Concept of Species

Various definitions of species have put forwarded by various workers. In ancient time and even upto 19^th century species were regarded as “man made categories of thought”. Taxonomists, evolutionists and geneticists have given different definitions of species.

1. Typological/Morphological Species Concept (Linnaeus): -

According to the taxonomists, a morphologically distinct population or a group of morphologically distinct organisms constitute a species. Therefore, a morphological species can be defined as “a group of individuals that resemble each other in most of their morphological characters, and which segregatethem from all other organisms of adjacent populations”.

The morphological species concept is not applicable in case of sympatric population and in allpatric populations. Stricking morphological differences may occur in sexual dimorphosim, larval stages, polymorphic stages of organisms of same species. Morphological species concept is also helpless in case of sibling species.

2. Nominalistic species concept (Ocam): -

This concept is of Ocam and his followers, who believed that only individual exists, while species are man’s own creations. This concept was popular in France in the 18^th century and it is still used rarely by some botanists. Now, it is established fact that species are the product of evolution. Two brothers are identical twins because they derived from same zygote. Thus, member of a species are similar to each other because of common heritage. Thus, any one who understands evolution will outright reject this concept.

3. Genetical Species Concept:

Lotsy (1918) and some other geneticists define species purely on the genetic basis. According to this concept “a species is a group of genetically identical individuals.”But the definition is incorrect because except for identical twins no two individuals are genetically similar.

4. Biological Species Concept: -

Biological concept was introduced to replace the faulty morphological species concept and has been propounded by Dobzhansky in 1937 and Mayr in 1942. According to Dozhansky “species are the more inclusive Mendelian populatios, which share in a common gene pool”. Mayr (1942) has given the most complete and very practical definition of species as “species are the groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups”. This definition treats the species as a dynamic unit, a stage in the process of evolution and not a ffixed entity; the main feature of distinctness between species being the reproductive isolation.

Drawbacks:

1. The definition is no-dimentional and is not applicable to the species living in geographical isolated areas or at different times.
2. The definition is not workable to the museum taxonomists and paleontologists, who work with dead specimens and fossils.
3. This definition is limited to the sexually reproducing organisms. Asexual organism can never be tested by this criterion

5. Evolutionary Species Concept:

Grant (1971) emphasized on evolutionary species concept to include asexual organisms. The same view point was also expressed by Simpson (1961) and defines species as “A species is a series of ancestor-desscendent populatios passing through time and space independent of other populations, each of which possesses its own evolutionary tendencies and historical fate”. The evolutionary species concept was most popular among paleontologists and is used extensively in comparative biology and phylogenetic systematic.

There is no empirical criteria by which evolutionary tendency of historical fate can be observed in a given fossil sample.

6. Phylogenetic Species Concept:

Phylogenetic species concept was proposed by Cracraft in 1983. According Phylogenetic species concept “. A species is the smallest diagnosable monophyletic group of populations within which there is a parental pattern of ancestry and descent”. Thus, according to this concept diagnosable geographic forms of the same basic kind of bird should be treated as distinct species, because those forms have evolved separately, and have unique evolutionary histories. This method is applicable even to unusual reproductive modes other than sexual reproduction.

The main drawbacks of this concept is that Phylogenetic species concept gives no indication as to what characters to use while delineating a species, nor does it defines as to what level of divergence constitute aspecies.

8. Ecological Species Concept:

Colinvaux (1986) emphasized on the ecological species concept. According to this concept “A set bof organisms exploiting a single niche is called a species”. Ecological niche creates discrete zone which creates dicretes phenetic cluster that recognize as species because ecological and evolutionary processes controlling the division of resources produce those clusters.

Drawbacks:

1. Niches tend to be assumed and are difficult to define completely. Many taxa expoloit overlapping resourses, or can suddenly swith if aresourse become scare.
2. Widespread species generally have local populations that differ in niche occupation.
   1. In cichlids within asingle set of offsprings from the same parents, individuals have been found to occupy different niches.
   2. Sympatric species occupy the same niche cannot be grouped together as the same species.

9. Recognition Species Concept:

This concept has been put forwarded by Paterson (1985) and Lambert et al (1987) as a replacement to the biological species concept. According to this concept species is “A group of organisms that share acommon fertilization system and are known to recognize one another as potential mates”.

