While the logic and mechanisms of allopatric speciation are relatively easy to grasp (we hope), there is a second type of speciation, known as sympatric speciation, which was originally more controversial. It occurs when a single population of organisms splits into two reproductively isolated communities within the same physical region. How could this possibly occur? What stop (or inhibits) the distinct sub-populations from inbreeding and reversing the effects of selection and nascent speciation? Recently a number of plausible mechanisms have been identified. One involves host selection104. In host selection, animals (such as insects) that feed off specific hosts may find themselves reproducing in distinct zones associated with their hosts. For example, organisms that prefer blueberries will mate in a different place, time of day, or time of year than those that prefer raspberries. There are blueberry- and raspberry-specific niches. Through a process of disruptive selection (see above), organisms that live primarily on a particular plant (or part of a plant) can be subject to different selective pressures, and reproductive isolation will enable the populations to more rapidly adapt. Mutations that reinforce an initial, perhaps weak, mating preference can lead to what known as reproductive isolation - as we will see this is a simple form of sexual selection105. One population has become two distinct, reproductively independent populations, one species has become two.