In London, peppered moths that were light colored blended in with trees of the
area, and were well populated. Over time, pollution caused the bark of the trees to darken, causing a
decrease in the number of light colored moths seen. The darker colored moths survived the predators
since they were able to blend in with the darkened tree bark and they reproduced rapidly.

In an experiment examining the effects tai chi on arthritis pain, callahan (2010) selected a large sample of individuals with doctor-diagnosed arthritis. half of the participants immediately began a tai chi course and the other half (the control group) waited 8 weeks before beginning. at the end of 8 weeks, the individuals who had experienced tai chi had less arthritis pain that those who had not participated in the course.