<P> In biology, a species is the basic unit of classification and a taxonomic rank, as well as a unit of biodiversity, but it has proven difficult to find a satisfactory definition . Scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties . If as Linnaeus thought, species were fixed, there would be no problem, but evolutionary processes cause species to change continually, and to grade into one another . A species is often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction . While this definition is often adequate, when looked at more closely it is problematic . For example, with hybridisation, in a species complex of hundreds of similar microspecies, or in a ring species, the boundaries between closely related species become unclear . Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies . Problems also arise when dealing with fossils, since reproduction cannot be examined; the concept of the chronospecies is therefore used in palaeontology . Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche . </P> <P> All species are given a two - part name, a "binomial". The first part of a binomial is the genus to which the species belongs . The second part is called the specific name or the specific epithet (in botanical nomenclature, also sometimes in zoological nomenclature). For example, Boa constrictor is one of four species of the Boa genus . </P>

When are two organisms considered to belong to the same species
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