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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.

Before Charles Darwin proposed his theory of evolution, it was believed that species were "fixed", meaning that they don't change, hybridize, or diverge into separate species.[1] However, it is now believed that the created kinds in the Genesis account of creation encompass families or orders.[2] Thus, speciation is consistent with creationism.

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

For some fungi, different stages of their life cycle used to be classified as separate species. In 2011, the International Botanical Congress decided to end this practice, with the change going into effect in 2013.[3]

Criteria for distinct species

A species is defined as a population that lacks gene flow with other populations.[4] There are three separate criteria commonly used by scientists to determine if species are distinct:[4]

  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".[4] 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.


Main article: Speciation

There are two main modes by which a species can diverge into two or more species.

Allopatric speciation occurs when two populations of one species are isolated geographically. The two populations can differ due to pre-existing differences in gene ratios.[5] Over time, more differences in gene ratios and traits can develop due to random mutations and differences in natural selection pressures.[5]

Sympatric speciation occurs without geographic isolation. An example is when plants gain sets of chromosomes.[5]


  1. Mader, Sylvia S. (2007). Biology, Ninth Edition. New York, NY: McGraw-Hill. p. 283. ISBN 0-07-325839-3. 
  2. Bodie Hodge and Dr. Georgia Purdom (April 16, 2013). "What Are “Kinds” in Genesis?". The New Answers Book 3. Answers in Genesis. Retrieved 22 August 2018. 
  3. Hawksworth, D. L. (2011). "A new dawn for the naming of fungi: impacts of decisions made in Melbourne in July 2011 on the future publication and regulation of fungal names". MycoKeys. 1: 7–20.
  4. 4.0 4.1 4.2 Freeman, Scott (2002). Biological Science: Instructor's Edition. Upper Saddle River, NJ: Prentice Hall. p. 450-451. ISBN 0-13-009338-6. 
  5. 5.0 5.1 5.2 Mader, p. 312