Directional selection

To use the Fundamental Theorem we plot $\bar w$ as a function of $p$ (Figure 1). The Fundamental Theorem now tells us that allele frequencies have to change from one generation to the next in such a way that $\bar w' > \bar w$, which can only happen if $p' > p$. So viability selection will cause the frequency of the $A_1$ allele to increase. Ultimately, the population will be monomorphic for the homozygous genotype with the highest fitness.¹⁴

Figure 1: With directional selection (in this case $w_{11} > w_{12} > w_{22}$) viability selection leads to an ever increasing frequency of the favored allele. Ultimately, the population will be monomorphic for the homozygous genotype with the highest fitness.