<Li> When organisms reproduce asexually, as in single - celled organisms such as bacteria and other prokaryotes, and parthenogenetic or apomictic multi-celled organisms . The term quasispecies is sometimes used for rapidly mutating entities like viruses . </Li> <Li> When scientists do not know whether two morphologically similar groups of organisms are capable of interbreeding; this is the case with all extinct life - forms in palaeontology, as breeding experiments are not possible . </Li> <Li> When hybridisation permits substantial gene flow between species . </Li> <Li> In ring species, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not . </Li>

Define species as a group of organisms that can reproduce to produce fertile offspring