Faunal Resources In India: Species Concept
Contents |
Species Concept
This is an extract from FAUNAL DIVERSITY IN INDIA Edited by J. R. B. Alfred A. K. Das A. K. Sanyal. ENVIS Centre, Zoological Survey of India, Calcutta. 1998 ( J. R. B. Alfred was Director, Zoological Survey of India) |
Taxonomists are mainly concerned with the comparison of the species rather than individuals or parts of individuals. Therefore, it is very essential to define and delimit a species first. The species and process of speciation and the conclusions drawn by several systematists have acquired immense importance in view of the vastness of available literature. In the beginning, it was believed that the species was supposed to be created as separate entities as static and monotypic. At this stage, there was neither confusion nor any seriolls species problem. Subsequently, a species was considered as a "Polymorph variant or morphologically different population or geographically isolated population" It was therefore, with the advancement of knowledge on the species, a well defined and acceptable definition was essential to avoid confusion.
The origin of the concept of species can be traced back to Aristotle's Scala Naturae, (Scale of nature) with its root in "Phylogenetic scale" describing primitive and advanced groups, based on evolutionary arrangement. Linnaeus's Philosophica Botanica (1751) has laid the foundation to the constancy of the species or the doctrine of individual organisms within a species. llliger (1800) spoke of species as a community of individuals, which produce fertile offspring. The first to recognise the biological variation and the application of the term "Kind" (for species) was Voigt (1817); Oken (1830) and Gloger (1833), the latter gave the definition, species is what belongs together either by descent or for the sake of reproduction. Baer (1828) defined the species as "sum of the individuals that are united by common descent" Cuvier (1829) defined it as the assemblage descended from one another or from common parents and of those who resemble each other.
Darwin's "The Origin of Species" (1859), looked at the species from a viewpoint almost directly opposite to that of Linnaeus. Accordingly, the species concept was purely arbitrary and subjective. The theory of evolution presupposes that species are mutable, but mutability itself doesn't imply evolution. Species must be able not only to change but also to change in an orderly manner and with reasonable systematic limits, before they can be organised into an evolutionary sequence. Darwin in a letter to Hooker (Oct. 22. 1864) wrote that "The power of remaining constant for a good long period, I look at as the essence of species", thus recognising the morphological species concept.
Plate (1914) was the first to state that the "members of a species are tied together by the fact that they recognise each other as belonging together and reproduce only with each other" Mayr (1940) defines species as "group of actually or potentially interbreeding population, which are reproductively isolated from each other" Simpson (1943) gave the definition of a genetic species as a "group of organisms so constituted and so situated in nature that a hereditary character of anyone of these organisms may be transmitted to a descendent of any other"
Dobzhansky defined the species in various ways "there is a single systematic category which, in contrast to others, withstood all changes in nomenclature with an amazing tenacity and that is the category of species (1937)"; "as the largest and n.ost inclusive... reproductive community of sexual and cross fertilising individuals which share a common gene pool" (1950), and in 1951 he redefined the species as" the group of populations, the gene exchange between which is limited or prevented in nature by one or by a combination of several reproductive isolating mechanisms" Borgmeir (1963) describes species as "natural phenomenon and biological unit based on objective facts" Wilmoth (1967) is of the opinion that a species is "a well defined autonomous and persistent organic unit, living in a free state of nature, not grading freely into any other unit, and generally of less perfect fecundity outside than inside its limits" Recently, a species is defined as the unit or element of the biodiversily spectrum -from genes to large-scale ecosystems. On the one hand, species are the most recognisable expression of genetic diversity, at the same time; the species are the building blocks of ecosystems (Mc Neely et Ill., 1990).
