Paul Andersen explains the importance of genetic variation within a population. He begins with a discussion of the devil facial tumor that is a form of cancer transferred between Tasmanian devils. He then explains how a decrease in genetic variability nearly led to the extinction of the black-footed ferret. He finally discussed the importance of genetic variability in disease resistance. He explains how mutations in humans allow some of them to be resistant to HIV and how the allele frequency can be calculated in a population using the Hardy-Weinberg equation.