In biologya species is the basic unit of classification and a taxonomic rankas well as a unit of biodiversitybut 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 Carl 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 offspringtypically by sexual reproduction. While this definition is often adequate, when looked at more closely it is problematic.
For example, with hybridisationin a species complex of "Phylogenetic species concept asexual plants" of similar microspeciesor in a ring speciesthe boundaries between closely related species become unclear.
Among organisms that reproduce only asexuallythe concept Phylogenetic species concept asexual plants a reproductive species breaks down, and each clone is potentially a microspecies. Problems also arise when dealing with fossilssince reproduction cannot be examined; the concept of the chronospecies is therefore used in palaeontology.
Other ways of defining species include their karyotypeDNA sequence, morphologybehaviour or ecological niche. All species are given a two-part namea "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 nomenclaturealso sometimes in zoological nomenclature. For example, Boa constrictor is one of four species of the genus Boa. Species were seen from the time of Aristotle until the 18th century as fixed kinds 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 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 recombinationwhile organisms themselves Phylogenetic species concept asexual plants 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 Phylogenetic species concept asexual plants mutation and selectionand can be treated as quasispecies. As a practical matter, species concepts may be used to define species that are then used to measure biodiversitythough whether this is a good measure is disputed, as other measures are possible. These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean terms thus named; today the birds are a classthe cranes are a familyand the crows a genus.
A kind was distinguished by its attributes ; for instance, a bird has feathers, a beak, wings, a hard-shelled egg, and warm blood.
A form was distinguished by being shared by all its members, the young inheriting any variations they might have from their parents. Aristotle believed all kinds and forms to be distinct and unchanging.
His approach remained influential until the Renaissance. When observers in the Early Modern period began to develop systems of organization for living things, they placed each kind of animal or plant into a context.
Many of these early delineation schemes would now be considered whimsical: John Rayan English naturalist, was the first to attempt a "Phylogenetic species concept asexual plants" definition of species inas follows:. No surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from Phylogenetic species concept asexual plants. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa.
In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences. This view was influenced by European scholarly and religious education, which held that the categories of life are dictated by God, forming an Aristotelian hierarchy, the scala naturae or great chain of being. However, whether or not it was supposed to be fixed, the scala a ladder inherently implied the possibility of climbing.
Faced with evidence of hybridisation, Linnaeus came to accept that species could change, and the struggle for survival, but not that new species could freely evolve. Jean-Baptiste Lamarckin his Zoological Philosophydescribed the transmutation of speciesproposing that a species could change over time, in a radical departure from Aristotelian thinking. InCharles Darwin and Alfred Russel Wallace provided a compelling account of evolution and the formation of new species.
Darwin argued that it was populations that evolved, not individuals, by natural selection from naturally occurring variation among individuals. Darwin concluded that species are what they appear to be: I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other It does not essentially differ from the word variety, which is given to less distinct and more fluctuating forms.
The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for convenience sake. The commonly Phylogenetic species concept asexual plants names for kinds of organisms are often ambiguous: Another problem with common names is that they often vary from place to place, so that puma, cougar, catamount, panther, painter and mountain lion all mean Puma concolor in various parts of America, while "panther" may also mean the jaguar Panthera onca of Latin America or the leopard Panthera pardus of Africa and Asia.
In contrast, the scientific names of species are chosen to be unique and universal; they are in two parts used together: A species is given a taxonomic name when a type specimen is described formally, in a publication that assigns it a unique scientific name.
The description typically provides means for identifying the new species, differentiating it from other previously described and related or confusable species and provides a validly published name in botany or an available name in zoology when the paper is accepted for publication.
The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens. Books and articles sometimes intentionally do not identify species fully and use the abbreviation "sp. This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology.