In life sciences, the species is the taxon's basic systematic. The species is a vague concept that there are a multitude of definitions in the literature. The definition most commonly accepted is that the biological concept of the case statement by Ernst Mayr (1942): a species is a population or group of populations whose individuals can actually or potentially to reproduce them and produce viable offspring and fertile under natural conditions. Thus, the species is the largest unit of population in which the flow gene is possible when individuals of one species are genetically isolated from other sets equivalent in terms of reproduction.
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However the criterion can not always be verified: the case for fossil organisms asexual or for rare or difficult to observe. Other definitions can be used
- case morphology: group of individuals defined by structural characteristics (size, shape ...)
- species phylogenetic: the smallest line of a population can be defined by a unique combination of diagnostic characters;
- species ecological: group of organisms sharing the same ecological niche;
- species phenetic: all living organisms are more like them than other equivalent sets.