Biologists striving to save threatened animal species from extinction have developed some rules of thumb for estimating population sizes to overcome genetic 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?
- 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.
A case study
- By 1827, shipwrecked sailors, brides from St. Helena and South African settlers boosted the total from 15 to 24 (7 men, 6 women and 11 children)
- 1832: population of 34 with 6 couples and 22 children
- 1852: 85 people
- 1856: population reached 96, then fell to 71 when 25 left for Massachusetts after Glass' death
- 1857: population crashed to 28 (only four families) because 46 left on the rescue frigate HMS Geyser, to avoid starving to death.
- 1887 started off well with a population of 107, but then 15 out of the island's19 adult men died at sea trying to intercept the ship, West Riding, and barter for food. Their potato crop had failed, so, despite poor weather, the men launched their longboat and were never seen again. This left only 92 people (4 elderly men and 88 women and children)
- 1890: 34 emigrated to South Africa, leaving 58 on the island.
- 1987: population of 296. [Data from Steve Mack and Arnaldo Faustini.]