Characters Of A Species
According to Mayr (1976), the species should have the following characteristics: 1 Species characters are adaptive 2 Species are evolved and evolving 3 Species differ genetically 4 Species differ ecologically
According to Slobodchikoff (1976), the species should have the following: 1 All organisms to be sorted into discrete groups (i.e., should have operational component) 2 Account for morphological, behavioural, ecological, physiological and genetic differences among populations included within a species. 3 Account for differences between population groups of different species 4 Apply to all forms of genetic continuity and types of reproduction 5 Consistent with the theoretical framework of evolution
But today, greater emphasis on the dynamic concept of species is given. Various attempts have been made to define a species, and different species concepts are recognised. Grant (1981) identified five different types of species viz: 1. Taxonomic species (Morphological species or Phenetic species); 2. Biological species (Genetic species); 3. Micro species (Agamospecies); 4. Successional species (Palaeospecies or Chronospecies) and 5. Biosystematic species (Ecospecies, Coenospecies). However, presently, the following well recognized species concept has been adopted in the following discussion, based on the literature available from different sources. 1. Nominalistic species concept The dogmatic belief is that the "nature produces individuals and nothing more" or the spontaneous generation of many highly organised animals, a pre-evolutionary theory of order, that of the scala naturae, usually assumed that all species formed a continuum and were inherited arbitarily and non delimitable (Simpson, 1961). The theory met with lot of critisism, as it believes in "nature produces individuals and nothing more, species have no actual existance in nature" 2. Typological species, (taxonomic species, morphological species or phenetic species) The concept mainly based on the work of Aristotle, accordingly to which each species has an intrinsic nature that make up the species, Le. has essential properties with defined characteristics and accidental properties that individual vary from one another. The taxonomists describe a given species based on the morphological properties, by designating a Type specimen, according to the International Code of Zoological Nomenclature. Thus a type specimen provides pragmatic starting point for all subsequent studies of species. A species name provides a convenient method of communicating information about a group of organisms. It is therefore, recognised as the morphological species concept. The main criticism comes from the fact that there are morphological differences due to sexual dimorphism, polymorphism, difference due to age and other form of individual variations.
The basic unit of Linnaean hierarchy is the species, of course composed of populations, a group of organisms defined in certain ways. All other groups above the level of species are themselves composed of one or more species. These groups, including species, are collectively known as taxa and each of them is a taxon. Taxon is defined as "group of real organisms recognised as a formal unit at any level of hierarchic classification. U A taxonomic category or simply a category is aclass the membersofwhichareall the taxa placedatagiven level inahierarchicclassification. Therankofacategoryiseitheritsabsolute positioninagiven hierarchic sequence of categories or its position relative to other categories. The rank of taxon is that of the category of which it is a member. An outgrowth of typological species concept is the phenetic species concept (Sneath and Sokal, 1973), which is mainly based on the morphological characters, but the relationships between characters and between organisms are analysed by computer algorithms to produce meaningful groupings of organisms. Hence, it is defined by Sneath and Sokal (1973), as "smallest cluster that can be recognized on some given criteria as being distinct from other clusters" The operational definition is " A species is a group of organisms not itself divisible with the phenetic gaps resulting from concordant differences in character (except morphs, such as those resulting from sex, caste or age differences) but separated by such phenetic gaps from other such groups" (Michener, 1970). Linnaeus System of Classification The Linnean hierarchy developed in the Systema Naturae is based on the grouping of organisms into classes owing to their shared characteristics. It is the arrangement of individuals into groups based on the similarities among them. It has a sequence of seven levels i.e., Kingdom Phylum Class Order Family Genus Species Since the method adopted by Linnaeus was not of very practical use in the case of larger falma and difficulties were felt in classifying the animals. As a result, it. \vas replaced by post-Linnaean taxonomists by another method known as upward classification. In this method, taxa are assembled into groups of similar species (or related species) forming a hierarchy of higher taxa by again grouping similar taxa of the next lower rank (Blackwelder, 1967). In recent times, the deficiency noticed in this practice has been rectified, by adding SUpL"-(above), sub -and infra (below) to them. A complete hierarchy used in classification of a large group of animals (Simpson, 1945) commonly known as "Hierarchical System of Classification" is given below. It has 20 levels. Use of all possible 5uper-, 5ub-and infra¬levels between kingdom and subspecies would give thirty-four levels, probably more than needed in practice. Therefore, the use of any particular number of levels is a completely arbitrary convention. Kingdom Phylum Subphylum Super class Class Subclass Infraclass Super order Order Suborder Infra order Super family Family Subfamily Tribe Sub tribe Genus Subgenus Species Subspecies
Thus, it can be seen that the functional objective of any classification is the power of organisation or grouping of things or concepts for storage and retrieval of information. As can also be seen from the above listing of heirarchies, the various catagories (Kingdom to subspecies) are arranged in heirarchial manner. This means, each level of classification embraces all of the features characteristic of the level below it. It also means that the features which characterise each level of the classification include not only those which define that level but also each of its higher levels. This system allows the quick diagnosis of an animal and sorting a newly discovered one into existing.
