It is about all species concepts

1. SPECIES CONCEPT
1. 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 18th 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
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 bio-
logical 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