 If \(\Delta p \gt 0\), then natural selection has led the \(A_1\) allele to increase in frequency; if \(\Delta p \lt 0\) then selection has led the \(A_2\) allele to increase in frequency. If \(\Delta p = 0\) then no gene frequency change has occurred, i.e., the system is in allelic equilibrium. (Note, however, that the condition \(\Delta p = 0\) does not imply that no natural selection has occurred; the condition for that is \(w_{11} = w_{12} = w_{22}\). It is possible for natural selection to occur but to have no effect on gene frequencies.)