The Genetic History of Tristan da Cuna

In the fall of 1993, geneticist Noe Zamel arrived at Tristan da Cuna, a small remote island in the South Atlantic (< Figure 23.1). It had taken Zamel 9 days to make the trip from his home in Canada, first by plane from Toronto to South Africa and then aboard a small research vessel to the island. Because of its remote location, the people of Tristan da Cuna call their home "the loneliest island," but isolation was not what attracted Zamel to Tristan da Cuna. Zamel was looking for a gene that causes asthma, and the inhabitants of Tristan da Cuna have one of the world's highest incidences of hereditary asthma: more than half of the islanders display some symptoms of the disease.

The high frequency of asthma on Tristan da Cuna derives from the unique history of the island's gene pool. The population traces its origin to William Glass, a Scot who moved his family there in 1817. They were joined by some shipwrecked sailors and a few women who migrated from the island of St. Helena but, owing to its remote loca-

• The Genetic History of Tristan da Cuna

• Genetic Variation

Calculation of Genotypic Frequencies Calculation of Allelic Frequencies

• The Hardy-Weinberg Law

Closer Examination of the Assumptions of the Hardy-Weinberg Law

Implications of the Hardy-Weinberg Law

Extensions of the Hardy-Weinberg Law

Testing for Hardy-Weinberg Proportions

Estimating Allelic Frequencies with the Hardy-Weinberg Law

• Nonrandom Mating

• Changes in Allelic Frequencies

Mutation Migration Genetic Drift Natural Selection

• Molecular Evolution

Protein Variation DNA Sequence Variation Molecular Evolution of HIV in a Florida Dental Practice Patterns of Molecular Variation The Molecular Clock Molecular Phylogenies tion and lack of a deep harbor, the island population remained largely isolated. The descendants of Glass and the other settlers intermarried, and slowly the island population increased in number; by 1855, about 100 people inhabited the island. However, Tristan da Cuna's population dropped markedly when, after William Glass's death in 1856, many islanders migrated to South America and South Africa. By 1857, only 33 people remained, and the population grew slowly afterward. It was reduced again in 1885 when a small

23.1 Tristan de Cuna is a small island in the South Atlantic.

boat carrying 15 men was capsized by a huge wave, drowning all on board. Many of the widows and their children left the island, and the population dropped from 106 to 59. In 1961, a volcanic eruption threatened the main village. Fortunately, all of the islanders were rescued and transported to England, where they spent 2 years before returning to Tristan da Cuna.

Today, just a little more than 300 people permanently inhabit the island. These islanders have many genes in common and, in fact, all the island's inhabitants are no less closely related than cousins. Because the founders of the colony were few in number and many were already related, many of the genes in today's population can be traced to just a few original settlers. The population has always been small, which also gives rise to inbreeding and allows chance factors to have a large effect on the frequencies of the alleles in the population. The abrupt population reductions in 1856 and 1885 eliminated some alleles from the population and elevated the frequencies of others. As will be discussed in this chapter, the events affecting these islanders (small number of founders, limited population size, inbreeding, and population reduction) affect the proportions of alleles in a population. All of these factors have contributed to the high proportion of alleles that cause asthma among the inhabitants of Tristan da Cuna.

Tristan da Cuna illustrates how the history of a population shapes its genetic makeup. Population genetics is the branch of genetics that studies the genetic makeup of groups of individuals and how a group's genetic composition changes with time. Population geneticists usually focus their attention on a Mendelian population, which is a group of interbreeding, sexually reproducing individuals that have a common set of genes, the gene pool. A population evolves through changes in its gene pool; so population genetics is therefore also the study of evolution. Population geneticists study the variation in alleles within and between groups and the evolutionary forces responsible for shaping the patterns of genetic variation found in nature. In this chapter, we will learn how the gene pool of a population is measured and what factors are responsible for shaping it. In the later part of the chapter, we will examine molecular studies of genetic variation and evolution.

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