That understanding was greatly extended in the 20th century through genetics and population ecology. Charles Darwin's 1859 book On the Origin of Species explained how species could arise by natural selection. In the 19th century, biologists grasped that species could evolve given sufficient time. Species were seen from the time of Aristotle until the 18th century as fixed categories that could be arranged in a hierarchy, the great chain of being. This obliges taxonomists to decide, for example, when enough change has occurred to declare that a lineage should be divided into multiple chronospecies, or when populations have diverged to have enough distinct character states to be described as cladistic species. If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change. Although none of these are entirely satisfactory definitions, and while the concept of species may not be a perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, regardless of the theoretical difficulties. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies. For example, the boundaries between closely related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, and in a ring species. While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example, Boa constrictor is one of the species of the genus Boa, with constrictor being the species' epithet. The second part is called the specific name or the specific epithet (in botanical nomenclature, also sometimes in zoological nomenclature). The first part of a binomial is the genus to which the species belongs. Īll species (except viruses) are given a two-part name, a "binomial". However, only about 14% of these had been described by 2011. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour, or ecological niche. It is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity.
In biology, 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. Intermediate minor rankings are not shown. The hierarchy of biological classification's eight major taxonomic ranks.
For other uses, see Species (disambiguation).
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