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In biology, a species is one of the basic units of biological classification (see the diagram at the side - a genus contains one or more species, etc.) A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity. A species consists of individual organisms which are very similar in appearance, anatomy, physiology and genetics due to having relatively recent common ancestors. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species.