Species: Concept, Types and Intraspecific Categories

In this article we will discuss about Species:- 1. Concept of Species 2. Types of Species 3. Intraspecific Categories.

Concept of Species:

Linnaeus (1735) conceived “species” as a unchangeable unit. This monotypic or static concept prevailed till the 19^th century. The system of classification followed by Linnaeus is an artificial system. While defining a spe­cies only the morphological characters were considered by him. Later Lamarck (1809) and Darwin (1859) put forwarded their evolu­tionary thoughts.

As a result the monotypic concept was replaced by polytypic or dy­namic concept. The latter concept states that the species undergo modification in course of time, in order to adapt themselves to the ecological niches and may gradually form another species under favourable conditions.

But in recent years, the approach of biology has radically changed. Today, while ascrib­ing characters, physiological, genetic, ecologi­cal and phylogenetic points are taken into consideration. These new insights have moulded the idea of species.

Dobzhansky (1937) has defined the spe­cies as “a group of individuals which while passing through the ordeal of evolution has been physiologically and genetically incom­patible of inbreeding with other group of individuals”. Emerson (1941) proposed that “a species is that which has evolved by re­productive isolation and a genetically dis­tinct group of natural population”.

Mayr (1963) called a species as “groups of actually or potentially interbreeding natu­ral populations which are morphologically distinct and reproductively isolated from the neighbouring natural groups”.

Authori­ties on this line, however, laid much less emphasis on morphological distinctness but have given much emphasis on reproductive isolation. Mayr’s definition of the species is based upon the biological parameters, such as reproductive isolation and a common gene pool, hence it is called biological species concept.

Simpson (1961) viewed species as “a lin­eage (an ancestral-descendant sequence of populations) evolving separately from oth­ers and with its own evolutionary role and tendencies”.

This is the evolutionary con­cept of species in which the time factor has been added to and the concept has laid much emphasis on phyletic lineage. Christoffersen (1995) proposes that “a species is a single lineage of ancestral-descendant sexual populations genetically integrated by histori­cally contingent events of interbreeding”. This is the ontological species concept.

The most important finding has come from the studies of genetics. It is known that all the morphological, physiological and eco­logical characters are based on genes which are present over the chromosomes.

The difference between one species and another, rests on the genetic make-up. In certain cases, e.g., butterfly, it has been re­corded that individuals having similar chro­mosomal make up (i.e., members of the same species), exhibit differences in morphologi­cal features. Such variations within the same species are now considered as infraspecies, which are not taxonomically recognised.

It is propounded that the expression of charac­ters of a particular species is not due to a definite set of genes in one individual but a collective gene pool of all the inbreeding in­dividuals. The species today is thus regarded as “the sum of all its variations”.

With the new concept of species in mind modern taxonomists want to establish a natural system of classification.

The goal of which is:

(1) To establish correct interrelationship amongst the existing forms (horizon­tal classification) and

(2) To find out the relationship of exist­ing forms with their ancestral forms (vertical classification).

This has made the task more complicated. Two factors—difficulty in the proper assess­ment of characters and non-availability of suitable fossil materials, are acting as hin­drances to complete the classification.

For this reason in various instances (e.g., insect) morphological features and phylogenetic speculations are still used, which has resulted into the coming of a mixed system of classi­fication. The mixture is between natural sys­tem and artificial system.

Types of Species:

Various types of species are recognised, of which the followings are:

(i) Allopatric species (Gk. allos = other, patris = native land):

The two or more related species that have disjunct geographical ranges are called allopatric species. Examples of such species are Indian lion (Panthera leo persica) and African lion (Panthera leo leo).

(ii) Sympatric species (Gk. syn = with, to­gether):

Two or more species are said to be sympatric when their geographical distribu­tions overlap, though they may segregate into different ecological niche. Examples of this type are the fig-frog (Rana grylio) and the gopher frog (R. areolata). The former is ex­tremely aquatic, while the latter species is restricted to the margins of swampy areas.

(iii) Parapatric species:

These are the species which have the geographical ranges with a very narrow region of overlap. Example of this type is the flightless Australian grass­hoppers, Moraba scurra and M. viatica.

Remarks:

Parapatric species are formed in nature mostly through chro­mosomal rearrangements.

(iv) Sibling species:

Two or more than two closely related species which are mor­phologically alike but behaviourally or reproductively isolated from each other. Examples are Drosophila persimilis and D. pseudoobscura. The mosquito Anopheles maculipennis complex consists of several sub­species, of which a few are vector of malaria and the rest are harmless.

(v) Cryptic species:

The species which are alike on the basis of observed features but are genetically and sexually they are differ­ent are cryptic species. There is a confusion between the terms sibling species and cryp­tic species. The cryptic species are incapable of interbreeding but the sibling species can interbreed and are incapable of producing fertile hybrids.

(vi) Monotypic species:

When a genus in­cludes a single species but does not include any subspecies, e.g., Vampyroteuthis, a vam­pire squid which is a single monotypic genus and also contains a single species, V. infernalis (monotypic species). Blackwelder (1967) states that the species with a single subspecies, called monotypic species.

(vii) Polytypic species:

When a species con­tains two or more subspecies, it is called polytypic species. Examples are tiger, Panthera tigris which has several subspecies; such as—(i) Indian tiger, Panthera tigris tigris, (ii) the Chinese tiger, P. t. amoyensis, (iii) the Siberian tiger, P. t. altaica, (iv) the Javan tiger, P. t. sondaica, etc.

(viii) Endemic species:

The species which are found in a particular region, called endemic species. Usually the species of oceanic is­lands which are found in a limited geographic area are called endemic species. The Dar­winian finches are the endemic species of Galapagos Islands. The lungfish, Neoceratodus forsteri occurs in Mary and Burnett rivers of Queensland in Australia, is an example of endemic species.

(ix) Transient species:

Species among con­temporaneous organisms, fossil or recent, called transient species (Imbrie, 1957). Blackwelder (1967) has defined that the species are the ones existing contemporane­ously, as a cross section of the lineages of evolutionary species.

(x) Agamo species:

Species are those which consist of uniparental organisms. They may produce gametes but fertilization does not take place. They reproduce by obligatory parthenogenesis. In case of bees, wasps, rotifers the haploid eggs develop into haploid individuals and the haploid eggs are not fertilized by sperms.

(xi) Panmictic species:

Species in which a single interbreeding population occurs (Blackwelder, 1967).

(xii) Apomictic species:

Species in which there is mixing of gametes between different indi­viduals.

(xiii) Incipient species:

A natural population which are about genetically isolated from the rest of population of the parent species due to geographical barrier but has not accom­plished all qualities for reproductive isola­tion from the parent population.

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 19^th 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, com­bined 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.