The Hardy-Weinberg principle

After a single generation in which all eight of the above assumptions are satisfied

$$\hbox{freq.}(A_1A_1\hbox{ in zygotes}) = p^2$$ (1)

$$\hbox{freq.}(A_1A_2\hbox{ in zygotes}) = 2pq$$ (2)

$$\hbox{freq.}(A_2A_2\hbox{ in zygotes}) = q^2$$ (3)

It's vital to understand the logic here.

1. If Assumptions #1-#8 are true, then equations 1-3 must be true.

2. If genotypes are in Hardy-Weinberg proportions, one or more of Assumptions #1-#8 may still be violated.

3. If genotypes are not in Hardy-Weinberg proportions, one or more of Assumptions #1-#8 must be false.

Point (3) is why Hardy-Weinberg is so important. There isn't a population of anything anywhere in the world that satisfies all 8 assumptions, even for a single generation.⁶But all possible evolutionary forces within populations cause a violation of at least one of these assumptions. Departures from Hardy-Weinberg are one way in which we can detect those forces and estimate their magnitude.⁷