1. No mutations occur.
2. There is no net movement of individual--with their genes--into the population (immigration) or out of it (emigration).
3. The population is large enough that the laws of probability apply. (This means it would be highly unlikely that chance alone can alter allele frequencies.)
4. Mating is random.
5. All alleles are no different in reproductive success. The offspring of all posible matings are equally likely to survive to reproduce in the next generation.
Hardy and Weinberg demonstrated mathematically that if the five conditions listed above are met, the frequencies, or relative proportions will continuesto be the same, generation after generation.
The discovery of this law was especially significant in proofing that natural selection is the primary mechanism of evolution. If the proportions of gene formed in a population do not change, the rate of evolution will be zero. Individual variations occur because of the various genetic combinations that result from the random mating of individuals. However, non-random, or selective, mating must occur for natural selection to take place. Certain gene-controlled traits are selected for or selected against by the partners involved. Over a long period of time, the traits will become more common or more rare in the population because the selective pressure will change the frequency of appearance of certain gene forms.
Medical geneticists can use the Hardy-Weinberg law to calculate the probability of human matings that may result in defective offspring. The law is also useful in determining whether the number of harmful mutations in radiation from industrial processes, medical techniques, and fallout.