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.
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.

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 ver-

tebrates. 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.

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.

Intraspecific Categories of Species:

Subspecies:
Linnaeus used the term “subspecies” when he classified subgroups of
man. He recognised four subgroups or variations such as (i) the
American-Indians (Homo sapiens americanus), (ii) the Europeans
(Homo sapiens europaeus), (iii) the Orientals (Homo sapiens asiaticus)
and the African Negroes (Homo sapiens afers). Subspecies is a
deviation from the type of species.

Early taxonomists applied the term ‘variety’ indiscriminately for any

variation in the population of a species. In the 19th century the term
subspecies replaced ‘variety’ and the term ‘variety’ is obsolete today.
Subspecies is actually a category below species.
When a population of a species splits up by natural barriers such as
mountains, islands, climate, etc., each isolated group may evolve
different characteristic features, so as to become recognizable as a
separate geographical race or subspecies.

With the publication of Charles Darwin’s “Origin of Species” in 1859,

an impetus was spread among later taxonomists because the book
provided many examples of variations or varieties. A lot of names
appeared in the Zoological Nomenclature for the same species. So
Wilson and Brown (1953) proposed the abolition of trinomial
nomenclature which is considered as subspecies concept.
Grant (1960) has defined the subspecies as “the groups of
interbreeding populations with some morphological differences, com-
bined with geographical, ecological or physiological distinctions which
give it species-like distinctness”.

The scientific name of the race (subspecies) of Indian lion is Panthera

leo persica, and the name of the African lion (race) is P. I. leo. The
distinguishing features of Indian race are—(i) scantier mane than that
of the African race (ii) a longer tassel of hair at the tip of the tail than
that of the African race (iii) a well-pronounced tuft of hairs on the
elbow joints and (iv) the abdomen bears a fuller fringe of hairs. Two
subspecies (races) of the same species can interbreed if they meet and
professional taxonomists can only recognise the differentiating features
of the subspecies of a species.

With the establishment of polytypic concept (Beckner, 1959), it is well

accepted that some species are distributed in different geographical
areas and form different local populations.

It is widely accepted that genotypic variation within allopatric species

occurs. It is widely accepted that these populations become different
from each other in morphology, biochemical or genotypic variations
that help to mark a taxonomic level sufficient to designate them as
subspecies.

If species which contain two or more than two subspecies, are called
polytypic species and the species which is without subspecies is called
monotypic species. All species are not polytypic and some are
monotypic. The polytypic concept clearly states that many recognised
morphospecies are not reproductively isolated and hence not separate
species; they are not considered as subspecies.
The Supra- and Infraspecific categories

Polytypic species (Rassenkreis): Rensch (1929) proposed the German term Rassenkreis (which
literally means “Circle of races” for those species, which have two or more geographically isolated
interbreeding populations.

Superspecies (Artenkreis): This German term literally means “Circle of species”. They are
monophyletic groups of closely related and largely or entirely allopatric species, distributed over a wide
range. Mayr (1931) introduced the term “Superspecies” for such closely related but distinct species that
have apperently evolved from a common ancestor. Examples of superspecies are paradise
magpies (Astrapia) in the mountains of New Guinea, e.g. Astrapia nigra, A. splendidissima, A. mayeri, A.
rothschildi and A. stephaniae. Species of Anopheles maculipennis complex in Europe are also
considered superspecies.

Sibling species: They are true sympatric species that are morphologically identical or nearly so but are
reproductively isolated. For example, Drosophila pseudo-obscura and D. persimilis are sibling species.

Subspecies: This term was used by Schlegel in 19th century as a replacement for variety. Subspecies
is a geographically isolated (allopatric) population of a species that differs morphologically from other
populations of the species but does not exhibit reproductive isolatA trinomen is used to designate a
subspecies. For example, Cervus elephuselephus is the continental red deer and Cervus elephus
scoticus is the red deer of Great Britain

Variety: This was the only subdivision of species recognized by Linnaeus, who used it to identify any
variation from the “type” and for individual variants or for anything that did not fit in a fixed pattern of a
species. The term is no longer used and is not recognized as a valid taxonomic category.

Deme: This is a minimal interbreeding local population unit of a species which share a single gene pool.
Demes live in most suitable areas that are separated by regions of unsuitable conditions and are
affected by the gene flow from adjacent areas.
Cline: This is an evolutionary concept proposed by J.H. Huxley (1939) and is defined as a gradation in
measurable characters. Cline is formed by a series of contiguous populations in which a given character
changes gradually. Two opposite ends of the series may be very different but difficult to be called
subspecies due to the absence of geographical isolation. The terms geocline (geographic), ecocline
(ecological) and chronocline (succession lines) are also used.

Race: This is not a recognized taxonomic category. They are local populations of a species which are
affected by the local conditions and therefore develop morphological differences. For example human
races.

Hypodigm: All specimens personally known to taxonomists at the time of describing species and used
by him collectively as a sample on which his inferences are based.