Species concept

It is about all species concepts

1. Species Concept Typological Species Concept : Def. According to it species is “A very natural group of organisms hence a natural taxon in classification has an invariant generalized or idealized pattern shared by all members of the group ”. 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 . Noor Zada, M.Sc Zoology, KUST, Kohat

2. 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 recognized 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 recognized by their mor­phological features. This is also called mor­phological species concept . Noor Zada, M.Sc Zoology, KUST, Kohat

3. Criticisms Due to several phenomena such as sexual dimorphism, polymorphism, and age differences, the same species develop strikingly morphological differences. This concept is not applicable in case of sibling species because sibling species are alike but belong to differ­ent species. Noor Zada, M.Sc Zoology, KUST, Kohat

4. 2. Nominalistic Species Concept : Def. It believes that “Nature produces individuals and nothing more”. ( Linneaus 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 18 th  century and still now is used among some botanists. Noor Zada, M.Sc Zoology, KUST, Kohat

5. Criticisms 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 . Noor Zada, M.Sc Zoology, KUST, Kohat

6. 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. Def. It was presented by Dobzansky in 1937. He suggested that “species is a group of interbreeding natural populations, that are reproductively isolated from such other group.” Noor Zada, M.Sc Zoology, KUST, Kohat

7. 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 . These are: 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 . Noor Zada, M.Sc Zoology, KUST, Kohat

8. 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 . Noor Zada, M.Sc Zoology, KUST, Kohat

9. 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: 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 ’. Noor Zada, M.Sc Zoology, KUST, Kohat

10. 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. Noor Zada, M.Sc Zoology, KUST, Kohat

11. Hybridization: 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. Noor Zada, M.Sc Zoology, KUST, Kohat

12. Lack of information: Due to lack of proper information systematists face some problems when applied to some cases . 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. Noor Zada, M.Sc Zoology, KUST, Kohat

13. 4. Evolutionary Species Concept : Def. According to this concept, species is “a spatio temporal lineage of populations that evolves separately from other lineages and has its own ecological niche ”. 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 ”. Noor Zada, M.Sc Zoology, KUST, Kohat

14. 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 . Noor Zada, M.Sc Zoology, KUST, Kohat

15. 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 . Noor Zada, M.Sc Zoology, KUST, Kohat

16. Difficulties IN THE Application of Biological Species Concept Difficulty # 1. Insufficient Information: Sexual dimorphism, age differences, polymorphism and other such types of variations often give rise to doubts as to whether a certain morphotype is a separate species or only a phenon within a variable population . Proper studies of life-history, population analysis etc. can unmask such doubts. However, such difficulties are also faced by the neontologists who normally work with preserved material and by the paleontologists who also must assign phena to species . Noor Zada, M.Sc Zoology, KUST, Kohat

17. Difficulty # 2. Uniparental Reproduction : Self-fertilization, parthenogenesis, pseudogamy , vegetative reproduction, are some forms of uniparental reproduction that do not fulfill the criteria of interbreeding. As per definition, a population is an interbreeding group, and, therefore, the term population in “an asexual biological population” is a contradiction. The biological species concept based on the pre­sence or absence of interbreeding between populations is, therefore, inappropriate for uniparental reproducing organisms. Noor Zada, M.Sc Zoology, KUST, Kohat

18. Difficulty # 3. Evolutionary Intermediacy: It is important for a taxonomist to have a tho­rough knowledge of all stages of differentia­tion between the individual variant and the well- characterised distinct biological species. Many species pass through intermediate stages like biotypes, races, subspecies, ecotypes or semi-species. In such incipient speciation, populations will be found which are in the process of becoming separate species and have acquired some but not yet all of the attributes of distinct species. The taxonomist, thus, may encounter various difficulties which may result from such evolutionary intermediacy . Noor Zada, M.Sc Zoology, KUST, Kohat