As you remember, population genetics is the study of genetic variation within populations.
Below are some good handouts which will help you learn more about allele changes over time at the population level:
http://www2.le.ac.uk/departments/genetics/vgec/schoolscolleges/topics/population-genetics
www.life.illinois.edu/ib/201/lectures/PopGen.pdf
www.csun.edu/~dgray/Evol322/Chapter6.pdf
