The Story of Tristan de Cunha

The small, isolated island of Tristan de Cunha in the South Atlantic was first successfully settled in 1817 by William Glass of Scotland and his family. Later, they were joined by a few additional settlers and some women from the island of St. Helena but thereafter they remained genetically isolated for many years. In 1961, a volcano on the island erupted and the inhabitants (around 300 in numbers) were evacuated to England. They remained there for two years during which time geneticists studied their gene pool and reconstructed their genetic history. The studies revealed that the population had been strongly influenced by a process known as genetic drift (random change in gene frequency due to chance). There were three forms of genetic drift identified:

1. The "founder effect" took place at the initial settlement. By 1855, the population of the island was about 100 people, but 26% of the genes at the time were contributed by William Glass and his wife. Even in 1961, these two founders contributed 14% of all the genes of the 300 people evacuated. The genes of the founding members heavily influenced the subsequent gene pool of the population.
   
   
2. The population size remained small throughout the history of the island and "sampling error" (chance deviations from expected gene frequencies) continually occurred. Sampling error is analogous to flipping a coin: we expect to get 50% heads, 50% tails. If we flip the coins 2000 times, we will get very close to that ratio. However, if we only flip the coin four times, we would not be surprised to get Heads three times and Tails only once, even though it is not predicted to be so. The smaller the sample (in this case the number of flips, but mating follows the same rules), the larger the sampling error.
   
   
3. The last form of genetic drift that occurred on the island is called the bottleneck effect. This occurs when a population is drastically reduced in size. Sometimes this can result in genes being lost from the gene pool by chance. For example, if 10 people live on an island, but a storm kills 5 of them, and those happen to be the five people with brown eyes, this means the genes for brown eyes are effectively lost from the gene pool, so the chances of blue or green eyes becomes much greater. Two such bottlenecks occurred during the history of the island of Tristan de Cunha. The first was the death of William Glass and the arrival of a missionary who encouraged the people to leave the island. At this point, many people did leave to America and South Africa, so the population of the island dropped from 103 individuals to 33 in just two years, between 1855 and 1857. The next bottleneck occurred in 1885. As the island had no natural harbour, islanders intercepted passing ships to trade with by rowing out to them in small boats. In November 1885, fifteen young men went out to intercept and ship, and in front of the whole community, their boat capsized and they all drowned. This left only four adult males on the island, one of whom was insane and the other two of whom were old. Many widows and their families left the island and the population dropped to 59 from 106. These bottlenecks had major effects on the gene pools of the remaining inhabitants. All the genes from several settlers were lost and the gene frequencies of others were altered.

(From pages 741-743-809 of reference 5)