Drawbacks:

• 1. Not applicable to asexual species.
• 2. Recognition species concept cannot be applied to the fossils.
• 3. It is difficult to know whether geographically isolated populations can potentially interbreed or not.
• 4. Sometimes species fail to recognize their own kind and attempt mating or actually mate with other species, i.e., hybridization.

These species concepts considerably overlap each other; for some organisms one definition is more suitable than another and for some the definition will coincide. In many cases the biological distinctness is primary and the morphological differences is secondary (Mayr, 1957). In 1995 Christoffersen defines a species as “an irreducible cluster of sexual organisms within which there is a parental pattern of ancestory and descent and that is diagnosably distinct from other clusters by a unique combination of fixed characters.

Aberrant species concepts:

Which species concepts are not attented by too much by the zoologists are called aberrant species.

1. Agamospecies: Agamospecies are asexual taxa. Cain (1954) defined agamospecies as “those forms to which the biological species concepts cannot apply because they have no true sexual reproduction.”

2. Nothospecies: Wagner (1983) defines nothospecies as “species formed by the hybridization of two sexual species”.

3. Compliospecies: According to Harlan et al. (1963) a compliospecies is a “species that plunders the genetic resource of another species through introgressive hybridization.”

4. Quasispecies: Quasispecies are clouds genotypes that apper in a population at mutation –selection balance (Bull et al. 2005).

Other species terms:

Monotypic Specis and Polytypic species:

Species that are not divided into subspecies are called monotypic species whereas species that contain two or more subspecies are called polytypic species. The term polytypic species was first coined by Beckner in 1865.

Eg. of monotypic species: Red panda is the only species of genus Ailurus: A.fulgens.

Eg. of polytypic species: Drosophila is a polytypic species – D. melagester, D. persimilis, D. pseudobscura.

Allopatric and sympatric species:

Organisms/species originating in or occupying different geographical areas are termed as allopatric species. Allopatric species do not overlap in their distribution. Again if the origin or area or occupation of two or more closely related species is the same, than they are called sympatric species.

Allotopic and Syntopic species:

If two species may have the same geographical distribution but not coexist in the same locality (macrohabitat) and they cannot interbreed than they will be termed as allotropic species. And if the two or more closely related species occupy the same locality and are observably in close proximity and there is a possibility of interbreeding than these species are called as syntopic species.

Allohospitalic and Synhospitalic species:

Eicher (1966) coined the terms allohospitalic species and synhospitalic species to the parasitic forms. Allohospitalic –if two or more related parasitic species do not occur in the same host are called Allohospitalic species. Synhospitalic: if two or more related species occur in the same host are called Synhospitalic species.

Parapatric and Peripatric species:

Closely related species exhibit a geographic distribution but their ranges do not significantly overlap but are immediately adjacent to one another. Such distributions are called parapatric distribution and the species are called parapatric species. In peripatric distribution a small fraction of population become geographically isolated from the parent population (Founder effect) and become a new species in course of time.

Cryptic species and Sibling species:

Saize and Lozano (2005) have defined cryptic species as “two or more groups of organisms that are morphologically indistinguishable from each other, yet found to belong to different evolutionary lineages”. They are also reproductively isolated from each other. They may be parapatric, sympatric or sometime allopatric. Eg. The African bush elephant and the African elephant are the cryptic species.

Sibling species are defined as the sympatric population that are morphologically similar but are reproductively isolated. These populations cannot be classified as separate species, but are called “biological races”. These biological races are indistinguishable from each other except for slight morphological differences. Mayr (1942) used the term “sibling species” to these biological races. Sibling species is found in Drosophila, ie, D. pseudobscura and D. persimilis are so identical in their morphology but the salivary gland chromosomes are different in their structure. These two races coexist in the same geographical region, without occurring natural hybrids.

Significance of sibling species:

These provide an opportunity to test the validity of biological species concept with regard to morphological species concept.

1. These are of great practical importance in applied biology, medical entomology and in agricultural pest control.

2. Sibling species help in the understanding of the process of speciation.

Paleospecies Chronospecies:

Fossil species are called paleospecies. They may represent remains of several species.

A chronospecies is a species which changes in course of time on an evolutionary scale and the species could not be classified as the same species. Throughout the change, there is only one species in the lineage living at any point in time.

Superspecies (Artenkreis):

The German term Artenkreis literally means “Circle of species”. The term Superspecies is applied to a monophyletic group of allopatric organisms, or two populations, or two varieties, species or taxa.

