Most populations are not isolated, which is contrary to the Hardy-Weinberg Theorem. These changes in the genome of an organism are an important source of natural selection. Individuals will mate more frequently with close individuals than more distant ones. Assortive mating, is another form of nonrandom mating. Here the individuals will mate with partners that closely resemble themselves in certain characteristics.
Populations vary in the types of individuals and their reproductive success.
Those individuals who leave more offspring behind than others, pass on more of their alleles and have a better success rate in dominating the population. Link to Darwin's books .