Species

In biology, a species is:

• a taxonomic rank (the basic rank of biological classification) or
• a unit at that rank (in which case the plural is "species". This is sometimes abbreviated: "spec." or "sp." singular, or "spp." plural).

There are many definitions of what kind of unit a species is (or should be). A common definition is that of a group of organisms capable of interbreeding and producing fertile offspring, and separated from other such groups with which interbreeding does not (normally) happen. Other definitions may focus on similarity of DNA or morphology. Some species are further subdivided into subspecies, and here also there is no close agreement on the criteria to be used.

Biologists' working definition

A usable definition of the word "species" and reliable methods of identifying particular species is essential for stating and testing biological theories and for measuring biodiversity. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species. Extinct species known only from fossils are generally difficult to give precise taxonomic rankings to.

Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Although properties such as DNA-sequences and morphology are used to help separate closely-related lineages, this definition has fuzzy boundaries. However, the exact definition of the term "species" is still controversial, particularly in prokaryotes, and this is called the species problem. Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species concerned.

Common names and species

The commonly used names for plant and animal taxa sometimes correspond to species: for example, "lion", "walrus", and "Camphor tree" – each refers to a species. In other cases common names do not: for example, "deer" refers to a family of 34 species, including Eld's Deer, Red Deer and Elk (Wapiti). The last two species were once considered a single species, illustrating how species boundaries may change with increased scientific knowledge.

Because of the difficulties with both defining and tallying the total numbers of different species in the world, it is estimated that there are anywhere between 2 and 100 million different species.

Placement within genera

Ideally, a species is given a formal, scientific name, although in practice there are very many unnamed species (which have only been described, not named). When a species is named, it is placed within a genus. From a scientific point of view this can be regarded as a hypothesis that the species is more closely related to other species within its genus (if any) than to species of other genera. A genus is commonly included in a hierarchy, with as the best-known taxonomic ranks: life, domain, kingdom, phylum, class, order, family, genus, and species. This assignment to a genus is not immutable; later a different (or the same) taxonomist may assign it to a different genus, in which case the name will also change.

In biological nomenclature, the name for a species is a two-part name (a binomial name), treated as Latin, although roots from any language can be used as well as names of locales or individuals. The generic name is listed first (with its leading letter capitalized), followed by a second term, the specific name (or specific epithet). For example, the species commonly known as the Longleaf Pine is Pinus palustris; gray wolves belong to the species Canis lupus, coyotes to Canis latrans, golden jackals to Canis aureus, etc., and all of those belong to the genus Canis (which also contains many other species). The name of the species is the whole binomial, not just the second term (which may be called specific name for animals).

This binomial naming convention, later formalized in the biological codes of nomenclature, was first used by Leonhart Fuchs and introduced as the standard by Carolus Linnaeus in his 1753, Species Plantarum (followed by his, 1758 Systema Naturae, 10th edition). At that time, the chief biological theory was that species represented independent acts of creation by God and were therefore considered objectively real and immutable, so the hypothesis of common descent did not apply.

Abbreviated names

Books and articles sometimes intentionally do not identify species fully and use the abbreviation "sp." in the singular or "spp." in the plural in place of the specific epithet: for example, Canis sp. This commonly occurs in the following types of situations:

• The authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong. This is particularly common in paleontology.

• The authors use "spp." as a short way of saying that something applies to many species within a genus, but do not wish to say that it applies to all species within that genus. If scientists mean that something applies to all species within a genus, they use the genus name without the specific epithet.

In books and articles, genus and species names are usually printed in italics. If using "sp." and "spp.", these should not be italicized.

Difficulty of defining "species" and identifying particular species

It is surprisingly difficult to define the word "species" in a way that applies to all naturally occurring organisms, and the debate among biologists about how to define "species" and how to identify actual species is called the species problem.

Most textbooks follow Ernst Mayr's definition of a species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups".

Various parts of this definition serve to exclude some unusual or artificial matings:

• Those which occur only in captivity (when the animal's normal mating partners may not be available) or as a result of deliberate human action.
• Animals which may be physically and physiologically capable of mating but do not normally do so in the wild, for various reasons.
• Animals whose offspring are normally sterile.

The typical textbook definition above works well for most multi-celled organisms, but there are several types of situations in which it breaks down:

• By definition it applies only to organisms that reproduce sexually. So it does not work for asexually reproducing single-celled organisms and for the relatively few parthenogenetic multi-celled organisms. The term "phylotype" is often applied to such organisms.
• Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding.
• There is considerable variation in the degree to which hybridization may succeed under natural conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed.
• In ring species, members of adjacent populations interbreed successfully but members of some non-adjacent populations do not.
• In a few cases it may be physically impossible for animals that are members of the same species to mate. However, these are cases in which human intervention has caused gross morphological changes, and are therefore excluded by the biological species concept.

Horizontal gene transfer makes it even more difficult to define the word "species". There is strong evidence of horizontal gene transfer between very dissimilar groups of prokaryotes, and possibly between dissimilar groups of single-celled eukaryotes; and Williamson argues that there is evidence for it in some crustaceans and echinoderms. All definitions of the word "species" assume that an organism gets all its genes from one or two parents which are very like that organism, but horizontal gene transfer makes that assumption false.

(Read more)