Use of seven basic Linnaen levels is required by convention, that is, no animal is considered to be satisfactorily classified unless, it has been placed implicitly or explicitly in some definite groups at each of the seven levels, i.e., use of any levels other than the seven mentioned above is optional and depends on the nature of the requirement by the author. One should carefully understand, that an individual never is and cannot be classified. Classification involves groups, i.e., what is clasSified is always a group or population, defined in the broadest sense as any group of organisms systematically related to each other. The concept of Linnaen species led to the mathematical formula by Ginsburg (1938) "a given population is to be considered a species with respect to another closely related population when the degree of intergradation (overlap of the observed samples) is not more than 10 per cent" The revival of this concept is taken up recently in "numerical taxonomy" Since numerical taxonomy is mainly based on the principles propounded by Adanson (1727-1806) it is often called Nee-Adansonian principle. Van valen (1988) defines species thus "a population is a species to the extent that most of its members share a set of criteria such as single origin and single extinction with reproductive continuity between these events, capacity to evolve, a mechanism to recognise other individuals or the gamates of the same species, and reproductive islolation from other species"
Classification And Naming
A taxonomist forms classifications employing a variety of methodologies. However, in all these methodologies, the basic tools of the taxonomist's job are attributes of the organism that he studies such as its structural features, external and internal morphology etc. For instance, birds are characterised by the presence of feathers, mammals by hair or fur, reptiles as well as fishes by scales (but fishes with fins and reptiles without fins) and amphibians by their skin lacking feathers, fur and scales. But all these animals are vertebrates because, of the possession of a backbone quite contrary to insects, spiders and worms, which lack a back bone (invertebrates). Yet, all these are animals. Therefore, all of them can be placed under a single kingdom called Animalia. Further, they can be segregated as vertebrates and invertebrates. All the vertebrates since they possess a backbone (a common character) can be grouped under one phylum, the phylum: Chordata. However, since, invertebrates donot possess such a common character, different groups of animals are placed under different phyla. Each phylum is further split into various categories, all based on character of animals, into class, order, family, genus and species. RAMAKRISHNA and ALFRED: Species L'oncepl 25 To illustrate, how this is done, classification of Amphibians is considered here. Amphibians comprise of tailed salamanders and newts, limbless snake-like forms and quadruped frogs and toads. Since, salamanders and newts retain tail even in their adult stage, they are grouped under the order named Caudata. Since frogs and toads do not retain tail in adult stage, they form a different order namely, Anura. The snake-like amphibians that donot have legs are grouped under the order Gymnophiona (Apoda). Each of these orders can further be divided into families. In this context, for convenience in explaining, we will confine our discussion to the Anuran amphibains of the Western India in identification key to its families, as shown below: 1 Upper jaw toothed, tongue bifid 2 Jaws toothless, tongue entire 3 2 Digits with an intercalary cartilage between penultimate Phalanges Family RHACOPHORIDAE Digits without intercalary cartilage Family RANIDAE 3 Pupil horizontal, tongue pyriform, skin tubercular Family BUFONIDAE
Pupil circular, tongue oval, skin smooth or granul;u Family MICROHYLDAE Each of the above families can be further split into genera. We will for convenience consider here the family Bufonidae alone as shown in the key to the genera of Family Bufonidae of Western India, as shown below: 1 Parotid glands absent Genus Ansonia Parotid glands present 2 2 Fingers webbed, discs present Genus Pedostibes Fingers free, no discs Genus Buto
Members of each of the above genera can again be categorised into distinct species as shown below in the key to the speies of the Genus Bufo of Western India.
1. Head without bony ridges 2 Head with bony ridges 4 2. Tymphanum very small or indistinct, toes fully webbed Species beddomeii Tymphanum larger 3 3. Tymphanum nearly as large as eye, toes webbed at base Species '/Olioills Tymphanum 2/3 diameter of eye, toes 2/3 or 'h webbed Species sloma/klls 4. Parietal ridges present 5 Parietal ridges absent 7 5. Size large, first finger longer than the second 6
Size small, first finger equal to second Species frrgllsonii
6. Dorsum of skin genrally smooth but with a few tubercles behind angle of jaws, limbs and flanks Species silentvaleyt'1lsis
Dorsum of skin warty Species parietalis
7. Tymphanum 2/3 diameter of eye Species metallostrictlls 26 Fmmal Resources in India Tymphanum small less than 'h diameter of eye Species miaotymphnllllm The identification key (as shown above) divided into two parts (couplets) at each stage is known as the dichotomous key. The species key presented above covers all the species of the genus Bllfo known from the Western India. If the character of the species of Bllfo collected from Western India does not fit into the above key, the identification key of Bltfo from other regions should be consulted and yet, if it does not fil into any of the keys pertaining to Bufo, it is a pointer that the species in question is likely to be new and hitherto unknown species. In such a case, a detailed study of the specimen is required to ascertain its own species specific characters and once ascertained, it can be described as a new species and the existing key to the identification further modified appropriately to accomodate the new species. Having come this fact, let us now examine how a particular species fit into a classification. We will take Bufo melanosticus, as a typical case. Kingdom ANlMALIA Phylum CHORDATA Subphylum VERTEBRATA Class AMPHIBIA Order ANURA Family BUFONIDAE Genus Bllio Species me/anostictus Depending on the groups and individual cases dealt with, it may be sometimes necessary to bifurcate the above taxonomic categories further. Taxonomists use different kinds of keys to separate and segregate characters in such a way as to provide a series of alternative choices. This includes dichotomous key, bracket key, indented key, serial key, branching key, circular or the box key, depending on their convenience of arrangement. RAMAKRISHNA and ALFRED: Species concept 3. Biological species (Genetic species) The definition that stress on the reproductive isolation and community of gene pools are usually considered as the biological species. Mayr (1969) stresses that the biological species concept should have the following three criteria viz., I. Species ilre defined by distinctness rather than by difference. 1 Species consists of population rather than unconnected individuals. 2 Species are more unequivocally defined by their relation rather to non conspecific population (isolation) than by the relation of conspecific individuals to each other. The decisive criteria is not the fertility of individuals but the reproductive isolation of the population.
The term biological species existed as early as 1828 (Von Baer), accordingly the species is "the sum of individuals that are united by a common descent" Plate (1914) defines "the members of a species are tied together by the fact that they recognise each other as belonging together and reproduce only with each other" Dobzhansky (1950) defines the species as the largest and most inclusive reproductive community of sexual and cross fertilising individuals which share a common genepool. Mayr (1963) defines species as "groups of actull/ly or potentill/ly interbreeding nllturnl popuilltions which lire reproductively isolated from other groups", and further in 1976, redefines biological species based on the non dimensional (reproductive gap) and multidimensional (gene flow) species concept. Florkin (1964) gave the biochemical definition for the species as "groups of individuals with more or less similar combinations of sequences of purines and pyramidine bases in their macromolecules of DNA, and with a system of operators and repressors leading to the biosynthesis of similar amino acid sequences. This was later modified by Dobzhansky (1970), thus "a biological species is an inclusive Mendelian population, integrated by the bonds of sexual reproduction and parentage" Barton (1989) defines species as "a group of population possessing inherited differences and prevent gene exchange with each other such groups" In contrast to the Mayr's isolation concept, Patterson (1988), Patterson and Lambert (1984) recognises "Species Mllting Recognition System" (SMRS), which are specific morphological and behavioural traits that allow an individual of a sexual species to differentiate between potential partners and organisms belonging to every other reproductive community.
