Species
species (noun, pl. species;nomenclature term) – the lowest principal taxonomic rank of biological nomenclature; although most taxonomist agree that species represents real, evolutionary entities, no universal species concept exists and no general agreement has been reached how to define a species; different species concepts were developed over time and are promoted by different taxonomists; (1) phenotypic species concept: phenotypic species (sometimes also called morphospecies or morphological species) are defined by a similar morphology and / or anatomy that is represented across a group of organisms; a strictly phenotypic species concept is continued to be applied in groups, where other methods to distinguish species are difficult to apply, however, it is usually recognized that this concept is rather limited; (2) biological species concept: species are defined as populations of freely interbreeding groups, different species can thus be distinguished as groups, which are reproductively isolated from one another, thus even cryptic (or sibling) species can be recognized even though these species show now distinct morphological or anatomical differences; the biological species concept is strongly promoted by taxonomists studying organisms which predominantly or exclusively reproduce sexually; the concept has limited practical value in organisms that reproduce predominantly or exclusively asexually, especially if lateral gene exchange and frequent mutations allow for evolutionary variation that is not limited by sexual processes; (3) phylogenetic species concept: species are defined as evolutionary lineages that share a common ancestry (phylogeny) which may be deducted from morphological, anatomical, sexual, asexual, molecular or any other relevant evidence using cladistic methods; this concept is theoretically limited only by the ability to reliably deduct evolutionary lineages; thus, the concept heavily relies on (hypothetical) evolutionary models, e. g., some, especially traditional cladistic models do not adequately incorporate lateral gene transfer and other asexual evolutionary processes; (4) ecological species concept: ecological species are defined as functional ecological entities, thus populations of organisms that theoretically could reproduce with one another but occupy different habitats or environments may be regarded as different species; in practise the ecological species concept is limited by organisms with a very variable ecology that changes over time, ecospecies are thus difficult to delimit from one another; (5) polythetic species concept: polythetic species are defined as entities that can be recognized by a variety of characters such as morphology, anatomy, ecology, breeding systems or phylogeny, typically a combination, but not all of these characters must be present to define a polythetic species, scientists promoting this concept argue that it is the most integrative concept that can even be applied to viruses, i. e., objects that show some but not all characteristics of a living organism; limitations of this concept are obvious: since any combination of characters can be used to define a species on a case by case basis, the choice of these characters and thus the entire concept appears entirely arbitrary; nevertheless this is still effectively the way taxonomy distinguishes between different species.
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