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Species concepts
== Defining species ==
== Defining species ==
There is not a single universally-agreed definition of "species", although all definitions agree that successful interbreeding is a key factor.{{fact}}
There is not a single universally-agreed definition of "species." This is because different definitions of "species" are germane to different contexts. For living organisms, the biological species concept is considered the norm, but it is not always the easiest definition to apply.
Examples of species concepts include:
;the ''biological species'' concept: A species is a group of organisms capable of interbreeding. This concept only applies to extant organisms which undergo a significant amount of sexual reproduction; moreover, successful reproduction (or, at least, the lack of barriers to it) must be observed. When several reproductively isolated populations are observed within an apparently similar group of organisms, they are often considered different species. There are several different ways to define this concept depending on which methods of reproductive isolation are being considered.
;the ''morphospecies'' concept: A species is a group of organisms characterized by common morphological features. While it is easy to observe and categorize species this way, especially fossil species, such categorizations are not foolproof, and this concept often yields faulty results. For example, a two-legged goat born to a four-legged goat mother is not a member of a new species.
;the ''genetic species'' concept: A species is a group of organisms defined by [[DNA]] sequence traits. The cutoffs between species defined this way are often arbitrary, but this species concept is often considered the best one for microorganisms like [[bacteria]].
=== Common third species ===
=== Common third species ===