Charles Darwin, when developing his theory of adaptation by natural selection, incorporated the observation that
a.) offspring are adapted to avoid resembling their parents in order to avoid mate confusion.
b.) offspring tend to resemble their parents.
c.) all competition between individuals within a species is over mates.
d.) variation in offspring is shaped by the behavior of the parents during their lifetimes.

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.