taxonomic group of organisms composed of individuals of the same genus that can reproduce among themselves and have similar offspring.

Merriam-Webster Online Dictionary
species (noun)
1.
a) - kind sort
b) a class of individuals having common attributes and designated by a common name , specifically a logical division of a genus or more comprehensive class - confessing sins in species and in number
c) the human race human beings - often used with the survival of the species in the nuclear age
d) (1) a category of biological classification ranking immediately below the genus or subgenus, comprising related organisms or populations potentially capable of interbreeding, and being designated by a binomial that consists of the name of a genus followed by a Latin or latinized uncapitalized noun or adjective agreeing grammatically with the genus name
(2) an individual or kind belonging to a biological species
e) a particular kind of atomic nucleus, atom, molecule, or ion
2.
the consecrated eucharistic elements of the Roman Catholic or Eastern Orthodox Eucharist
3.
a) a mental image , also a sensible object
b) an object of thought correlative with a natural object
species (adjective)
belonging to a biological species as distinguished from a horticultural variety - a species rose
species (Wikipedia)

LifeDomainKingdomPhylumClassOrderFamilyGenusSpecies
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 (/ˈspʃz/ (About this soundlisten)) is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. 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. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined.

All species (except viruses) 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 genus Boa.

While the definitions given above may seem adequate, when looked at more closely they represent problematic species concepts. 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. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies. Though none of these are entirely satisfactory definitions, scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If 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.

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. In the 19th century, biologists grasped that species could evolve given sufficient time. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection. That understanding was greatly extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures. Genes can sometimes be exchanged between species by horizontal gene transfer; new species can arise rapidly through hybridisation and polyploidy; and species may become extinct for a variety of reasons. Viruses are a special case, driven by a balance of mutation and selection, and can be treated as quasispecies.

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