The isolation concept of Mayr and recognition concept of Patterson are regarded as complementary, instead of alternative (Templeton, 1989). He further presents his own concept regarding the species. According to Templeton (ibid), species is "the most inclusive group of organisms having the potential for genetic and/or demographic exchangability" The Biological species concept has created more confusion in plant taxonomy than in animals, for eg., in the genus Epilobium with about 160 species, interspecific hybridisation is very common, therefore, according to biological species concept the whole group is to be placed under a complex species. It should be noted that all populations tend to vary and no two populations are alike, variations observed are adaptive and has !}urvival value, variations shown by the individuals within a population must be heriditary if successive generations are to be modified from ancestral types, the forces of nature results in extinction and survival, and lastly the environment of the individuals must not be static. Biological species concept solely depends on the population studies which involves fertility tests and such tests are not applicable to all groups. Further to this, there are certain interspecific and intergeneric crosses, which pose several problems to the evolutionary biologists. For this reason, Sokal and Crovello (1970) concluded that the biological species concept is "neither operational nor heuristic nor of practical value" They are of the opinion that the phenetic species concept is the most appropriate one to be associated with the taxonomic category "species" It should be noted that the genetic species concept of Hugo de Vries, Lotsy, Shull, Bateson and other Mendelian followers is only a typological species concept that had nothing to do with the biological species concept (Mayr, 1963). The biological species concept is not perfect, as Wilson (1992) notes, "the concept has been corroded by exceptions and ambiguities." According to Nelson (1995), the species (or the populations of a species) at some point in its evolutionary history may not be reproductively isolated. Hybridisation between species does occur, and in some cases especially in plants population may partially interbreed enough to produce a good many hybrids (semi -species) on a persistant basis. 4. Agamospecies or microspecies Agamospecies are species of uniparental organisms (Ghiselin, 1974). Normally, ordinary mutations, or micromutations cannot produce new species in sexually reproducing organisms. The situation is different in uniparental condition, this is well established in botany by Mayr, (1969) [as in Hawkweed (Hierncillm) and Hawthorn, (Crataeglls)] who considers morphologically distinct clones as microspecies. In case of animals, Genermont (1980) recognises five modes of uniparental reproduction viz., 1) Asexual (rotifers, cladocerans and nematodes) 2) Self fertilisaton as in parasitic hymenopterans (Telmomlls lariai) and trematodes (Didimozoon, Wedlia bipartita) 3) Thelitoky (Le. Meiotic and ameiotic parthenogenesis) 4) Gynogenesis as in Dugesia (Planaria ), LumbridlIlIs (Oligochaete), Poedla formosa (fish), Ambystoma (newt) and 5). Hybridogenesis (Poecillopsis, a freshwater fish). Mayr (1987) considers these uniparentally reproducing organisms as a separate type of taxon and suggests the name Paraspecies, which fall within Simpson's concept of evolutionary species Minelli (1993). 5. Biosystematic species (Ecospecies, Coenospecies) Turesson (1922) was the first person to recognise differences between Linnaen species, (the species based on the natural population, which he calls ecospecies) and the species based on the experimental studies on the population as biosystematic species or coenospecies. He later in 1936 contended that most species are mosaics of "discontinuous adaptational" ecotypes and this was mainly based on the field experiments with plants in strikingly different habitats. He further believes in the ecospecies concept that the phenotype responds as a unit to the environment. Van valen (1976) while supporting the Turesson (1922) held that species are maintained for the most part ecologically and not reproductively and according to him the assumptions are 1) The genes are of minor importance in evolution 2) The control of evolution is largely governed by ecology and the constraints of individual development and 3) Selection acts primarily on phenotypes, which are the building blocks of the community. Hiesey (1945) redefined the coenospecies as "all the ecospecies so related that they may exchange genes among themselves to a limited extent by hybridization" However, these two differences are not accompanied by satisfactory definitions and hence these are also called "species in statu nascendi" or intermediate entities. Bernardi (1980) and Stace (1989) considers these as biosystematic categories or taxonomic categories of evolutionary systematics.
