Charles Darwin viewed evolution as a gradual emergence of new varieties of life from previous forms over long periods. A gradual emergence, however, must be made up of a bunch of microscopic changes in much shorter periods, known as short-term or microevolution. These tiny evolutionary changes usually are found within a specific population of people and often are due to adaptations that are made to help cope with environmental pressures like increased solar radiation, high altitude and dietary differences.
Biological anthropologists explore many of these environmental pressures and investigate the adaptations that populations undergo in response to them. Let's take high-altitude living as an example to see exactly what they are learning about populations who live high in the mountains. The 25 million people who live at altitudes above 10,000 feet (3,048 meters) cope with daily stressors to their bodies, the most challenging of them being the low oxygen pressure in the air [source: Nelson and Jurmain]. Natives of the Peruvian Andes have up to 30 percent more red blood cells than populations at sea level to allow the blood to carry more oxygen [source: Nelson and Jurmain]. Other physiological adjustments in this population include lower birth weights, larger placentas and delayed sexual maturity. Some of these changes can be attributed to individuals' bodies making physiological adjustments to their environment (acclimatization), not an actual adaptation in the population that demonstrates evolution.
Acclimatization is a reversible process, should individuals be taken back into an environment that no longer contains the stressful element to which their bodies were responding. This type of change does not leave a lasting impression on a species the way an adaptation does. This debate between acclimatization and adaptation is at the crux of what a biological anthropologist might study. They employ all sorts of models to compare genetically related populations living under different environmental conditions to find out more about how we, as humans, have evolved and continue to evolve.