end of this discussion.

Given this view of viability, it seems to me that the issue that will determine the viability of the colony will be the effect of inbreeding among the descendants of the colonists. Inbreeding has the potential to , dramatically increase the frequency of genetic diseases and defects by giving rise to individuals who are homozygous for recessive alleles which might exist in the population. You might expect that all of the potential colonists on the expedition would be rigorously screened for potentially dangerous dominant and recessive alleles before the complete set of colonists was chosen. In this case, the potential for inbreeding to have dangerous ramifications is much lower than it would be if the colonists were chosen randomly from the general population. All of this is a way of saying that the answer to your question really depends on the genetic nature of the founders of the colony.

As an example, consider the Cheetah. There are thought to be somewhere between 10,000 and 25,000 Cheetahs alive today. While there used to be different species of Cheetah all over the world, most of them died out about 10,000 years ago, and modern Cheetahs are considered to be endangered ; their habitats are being destroyed and they have difficulty breeding in captivity. In addition, all, Cheetahs alive today are almost genetically identical, as if they were all identical twins, and they all share a set of asymmetrical skull deformities due to their genetic identity. In fact, it seems possible that all 10,000-25,000 Cheetahs alive today are the descendants of a single pregnant female who survived the extinction of all other Cheetahs about 10,000 years ago. Now, even if this scenario is not exactly correct, the genetic evidence certainly indicates that all modern Cheetahs are descended from a small number of closely related individuals. So, this is a case where you had a founding population of closely related individuals, and their descendants managed to expand their range considerably.

Let's consider an example from human history. The best example of a small founding population that I can think of is the population of the island of Tristan de Cunha. This southern Atlantic island was colonized in 1814 by 15 people (not all of whom were adults). This group was joined over the next several decades by sailors from 17 shipwrecks as well as immigrants, mostly from South Africa. By 1827, the population numbered 24, with 7 men, 6 women, and 11 children. By 1856, the population had grown to include 71 people, but then crashed the following year to 28, due to starvation. In 1887, the population was 107. In this year, 15 out of the 19 adult men on the island died at sea when their boat sank. This remaining population of 92 consisted of four elderly men and a number of women and children, and by 1890, 34 of them had left the island for South Africa. In 1987, the population numbered 296 people. The modern islanders share 7 family names, and DNA studies have shown that they are descended from five female founders, suggesting that the modern population is descended from at least 12 individuals.

Everyone on Tristan de Cunha is related. In fact, due to extensive inbreeding over the last 200 years, any pair of islanders are as closely related as first cousins. Since the colonists were chosen in an essentially random fashion, we can expect that there are some deleterious alleles in the population. Asthma is quite frequent in this population; 55% of the islanders seem to have asthma. Pedigree studies and genetic studies indicate that asthma was brought to the island by three colonists who arrived in 1827, two sisters and the daughter of one of these women. Similarly, one of the 15 original colonists was a carrier of the recessive allele for the disease known as for retinitis pigmentosa, a form of blindness. In the modern population, 4 individuals have the disease, and at least 9 others are carriers of the recessive allele. The frequency of these diseases in Tristan de Cuhna islanders is much higher than they were in the original population the colonists were drawn from because of the degree of inbreeding made necessary by the small number of founders.

So, lets take a closer look at your questions:

(1) If I wanted to send an expedition into deep space to establish a colony, how many unrelated, fertile mating pairs (from the same ethnic background) would be needed to form the basis of a viable colony, assuming that they would never have contact with other humans again?

As I suggested above, the answer to this question depends on your initial colonists. If they are lucky, like Cheetahs were, they could get away with a very small number of founders. In this case, luck could be enhanced by genetic screening for individuals with a minimal number of dangerous recessive alleles. On average, each of us is supposed to have on the order of 10-100 defective genes out of the 30,000 or so in the genome. However, careful screening of the entire human population, or perhaps a selective breeding program, could possibly come up with a set of individuals with a smaller number. If you look at the islanders of Tristan de Cunha, you can see that success is possible with a small number (on the order of 10 each) of founding males and females, even with disease-causing genes in the population. Similarly, the Polynesian island of New Zealand seems to have had between 50-100 founding females, with a corresponding number of males, and populations in eastern Polyensia may have been founded by even smaller numbers of people.