Plant taxonomists believe that the ecotype or ecospecies represent more precisely the biological description of geographic variation rather than the division of species into subspecies. Ecotypes according to Mayr (1969) are rarely discontinuous, rarely well defined and often polyphyletic and always full of intraecotype variablity. The ecotype concept, thus, suffers from the same weakness as that of the subspecies concept.
In animal taxonomy, the term ecotype is rarely used and prefer to use the term "ecological race", which is defined as the adaptation of local population to the local habitat. These in recent terminology, are recognised as sibling species (Mayr, 1963; Stebbins,1981) or ecophenotypes (Le.,the nongenetic modifications of phenotypes). Mayr (1963) defines sibling species as "morphologically similar or identical natural population that are reproductively isolated" Thus sibling species can be recognised by biological differences, such as host specificity, habitat specificity, fecundity, voltinism, diapause, longivity, temperature tolerance etc. Taxonomists, thus are unable to recognise sibling species in comparison to field biologists or ecologists. For this reason, those who adhere to purely morphological species concept usually refer to sibling species as "biological races" Savanna sparrow (Passerculus sandwichensis ), Redwing (Agelaius phoeniceus) and the Swainson's Warbler (Limnothlypis swainsoni) are some of the examples of ecological races cited by Mayr (1969).
Taxonomic Diversity Within The Species
Due to the absence of a clear definition on the species and the difficulty in draWing conclusion on the closely related species (species in statu nascendi), Bernardi (1980) and Stace (1989) considers these as biosystematic categories in animals and include the following types :
(A) Good species -forming a monophyletic group, the mutual relationship of which offers some peculiarities but specialed from the same slock. (B) Sibling species -also known as microspecies (Gates, 1951), biospecies (Scheel, 1968) showing the absence of any morphological differences between the species. Mayr (1963) defines sibling species as morphologically similar or identical natural populations that are reproductively isolated. (C) Semi species also known as allo species (Amadon, 1976) having closely related allopatric species. Mayr(1963) applies the term semispecies to conspecific population which are partially reproductively isolated, perhaps to the extent of being thought of as incipient species. (D) Coenospecies or coenogamodeme (Gilmour and Heslop-Harrison, 1954) -wherein the genetic isolation between closely related species is not complete and irreversible.
Accoring to Minelli (1993) members of two comparia cannot be crossed, because they are separated by genetic incompatability barriers. This comparia consists of two or more coenospecies. Coenospecies belonging to same comparium can be artificially crossed, which produce sterile F, hybrids. Similarly, at the subspecific level, ecospecies can be subdivided into several ecotypes. Members of a subspecies share a unique geographical range or habitat, a group of recognizable genetically controlled characteristics, morphological or molecular, and a unique natural history as compared to other subspecies. Since subspecies are not distinct species, they are reproductively compatible and will periodically interbreed with adjacent subspecies. All subspecies have the potential to acquire suitable adaptations to their specific ecological habitat and the longer they are separated the more cumulative adaptation we might expect. All subspecies also have the potential to one day evolve into new species as suggested by Charles Darwin in "On the Origin of Species" in 1886. Avise and Ball (1991) suggested that valid criteria for subspecies include concordant distribution of multiple independent genetic traits. These can be morphological or molecular or both.
