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There are several definitions for species.
The morphological species definition
A species has several features that are not shared by other groups. This is the oldest
definition which is solely based on "visible" features. The main problem with this
definition is the question how different to groups have to be before they can be
considered as species.
The Evolutionary species concept
A species descents from a common ancestor and shared evolution.
The biological species concept
A species is a group of (potentially) interbreeding individuals that
are reproductively isolated (in place, time or behavior) from other
groups. All members of the group share a common ancestor.
The species is the fundamental unit of evolution, while higher
taxa (families, orders, ..) are artificial groups reflecting a possible
evolutionary relationship.
Remarks:
- Because fossils and asexual organisms do not interbreed they cannot be
considered as a species.
- This distinct reproductive group of organisms which naturally mates in the wild
to produce viable offspring. This means that, if a group of fish from distinct populations
do mate in an aquarium and produce viable offspring, they cannot be considered
to belong to the same species.
The Genotypic cluster definition
A species is a common genetic pool that are identified by their genetic
differences from other groups. This is the latest definition which is based
on differences in DNA sequences.
Subspecies are commonly used to recognize the existence of regional variants of a species,
that is some degree of morphological differentiation (not as great as that between species)
accompanies geographic (or ecological, or temporal) separation from other subspecies.
Varieties (var.) are often used to recognize the existence of local variants, although
the choice between subspecies and varietal status in some groups is
sometimes more a matter of personal taste than of adherence to a strict definition.
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