New species arise after two individuals can no longer produce fertile offspring together. (sciencemag.org)
If the basic definition of a species is a group of organisms capable of mating and producing fertile offspring only within their own group, the polar bear and brown bear fail to qualify. (scientificamerican.com)
Rainbow trout (Oncorhynchus mykiss), the most widely introduced salmonid in the world (Lever 1996), produces fertile offspring when crossed with cutthroat trout (Oncorhynchus clarkii), and introgression often continues until a hybrid swarm is formed and the native cutthroat genomes are lost (Allendorf & Leary 1988). (rsbl.royalsocietypublishing.org)