Change is the only constant
Over the last years, many populations evolved genetic adaptations to their local environments
changes in genes improve people's chance of surviving till reproductive age
which is what drives natural selection.
Genetic adaptations to environment
Visitors at the Tibetan high plateau, which has just 60% of oxygen found below, struggle with altitude sickness while native Tibetans sprint up the mountains. It's due to changes to some genes that allows their body to make the most of limited oxygen.
Genetic adaptations to environment
People in Siberia are adapted to survive in extreme cold and are slower to develop frostbite. They've undergone selection for a higher metabolic rate that increases heat production
Genetic adaptations through diet
All mammals can drink milk from their mother as babies but after weaning they switch off the gene that allows them to digest milk. Some communities that used cows for milk have adapted themselves to prevent that gene from switching off.
Role of medicine
Modern medicine has removed several selective pressures. Antibiotics, vaccines, and clean water make the differences between our genes less important. These tend to stop selection by allowing more people to survive till a reproductive age.
Has medicine stopped human evolution?
There are other aspects of evolution besides natural selection. On a genetic level, modern medicine might increase variety because harmful mutations don't kill people and aren't thus eliminated.