In contrast to the above, certain other categories are errected in taxonomic levels between the species and the subspecies. These include quasi species (partial infertility whereever two forms live in sympatry); vice species having sterility barriers in some areas of sympatry but no barriers in other areas; citra species (Dujardin, 1965) or mega subspecies (Amadon and Short, 1976). These are monophyletic groups of subspecies of the same species, morphologically and geographically isolated from other species within the species.
Subspecies, Races and Varieties Progress in science has led to an increasing appreciation of diversity within species, as well as understanding of the complex patterns of distribution. Species vary in space and time Le., variation of animal species occur in non isolated, incompletely isolated, temporarily isolated or isolated local populations and in the course of their evolution. The geographical distribution and variation was well understood after Darwin's Theory of Natural Selection. The term "variety" was first used by Linnaeus (1751) in his Philosophill botanica and stated that there are as many varieties as there are different plants produced from the seed of the same species. The variety is thus defined as "nongenetic modification of the phenotype" Linnaeus included the nongenetic variety for "races" of domestic and intra population genetic variants in animal kingdom. Race is thus synonymous with the subspecies, however, "race" is also colloquially used to designate populations or aggregates of populations within formally recognised subspecies (Mayr, 1969). Local populations that are conspeciously adapted to a local habitat are often referred as "ecological races" Contrary to this, morphological, ecological as well as physiological characterstics of adaptation of infraspecific units are known as "geographic races" Ornithologists in the later part of the nineteenth century began to recognise geographic races as taxa in classification and began to use them as trinomials (Schlegel, 1844) and the subspecies concept was first defined in ornithological circles (Allen, 1877). According to Mayr (1985) subspecies "is an aggregate of local populations of a species, inhabiting a geographical subdivision of the range of the species, and differing taxonomically from other populations of the species" Bohme (1978) states that subspecies are those cases where "intra-specific evolutionary divergence can potentially give rise to new species" Grant (1981) considers subspecies as groups of interbreeding populations with strong morphological differences, combined with geographical, ecological, edaphic or physiological distinctions which give such groups a species like distinctiveness.
The definitions of ecological races and geographic races which are local populations of intraspecific units show possible phylogenetic trend in a polytypic species (Grant, 1960). It should be noted that the characters expressing infraspecific divergences should be adaptive characters necessary for evolutionary process. Few taxonomists are also of the opinion to avoid the usage of subspecies (La greca, 1987; Minelli, 1993). Therefore, there is great deal of confusion at the level of infraspecific taxonomic hierarchy and whether subspecies or varities are to be recognised or not. As far as the plant taxonomists are concerned there is a great deal of usage of intraspecific level as can be seen in the works of Hamilton and Richards (1992), who fOlmd nearly 8% of the plant species discussed in 26 major journals published between 1987 -1990 subdivided into subspecific entities. Of these subdivided species, 42% were divided into subspecies, 52% into varieties, 3% into formae and 3% into infraspecific taxa of two or more levels. Minelli (1993), the Australian systematist observes that infraspecific taxa are more commonly recognised in developing countrires, using European classic reference as source and adds "given that, experimental evidence shows monotypic variation in response to environmental variables such as predators, pH, temperature, and naming of infraspecific taxa is neither justified nor useful" He prefers to call them ecotypes rather than subspecies. At the same time, there are certain defenders for the retention of the intraspecific level in groups like odonata, orthoptera, lepidoptera, hymenoptera, a few families of coleoptera, birds and mammals. In animal taxonomy, according to lnternatonal Code of Zoological Nomenclature, the Article 16 and Article 45 relates to species groups, which include taxa at the rank of species and subspecies. The application of names, formation, treabnent and infrasubspecific names are all in accordance with the provisions of various articles of the code. The zoological code does not recognise the names of the taxa of infrasubspecific rank i.e, a scientific name proposed expressly as the name of a "variety" or "form" after 1960, is excluded from Zoological Nomenclature (Article Ib (5); Article -16, but those designated. before 1961, to be regarded as sUbspecific names (Article 45 g). Hybrids
