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 19th 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.
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:
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, 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.