The following points highlight the four important species concept. The important species concept are: 1. Typological or Essentialist Species Concept 2. Nominalistic Species Concept 3. Biological Species Concept 4. Evolutionary Species Concept.
1. Typological Species Concept:
According to this concept, there are a number of diversities on the surface of the earth that exist as a limited number of universals or types. These types do not bear any relationship to each other. The universals or types are called species. Variation is considered as trifling and irrelevant phenomenon.
This concept, was in the philosophies of Plato and Aristotle and was the species concept of Linnaeus and his followers. Cain (1954, 1956) regarded the above concept as the morpho-species concept. Another group of scientists refer to this as essentialist species concept because the members of a taxon or the species can be recognised by their essential characters.
This is why essentialist ideology is also referred to as typology. Again morpho-species or morphological species concept states that one species can be segregated from another species by physical features and can be recognised by their morphological features. This is also called morphological species concept.
Simpson (1961), Mayr (1969) and recent scientists have not accepted the above concept totally though it has some positive points:
(i) Due to several phenomena such as sexual dimorphism, polymorphism, and age differences, the same species develop strikingly morphological differences.
(ii) This concept is not applicable in case of sibling species because sibling species are alike but belong to different species.
2. Nominalistic Species Concept:
Occan, the proponent of this concept and his followers (Buffon, Bessey, Lamarck, etc.) believed that only individuals exist but do not believe in the existence of species.
Species are man’s own creations and have no actual existence in nature. They are mental concept and nothing more. Therefore, such mental concept (i.e., species) of man has no value. This concept was popular in France in 18th century and still now is used among some botanists.
Simpson (1961), Rollins (1965) and Mayr (1969) stated that no biologists can agree with the idea that man cannot produce species and it is the established fact that the species are the products of evolution.
3. Biological Species Concept:
Due to some incompleteness in the above mentioned concepts and continuous pressure from the naturalists, a new concept the biological species concept emerged in the middle of 18 century. The concept took a number of years to get its foot in the soil of biology.
K. Jordan (1905) first gave the definition of biological species concept. Later Mayr proposed the biological species concept in 1940, 1942, 1949. According to this concept, “a species is a group of interbreeding natural population that is reproductively isolated from other such groups”. Mayr explained that a species has three following properties.
1. Reproductive community:
The individuals of a species seek each other as potential mates for the purpose of reproduction and the members form a reproductive community.
2. Ecological unit:
The members of a species differ each other for many features but all members together form a unit, interact as a unit with other species in any environment.
3. Genetical unit:
The members freely interbreed consisting of an intercommunicating gene pool, whereas the individual is merely a temporary vessel holding a small portion of the contents of gene pool.
This definition of biological species concept has accepted by Dobzhansky (1951) and Hanson (1981) especially for two reasons— gene pool and reproductive isolation.
Dobzhansky, Ayala, Stebbins and Valentine (1977), have postulated more or less same definition. According to them, a species as a single or more Mendelian populations between which the gene exchange is limited or prevented by reproductive isolating mechanisms.
Most modern taxonomists and evolutionists consider the biological species concept as the widely accepted species concept because the maximum workers apply this concept during their work. This concept has no fixity, and always changeable and has the potentiality for modifications required by the evolution.
Paterson (1985) has proposed a definition which can overcome some defects present in the biological species concept. According to him, “a species is a population of biparental organisms, the members of which share a common fertilization system”. Mayr (1988) has remarked that Paterson’s species concept is not error-free and is based on the misinterpretation of the biological species concept.
Though Mayr’s biological species concept is widely accepted to the zoologists but the- shortcomings of the concept are criticised by the evolutionists when applied to certain groups:
(i) Lack of information:
Due to lack of proper information systematists face some problems when applied to some cases.
(a) The morphological differences are observed due to sexual dimorphism, age differences and genetical polymorphism and individual variation can be unmasked through the study of life history and through the population analysis. The taxonomists mostly work on preserved museum specimens. So reproductive isolation is not verified in the preserved specimens. Again biological species concept is not applicable in fossil specimens.
(b) The closely related two populations live in a continuous area but show preferences for different habitats. In this case, two populations fail to interbreed due to living in different habitats. So it is difficult to apply the biological species concept on these populations because these populations are either distinct species or failure of interbreeding due to living in different habitat.
An example of drongo birds is recorded in central Africa. Species A, Dicrurus ludwigii are found in the evergreen rainy forest areas and species B, D. adsimilis are found in the open grassy land areas. They live in two ecological niches with a distance of 50 m apart and do not interbreed.
(ii) Apomictic or asexual groups:
Biological species concept is not applicable in apomictic species (i.e., asexually reproducing groups) that do not fulfil interbreeding criterion which is the most important characteristic feature in biological species concept. Apomictic groups show uniparental reproduction by parthenogenesis, apomixes and budding, etc.
Uniparental reproduction is seen in lower invertebrates and lower vertebrates. The descendents of apomictic groups are termed agamospecies or binoms, paraspecies but Ghiselin (1987), Mayr (1988a) stated that these are not considered as ‘species’.
To solve this dilemma, Simpson (1961), Mayr (1963, 1969) and M.J.D. White (1978) discussed the problem on the basis of discussion of Dougherty (1955) and Stebbins (1966).
Attempts to define agamospecies or asexual species with or without using the word population have not been successful. There are well defined morphological discontinuity among the uniparentally reproductive organisms. These discontinuities are produced by natural selection among the various mutants which occur in asexual clones.
Sibling or Cryptic species:
Biological species concept is not applicable in sibling or cryptic species because members of sibling or cryptic species are all alike, not separated morphologically but reproductively isolated populations.
Incompleteness of speciation:
Evolution is a gradual and continuous process. To attain a new species, especially three attributes are necessary, such as reproductive isolation, ecological difference and morphological differentiation. There are many species which represents an incomplete stage during speciation. To apply the biological species concept in these cases becomes difficult.
According to biological species concept, two good species fail to interbreed. If the reproduction isolation breaks down, the two good species interbreed and produce fertile hybrid.
4. Evolutionary Species Concept:
Not all taxonomists specially palaeontologists are not satisfied with the biological species concept. They preferred a definition of species which are related to the evolution.
Simpson (1961) has proposed a definition with many modifications that is “an evolutionary species is a lineage (an ancestral- descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”.
Simpson has stated that the above definition not only is consistent with biological or genetical concept of species but it helps to clarify and to remove some limitations of the biological species concept. Mayr (1982) has stated that the above definition is related to the phyletic lineage, not indicates a species concept.
The evolutionary concept is applicable only to the isolated population and incipient species but not applicable to a single species. Simpson tried to solve the species definition by adding the time dimension in this species definition. Reif (1984) and Mayr (1987) have stated that there are many demerits in evolutionary species concept.
Wiley (1978) has provided a revised definition of evolutionary species concept. He stated that “an evolutionary species is a single lineage of ancestral-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”.
Mayr and Ashlock (1991) stated that the concept has developed on the basis of a species taxon, not of the species category.
Christoffersen (1995) proposed the ontological species concept that is “a species is a single lineage of ancestral descendant sexual populations genetically integrated by historically contingent events of interbreeding”. This definition of Christoffersen has given stress on the interbreeding nature of a species.