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frojack writes:

The Scientist reports on a study of a villages in Argentina, where the people have been drinking poison—arsenic, to be specific—for thousands of years. The levels in the principal water source is up to 80 times the level considered to be safe by the World Health Organization (WHO). Even the best wells exhibit over 20 times the arsenic allowed in the WHO limit.

And it doesn't seem to bother them at all. There is every indication that these Andean communities may have evolved the ability to metabolize arsenic.

Swedish biologist Karin Broberg, of Stockholm’s Karolinska Institute, and colleagues at Uppsala and Lund Universities have been trying to figure out how generations of villagers in the Andean village of San Antonio de los Cobres (SAC), an area of nearly 6,000 residents, have been able to survive this chronic exposure to toxic levels of arsenic.

The researchers knew that a particular allele, AS3MT, located on chromosome 10, was suspected as the main gene involved in arsenic metabolism in humans. But the metabolism rate in these Andean villagers was sky high compared to people elsewhere.

By comparing genetic samples and urine from a wide selection of South American populations in Peru, Argentina, and Columbia, they hoped to determine if the arsenic tolerance was simply due to genetic accident, (population drift) or if it was a byproduct of natural selection. Natural selection tends to exhibit itself via higher levels of homozygosity, where particular alleles come from one lineage. (See here for a primer on Drift vs Selection.

In the area around AS3MT, the SAC population differed dramatically from the comparison populations. Not only did the SAC women have higher levels of protective AS3MT alleles, but these alleles also had longer stretches of homozygosity—a telltale sign of selection.

The extremely strong difference in allele frequency is considered a clear result of selective pressure on a population.