The Hardy-Weinberg principle is based on the stipulation that there will be no change in allele frequency of a population over time. This is called genetic equilibrium. For genetic equilibrium to occur, a set of conditions must be met by the population. These conditions include:

• Random mating
• No mutations
• Large population size
• No migration
• No natural selection

Fortunately for evolutionary change, it is almost impossible to maintain these conditions in natural populations. As a result, it is expected that allelic change will occur all the time in almost all populations.

The accumulation of changes in allele or genotype frequencies in a population is called microevolution. Let's look at how failure to meet the criteria for genetic equilibrium results in microevolution.