Disruptive selection tends to increase genetic variation in populations, according to About.com. It is the opposite of stabilizing selection, which tends to drive a population toward a mean genotype and eliminate outliers on either end of the population average.
Disruptive selection is a special case of natural selection, which encourages the gene pool in a population to diverge into two or more extremes, About.com reports. In a population undergoing disruptive pressure, selection favors two or more traits simultaneously, as each has its advantages for the fitness of the organisms possessing it, and punishes the expression of mixed traits.
An example of disruptive selection is guppies in a creek. If the creek bed is a mixture of dark pebbles and light sand and the guppy population has the genes to express both camouflage patterns, selection works to isolate each camouflage pattern and prevent mixing. In this case, any guppy with strictly dark or light camouflage is able to find shelter among either type of background, but individuals with an intermediate camouflage, a mottled mixture of light and dark spots, stand out against both backgrounds and are easy for predators to spot. Under conditions like these, selection favors the divergence of the two populations and increases the overall genetic diversity of the gene pool.