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Sunday, January 8, 2012

How many people would it take to create a healthy community?

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?
A case study
Starting in 1816, fifteen British immigrants settled the most remote island on our planet, Tristan de Cunha ― located almost half way between Cape Town, Africa and Buenos Aires, South America in the Atlantic Ocean.  We know much about the islanders' founders and their lonely struggle for survival.  Colonizing a remote island is similar to colonizing a remote planet. 
It all started with Napoleon, imprisoned on St Helena, about 2,430 kilometers (1,510 mi) to the north.  The Brits had exiled Napoleon to this tiny windswept island in 1815. But now, rumors of rescue plans reached the halls of Parliament.  What if the French launched a rescue of Napoleon from Tristan? In August 1816, Britain annexed Tristan de Cunha to thwart the possibility.
The Royal Army sent five officers and 36 soldiers from South Africa to take possession of Tristan da Cunha in 1816. Corporal William Glass from Kelso in Scotland, with his South African wife and two children, asked to stay when the troops withdrew a year later. 
Glass and his family became the first settlers of Tristan.  In all, just 15 people (seven founders and eight men) founded the colony, says geneticist Himla Soodyall from the University of the Witwatersrand, South Africa.  They arrived at various times between 1816 and 1908.  Colonizing over the decades undoubtedly increased the colony's genetic diversity and helped them survive. 
We have traced the mitochondrial DNA of the present approximately 300 people back to just five female ancestors, including one pair of sisters, says Soodyall.  The population grew and fell with the flow of immigrants and disasters:
  • 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.]
Only seven family names are in use now, corresponding to the original eight male founders (minus one).  Any pair of the 300 islanders is as closely related as first cousins.  But it's a healthy human population since it survived.  The people (taken as a group) were able to cope with the hardships they encountered, and the few inheritable diseases were not life-threatening. 
By and large, this is an impressively successful (and lucky) colonization still viable almost 200 years later.

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