The term hybrid is normally applied to the individuals that are the offsprings of a cross between two good biololgical species (e.g., horse and donkey resulting in the sterile mule or hinnie). Plant hybrids are common and many stable hybrids are known in nature. Knobloch (1972) lists nearly 3,000 intergeneric crosses among a list of 23,675 intergeneric and interspecific hybrids. Hunt (1986) describes nearly 70,000 hybrids derived from 1100 natural species in orchids. In animal kingdom Grant (1992) reports that 9.5% of the bird species are known to have bred in nature with another species and produced hybrid offspring. Bullini (1985) observed exensive hybrid species in grasshoppers, stick insects, weevils, plant hoppers, freshwater snails, planarians, fishes, newts, frogs, and lizards. The¬question now arises about the topology of the phylogenetic tree in these hybrids. 6. Phylogenetic species Lamarckian theory (1809) elucidates that all taxa have arisen by evolution and are in phylogenetic continuum. Darwin's theory of evoution (1859) recognised that the taxa originated by evolution and evolutionary phylogeny could explain the order that had already found among organisms. The concept is based on the direct geological evidence and indirect deductions based on geographical distribution. It is now possible to estimate the time taken for one. species to split into two "good" Ones or for a species to change sufficiently for systematics to regard it as having become a different one. The concept includes the origin and linkage based on monophyly or polyphyletic lineages. Monophyly includes members arising from the common ancestor, but not all lineages• arising from that ancestor must be members of the designated category. Polyphyletic lineages include species that arise from two or more immediate ancestors, but which does not include the ultimate ancestor of these multiple immediate ancestors (Ross, 1974). Recently, biologists (Maddison and Maddison, 1992; Wiley et al., 1991) reconstruct the pattern of events that have led to the distribution and diversity of life; this is known as cladistics or phylogenetic systematics. The basic idea behind cladistics is that members of a group share a common evolutionary history, and are "closely related," more so to members of the same group than to other organisms. These groups are recognized by sharing unique features which were not present in distant ancestors. These shared dt!rived characteristics are called synapomorphie. There are three basic assumptions in cladistics:
1. All groups of organisms are related by descent from a common ancestor. It essentially means that life arose on earth only once, and therefore all organisms are related in some way or other. 2. There is a bifurcating pattern of dadogenesis. The most controversial assumption that, new kinds of organisms may arise when existing species or populations divide into exactly two groups. 3. Change in characteristics occurs in lineages over time. The convention is to call the "original" state of the characteristic plesiomorphic and the "changed" state apomorphic.
Conclusion
The key feature of the species based approach is their emphasis on analysing population sizes and geographic distribution of individual species to identify conservation priorities. The species based approach emphasizes biological individualism (Le., stressing the value of individual species), where as ecosystem based approach emphasizes the importance of interaction between genes, species and biophysical processes (Nelson, 1995).
In reality, there are only two recognised systems of species concept currently in use viz., typological and biolgical species, the former relies more on type designation recognised by the International Code of Zoological Nomenclature and the latter on the reproductive isolation between groups of organisms. Since both the concept have their inherant advantages and disadvantages, the species concepts should have to be dynamic, allowing organisms to be sorted into discrete groups; accounting for morphological, physiological, behavioural, ecological and genetic similarities among populations induded within a species as well as account for differences between population groups of different species. It should also apply to all forms of genetic continuity and types of reproduction, yet be consistant with the theoratical framework of evolution. With this view in mind, Slobodchikoff (1976) introduced the terminology "Selection Species Concept" Accordingly, a species is a system of genetically similar individuals and populations maintained as a cohesive unit by a set of selection pressures that balance the dismptive forces imposed by environmental factors, mutation or genetic recombination.
