The lander touched down some 200 years after it left Earth, 20 light years away. Descendants of the original pioneers looked out at a new planet ― a dark alien world. Some despaired; others stared with excitement at the challenge. More than a few wondered, 'What had their ancestors gotten them into?'
A red-dwarf Sun, fifty times dimmer than Sol, filled much of the sky. Low scrub vegetation, dark in color, almost black, crowded the landing site. The broad daylight seemed too dark to support plant-life, but shrubs absorbed nearly all light that hit their fuzzy leaves and flourished. A large, deep ocean shimmered in the dusky sunlight, lapping the shores, awaiting seafaring folk...
The voyagers comprise a healthy population able to populate the planet. Their forebears had done well.
How many? About 160 original pioneers can do the job. Actually a mere 15 can work if they're lucky. Five hundred might be enough. NASA's proposal of 10,000 is a more comfortable number. You pick 'em.
What ratio of men to women? Perhaps half men and half women or possibly two men for every three women.
That's the basic answer. Now let's delve into how we arrived at the numbers ― sex and genes. Also, what is a "healthy population?"
A healthy population is one:
- big enough to minimize inbreeding, (although this is not much of a problem if the original pioneers don't have genes that could cause harmful conditions, like hemophilia)
- small enough for space travel.
Genetic diversity may be helpful the "first several hundred years", as long as it doesn't interfere with the social organization of the colonists, says biochemist Lawrence Moran of the University of Toronto. After that, most genetic diversity will "disappear."
Anthropologist John Moore of the University of Florida thinks the traditional family is the best organization, since it has served us well for a million years. A ratio of one man to one woman works. He suggests about 160 people as a minimum number, which allows around ten potential marriage partners. He bases his population estimate on mathematical modeling and studies of small migrating groups of early humans.
A much smaller initial population is possible with the help of human embryos (and the technology to support in vitro embryogenesis and fetal development). Just two human females could manage the job, Moore says. Sperm banks can provide genetic diversity in years to come.
However, "don't expect human nature to change," Moran says. "Men and women will still want to pass on their own genes."
Genetic drift or random catastrophes can leave a population either without certain genes helpful to survival or with nearly identical genetic makeup, and therefore identically vulnerable.
Present-day cheetahs are an example of what can happen to a species struck by a calamity of some sort. About 10,000 years ago most cheetahs died out ― we don't know why. The population has since rebounded in size but their genetic structure, based on the few who managed to survive the catastrophe, is almost identical. Consequently, says biologist John W. Kimball author of Biology, cheetahs will accept skin grafts from each other just as identical twins do. Kimball wonders if a population so much alike has the potential to adapt to a changing environment.
On the other hand, Moran says, "The idea that you need to have lots of variation within a species in order to prepare for changing environment is a myth that doesn't stand up to scrutiny."
"Evolution is a very long-term process. It highly unlikely that there will be some rapid drastic change in the environment that will wipe out all existing cheetahs but would have saved some of them if they had more diversity."
Does natural selection or does genetic drift dominate evolutionary changes in a population? That debate rages among biologists these days.
Biologists striving to save threatened animal species from extinction have developed some rules of thumb for estimating population sizes to overcome genetic problems:
- A size of 500 to 5000 individuals may ensure overall genetic diversity (Frankham).
- Another figure (based on work by Soulé and Foose in 1986) states a founding population of 20 to 30 individuals that are unrelated and non-inbred would preserve 90% of the original genetic diversity for 200 years.
- An initial population size (the founders) of 50 (Frankham and Franklin) avoids inbreeding problems.
Rules of thumb, however, rest on theoretical modeling, without much experimental evidence to support the predictions. Unfortunately, this is especially true of projecting populations needed to colonize space. So, let's consider a bit of history. What worked on Earth?