The hierarchy of
biological classification's eight major
taxonomic ranks. A
genus contains one or more species. Intermediate minor rankings are not shown.
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 and clearly distinct from one another, 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 any two individuals of the appropriate sexes or mating types 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.