An instance of type of biological taxon. Each instance of species is the most general taxon from which two breeding organisms of appropriate sexes can conceivably produce fertile offspring, or, in the case of asexual reproduction, is conventionally defined. Members of different species of animals cannot produce fertile offspring by interbreeding. If there are only two breeds of a given species and one breed becomes extinct, the second breed by virtue of that fact becomes an instance of species -- since the only organisms instances can breed with to produce fertile offspring are, at that point, members of that collection. An instance of species has members who all have significant traits in common, and members of each biological species have other members as parents. Exceptions occur when a species is conventionally defined to start since parenthood could conceivably be traced back billions of years, yet new species came into existence. In biological taxonomy, related species are grouped into a particular instance of biological genus. Some genera have only a single species, but they remain different taxons.