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Species is the smallest grouping of organisms commonly recognized in taxonomy. If a species has multiple populations with significant morphological differences, it may be divided further into subspecies.

The species is the second part of an organism's scientific (Latin) name.

Criteria for distinct species

A species is defined as a population that lacks gene flow with other populations.[1] There are three separate criteria commonly used by scientists to determine if species are distinct:[1]

  1. The Biological Species Concept: Species are separate if they are reproductively isolated, meaning they cannot or do not interbreed. They are also isolated if they produce offspring that are not viable or fertile.
  2. The Morphospecies Concept: Species are separate if they differ in "size, shape, or other morphological features".[1] This criterion is used if reproductive isolation cannot be determined, as for fossils, but it is still based on the assumption that reproductively isolated species will have different phenotypes. Also, there are no objective standards for which features should be used or to what degree they should differ.
  3. Phylogenetic Species: Each tip on a phylogenetic tree is a distinct species. However, not all phylogenies are determined in the same way. This criterion would also lead to the identification of many more species than the other two.

Although the term "species" is defined by reproductive isolation, this standard is not always used in practice.


  1. 1.0 1.1 1.2 Freeman, Scott (2002). Biological Science: Instructor's Edition. Upper Saddle River, NJ: Prentice Hall. p. 450-451. ISBN 0-13-009338-6.