which natural selection reinforces reproductive isolation. In sympatry, reinforcement increases species discrimination and sexual adaptation in order to avoid maladaptive hybridization and encourage speciation. If hybrid offspring are either sterile or less-fit than non-hybrid offspring, mating between members of two different species will be selected against. Natural selection decreases the probability of such hybridization by selecting for the ability to identify mates of one's own species from those of another species. Reproductive character displacement Reproductive character displacement strengthens the reproductive barriers between sympatric species by encouraging the divergence of traits that are crucial to reproduction. Divergence is frequently distinguished by assortative mating between individuals of the two species. For example, divergence in the mating signals of two species will limit hybridization by reducing one's ability to identify an individual of the second species as a potential mate. Support for the reproductive character displacement hypothesis comes from observations of sympatric species in overlapping habitats in nature. Increased prezygotic isolation, which is associated with reproductive character displacement, has been observed in cicadas of genus Magicicada, stickleback fish, and the flowering plants of the genus Phlox. Differential fusion An alternative explanation for species discrimination in sympatry is differential fusion. This hypothesis states that of the many species have historically come into contact with one another, the only ones that persist in sympatry (and thus are seen today) are species with strong mating discrimination. On the other hand, species lacking strong mating discrimination are assumed to have fused while in contact, forming one distinct species. Differential fusion is less widely recognized than character displacement, and several of its implications are refuted by experimental evidence. For example, differential fusion implies greater postzygotic isolation among sympatric species, as this functions to prevent fusion between the species. However, Coyne and Orr found equal levels of postzygotic isolation among sympatric and allopatric species pairs in closely related Drosophila. Nevertheless, differential fusion remains a possible, though not complete, contributor to species discrimination. Examples Sympatry has been increasingly evidenced in current research. Because of this, sympatric speciation – which was once highly debated among researchers – is progressively gaining credibility as a viable form of speciation. Orca: partial sympatry Several distinct types of killer whale (Orcinus orca), which are characterized by an array of morphological and behavioral differences, live in sympatry throughout the North Atlantic, North Pacific and Antarctic oceans. In the North Pacific, three whale populations – called "transient",