Infraspecific categories:

Variety:

On typological principles each species has a fix pattern. Anything that do not fix into the idealized pattern is variety. The variety is not used by animal taxonomists, but botanists and baceteriologists still use the term.

Deme:

The smallest unit of population that has evolutionary significance is a group of individuals of same species or sub species and they are so localized that they are in frequent contact with each other. They are assemblage of taxonomically closely related potentially interbreed individuals of a species of a paticular locality.This minimal population is called deme (Simpson, 1961). It has no nomenclatural status.

Morphotype:

The aggregation of particular variations within population is called morphotype. These are the “distinguishable sympatric and synchronic interbreeding populations of a single species” (Edward, 1954).

Cline:

The term cline was coined by Huxley, 1939. Cline is a gradient of measurable characters. The term cline has been applied to the situations where a character varies more or less continuously with a gradual change in environmental terrain (Volpe, 1985). Different types of character give different clines in different directions within the same species. At present different cline concept are used in taxonomic literature. These are – geocline for geographic, ecocline for ecological, chronocline for successional clines etc. The term geocline is most frequently used in our modern taxonomy.

Race:

Races are equated with subspecies in taxonomy. In general the group of variant individuals within a species and differing slightly in characteristics from the typical members of the species is called race. In terms of Dobzhansky “races are populations which differ in the relative frequencies of gene or chromosomal structure. Races are simply geographical aggregates of populations with a slight genetic divergence from the original group of species or sub species. Howwever, the race is neither designated as a subspecies nor recognized in the taxonomic hierarchy.

Subspecies:

The term subspecies was first introduced by the ornithologist H. Schlegel in 1844 into the zoological nomenclature giving rise to the trinominal nomenclature and later it was approved by International Congress of Zoology for inclusion in ICZN. Presently it is the lowest taxonomical category and is defined as “a geographically separate aggregate of local population of the species”. In other words, subspecies are geographical races within the same species which are sufficiently different from the original species. The subspecies of a species are groups of interbreeding populations with strong morphological differences, combined with geographic, ecological, edaphic or physiological distinctiveness (Grant, 1960).

Some taxonomists designated subspecies on the basis of morphological, geographical and ecological characteristics. Edward (1955) explained different subspecies as follows –

1. Geographical subspecies: Macrogeographically isolated synchronic infraspecific populatios which are separated during mating time but under sympatric condition individuals of respective infraspecific population would cross breed freely and normally.

2. Temporal subsprcies: Therse are sympatric populations and are temporally isolated during mating season but whose members crossbreed freely and normally if the populatios become synchronic under natural condition.

3. Seasonal subspecies: If two distinct populations or aggregates of individuals of same species mature at different respective times during the same calendar year (e.g., one in spring and the other in the fall) with no period of time during which reproductive forms of both demes coexist, then interbreeding can occur between the members of the respective populations.

4. Annual subspecies: If the members of one distinctive population or aggregates of population witin a given species mature only during different years from those of another population of the same species, then therespective populations might be termed annual subspecies, if temporal isolation between their reproductive forms is complete.

5. Geological subspecies: Populations which function during different geological times respectively, have absolutely no chances of becoming synchronic. Hence the test of how freely and normally their members could interbreed is purely conjectural.

6. Ecological subspecies: Distinctive, different, macrogeographically sympatric, infraspecific population or aggregate of populations which are are isolated microgeographically, but whose members would crossbreed rather freely and normally if the populatios were to become microgeographically sympatric under natural conditions. These include different niches, biotopes or populations of biotopes. These are the faunistic maps of the sub species.

7. Polytopic subspecies: When subspecies of a species differ in a single diagnostic character like colur, size, or pattern, it may happen that several different and somewhat widely separated populations independently acquire an identical population. Although such visually identical populations are different genetically, yet since the subspecies is not an evolutionary concept, these are combined by taxonomists into a single subspecific taxon called polytopic subspecies (Mayr, 1969).

Shortcomings of the subspecies concept:

Some of the problems faced by the taxonomists in designating a population as a subspecies are –

1. What is the level of differences to recognize a population as subspecies?

2. It ignores about the intermediate populatios.

3. It ignores about the process of demitation of subspecies against the adjacent subspecies.

4. When should geographical isolates be called species and when subspecies?

Despite certain shortcomings the subspecies is still the most efficient way of referring to geographic separation of the species population.