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Science Frontiers ONLINE No. 82: Jul-Aug 1992 |
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EVEN TODAY NATURAL SELECTION IS MOLDING HUMAN POPULATIONS
Nauru is a remote Pacific atoll with a population of 5,000 Micronesians. Formerly, the Nauruans led energetic lives -- fishing, subsistence farming -- and they were slim and healthy. Then came colonization and phosphate mining; with these came wealth, imported caloriepacked food, sedentary lives, obesity, and, unhappily for this tropical paradise, diabetes. The incidence of diabetes mellitus shot up to 60%, an astounding statistic by world standards. On one of the wealthiest of the Pacific islands, the inhabitants have the shortest life spans! The same scenario is being played out in other parts of the world where life styles have changed drastically; for example, some Polynesians, American Indians, and Australian aborigines are similarly afflicted. Furthermore, an epi-demic of diabetes mellitus is anticipated as the "benefits" of civilization are brought to India and China.
Two questions must be answered: (1) Why is the incidence of diabetes mellitus only 8% among American junkfood-eating couch potatoes? Probable answer: natural selection has already modified the American genotype by eliminating those who are supersensitive to diabetes mellitus under conditions of rich diets and sedentary lives. (2) Why are modern populations still living under Spartan conditions so sensitive to diabetes in the first place? Possible answer: the so-called "thrifty genotype" hypothesis. In this view, the genotype that is sensitive to diabetes also confers survival advantages in societes where food supplies are meager and unpredictable. This genotype provides for a hair-trigger release of insulin for the rapid conversion of rare food gluts into body fat deposits that will sustain the individual during the next famine. Unfortunately, when rich food is continuously available, people with this "hairtrigger" genotype succumb to diabetes.
(Diamond, Jared M.; "Diabetes Running Wild," Nature, 357:362, 1992.)