(2) If the colonist practiced polygamy, with each person having three spouses, how many colonists would be needed then? The assumption here is that no couple would be allowed to mate if they had a common ancestor within three generations.

The number of spouses is largely irrelevant in this example. Polygamy will not compensate for a low genetic diversity in the colonists (or in any subsequent generation), although it could 'speed up' the process of genetic mixing that will occur over subsequent generations. What is probably more important is the number of children that each colonist has. Since there will be no further contact from Earth, the things that the colony will want to avoid are a loss of genetic diversity, either through catastrophe or through genetic drift, and inbreeding between individuals with the same recessive alleles. Since a parent contributes 50% of their genes to each of their children, the chance of a given gene being passed on to the next generation increases with each child that parent has.

In order to maximize the genetic potential of the colony, the colonists might not want to leave things to chance by selecting mates in the old fashioned, random way, or by leaving the genetic makeup of their offspring to chance. Instead of making this decision based on pedigree, they might use genetic screening to select colonists whose genetic makeup will complement their own in their offspring, reducing chances for homozygosity, as well as to select offspring who are heterozygous at as many loci as is possible.

(3) Is it possible to create a viable colony with a lower male-female ratio? What would the minimum ratio be?

This is a very interesting question. As you could see with the example of Tristan de Cunha, most of the men died when that boat sank, but the colony was able to survive. The same thing would probably not be true if the majority of the women had died. In general (at least with mammals), the ratio in size between males and females for a given species represents the mating ratio for that species. So, if males and females are the same size, then usually one male will be mating with one female (for a given mating period), and if males are twice the size of females, one male will mate with two females. The larger size is related to the number of other males they have to fight for access to the females. In our species, males are slightly larger than females, suggesting that populations with slightly low male to female mating ratios (3 men / 4-5 women) have been the 'optimal' state over the last five million years or so. Note that this is not necessarily the actual ratio of males to females in the population, nor does it mean that the colonists could not adopt an even lower ratio at the outset, since one male can fertilize a large number of females at one time. Of course, the fewer males you have, the lower the Y-chromosome diversity will be.

Now that we have these points in mind, let's return to the issue of 'viability' for the colony. It is difficult to predict what sort of conditions or challenges such a colony would face, either in a controlled synthetic habitat or on a new planet. Given the fact that there will be no further contact with Earth, and the enormous potential for disaster, the colonists would want to bring as many tools and supplies with them as were possible, to prepare them for as many contingencies as possible. Toward this end, I would think that the colonists would want to bring as much of the human species' genetic diversity with them as was possible, to maximize their potential to adapt to whatever environment they encountered, to maximize their viability, so to speak.

Think of it this way. If the colonists were genetically identical clones of one another, they would all be susceptible to the same things -- they would succumb to the same diseases, or react to radiation in the same way -- and the success of the colony would hinge largely on the appropriateness of that particular cloned individual to the new environment. If the environment were to change, the colony could suffer drastic consequences. This isn't even a question of genetic disease or inherited infirmity, just a question of the chance that a single individual could be perfectly adapted for a particular environment.

On the other hand, we have here on Earth an example of one species which has adapted to a large number of different environments, for the most part using only stone-age technology, and some of that adaptation has been made possible by regional genetic diversity. If you compare the genetic diversity of any given population of people to the genetic diversity of our species as a whole, you will find that about 86% of the total diversity of our species can be found in any given population. So, if everyone else on the Earth were to go extinct, leaving only the people living on Sardinia, or Madagascar, or Vancouver island, or New Zealand only about 14% of the genetic diversity of our species would be lost. However, for the colonists, that 14% could represent the difference between success and failure. Representing the hope of an entire species, I would think that the colonists would want to make sure that all of the diversity of our species was represented among their number at the outset. The most important thing for them to consider would not be the absolute number of colonists (of a given ethnicity), but that they had a comprehensive representation of the genetic diversity of our species.

I hope that this has helped to address the issues you had questions about. Feel free to contact me through the Mad Scientist network if you want to discuss this further.

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