A species is a reproductively isolated population that shares a common gene pool and a common niche. This definition defines a species reproductively, genetically, and ecologically. The definition of a species given above as taken partly from Mayr, is somewhat idealistic. Since it assumes sexual reproduction, it leaves the term undefined for a large class of organisms that reproduce asexually. Biologists frequently do not know whether two morphologically similar groups of organisms are “potentially” capable of interbreeding. Further, there is considerable variation in the degree to which hybridization succeed may under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist:
1. In practice, species are defined by easily recognizable phenetic characters that reliably indicate what species an individual belongs to.
2. The biological species concept defines a species as a set of interbreeding forms. Interbreeding between species is prevented by isolating mechanisms. Principal authors- John Ray, Buffon, Dobzhansky (1935); Mayr (1942). Such species are also termed as biospecies. The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.
Organisms may appear to be alike and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella manga) look almost identical to one another, yet do not interbreed with each other-thus, they are separate species according to this definition. Organisms may look different and yet be the same species.
3. The ecological species concept defines a species as a set of organisms adapted to a particular ecological niche or a lineage (or closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range.
4. The phenetic species concept defines a species as a set of organisms that are sufficiently phenetically similar to one another.
5. The biological, ecological, and phenetic (and several other) species concepts are all closely related, and are concerned to explain or describe much the same fact: that life seems to come in the form of distinct species.
6. Individuals mainly interbreed with other members of their own species because of isolating barriers that prevent interbreeding with other species. Isolating barriers can be prezygotic or postzygotic.
7. Geographic variation can be adaptive or neutral. The amount of genetic variation among geographic races of a species can be described quantitatively and is low in human beings relative to other species.
8. The theory of evolution justifies population thinking rather than typological thinking about intraspecific variation: all individuals in a population are equally good members of a species, rather than some being better specimens than others.
9. Character displacement occurs when two species have partly overlapping geographic ranges and the two species differ more in sympatry than in allopatry. Character displacement may be caused by ecological competition.
10. The biological species concept explains the integrity of species by interbreeding (which produces gene flow), the ecological concept by selection. The two processes are usually correlated, but it is possible to test between them in special cases. Selection can be strong enough to overcome gene flow, and selection can maintain a species integrity in the absence of gene flow.
11. Taxonomic entities such as biological species may be real or nominal. According to the biological species concept, species can be real, but lower and higher taxonomic levels are nominal. According to the ecological species concept, all taxonomic levels can have a similar degree of realism.
12. Cladospecies: Set of organisms between speciation events or between speciation event and extinction or a segment of a phylogenetic lineage between nodes. Upon speciation the ancestral species is extinguished and two new species are named.
13. Evolutionary species: is a unit of evolution or evolutionary group. A lineage (an ancestral-descendent sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies (Simpson).
14. Genetic species: Group of organisms that may inherit characters from each other, common gene pool, reproductive community that forms a genetic unit
15. Typological species: A group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties. The clusters of variations or phenotypes within specimens (ie: longer and shorter tails) would differentiate the species. This method was used as a “classical” method of determining species, such as with Linnaeus early in evolutionary theory. However, we now know that differentphenotypes do not always constitute different species (e.g.: a 4-winged Drosophila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies.
16. Morphospecies: Species are the smallest groups that are consistently and persistently distinct, and distinguishable by ordinary means. Contrary to the received view, this was never anything more than a diagnostic account of species. Morphospecies is a population or group of populations that differs morphologically from other populations. For example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the beginning of recorded history. This species concept is much criticised because more recent genetic data reveal that genetically distinct populations may look similar and, contrarily, large morphological differences sometimes exist between very closely-related populations. Nonetheless, most species known have been described solely from morphology.
16. Nothospecies: Species formed from the hybridization of two distinct parental species, often by polyploidy.
17. Phylospecies: The smallest unit appropriate for phylogenetic analysis, the smallest biological entities that are diagnosable and monophyletic, unit product of natural selection and descent. A geographically constrained group with one or more unique apomorphies (autapomorphies).
18. Phenospecies: A cluster of characters that statistically covary, a family resemblance concept in which possession of most characters is required for inclusion in a species, but not all. A class of organisms that share most of a set of characters.
19. Taxonomic species: Specimens considered by a taxonomist to be members of a kind on the evidence or on the assumption they are as alike as their offspring of hereditary relatives within a few generations. Whatever a competent taxonomist chooses to call a species.
20. Ring species are species with a geographic distribution that forms a ring and overlaps at the ends. The many subspecies of Ensatina salamanders in California exhibit subtle morphological and genetic differences all along their range. They all interbreed with their immediate neighbors with one exception: where the extreme ends of the range overlap in Southern California, E. klauberi and E. eschscholtzii do not interbreed. So where do we mark the point of speciation?
21. Chronospecies are different stages in the same evolving lineage that existed at different points in time. biological species concept-for example, It is not really Obviously, chronospecies present a problem for the possible (or very meaningful!) to figure out whether a trilobite living 300 million years ago would have interbred with its ancestor living 310 million years ago.
22. Phylogenetic / Evolutionary / Darwinian species: A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from a divergence becomes one another: when such sufficiently clear, the two populations are regarded as separate species.
23. Microspecies: Species that reproduce without meiosis or fertilization so that each generation is genetically identical to the previous generation.
