The following points highlight the three main concepts of species. The concepts are: 1. Morphological Species Concept 2. Genetic Species Concept 3. Biological Species Concept.
1. Morphological Species Concept:
This is the traditional concept of species. It was originally introduced by Carolus Linnaeus in his Systema Naturae in 1758. According to this concept, a population group of morphologically distinct organisms constitutes a species.
The main objections to this concept are that it does not take into consideration:
(i) The range of variation in size, colour, form & weight.
(ii) The genetic diversity
(iii) The common origin of related species and
(iv) The change of species in time.
Morphological species concept does not take into account of pronounced sexual dimorphism exhibited by different organisms. For example, the males and females of river duck, the mallard, were originally placed under separate species. The males were described as Anas boschas and females as A. platyrhyechos.
In many other birds like birds of paradise, humming birds, wood warblers etc. females differ more from the males of their own species than from the females of other related species. In many deep-sea angler fishes of the family Ceratioidei, the males are much smaller in size than females, live as parasitic forms upon them. Such dwarf males, originally mistaken as separate species, attach themselves by the mouth of the female and feed up their body fluids.
Insects and lower invertebrates like rotifers, echiuroids like Bonellia exhibit more pronounced morphological differences in the two sexes. Larval stages of several vertebrates and invertebrates are so markedly different from their parents that they were very often placed in separate species.
2. Genetic Species Concept:
According to Lotsy (1918) a species is a group of genetically identical individuals. This view was supported by some geneticists. But this definition of species is incorrect because even the off-springs of same parents have different genetic constitution. Only identical twins are genetically similar.
3. Biological Species Concept:
This is the modern concept of species proposed and developed by Dobzhansky in 1937 and Mayr in 1942. According to Dobzhansky mendelian population sharing a common gene pool constitute a species. The most uptodate and convincing definition of species has been given by Mayr (1942) to his book “Systematics and the origin of species”. According to him, species are the groups of actually or potentially interbreeding natural populations that are reproductively isolated from each such groups.
The general characters of animal species may be summarized as follows:
1. Each species possesses a common gene pool.
2. Each species is in a process of continually adjusting to its environment.
3. Each species fills an ecological niche not exactly utilized by another species.
4. Each species possesses a constellation of isolating mechanisms that indirectly or directly prevent exchange of genes with related species.
5. Each species has the capacity to give rise to new species.
Subspecies and varieties:
Subspecies is an aggregate of local breeding populations of a given species which has become recognizably different from another population of the same species. A subspecies usually differ from other similar breeding groups of the same species both taxonomically and with respect to certain gene pool characteristics.
The subspecies name is written immediately after species name. So the whole constitute a trinominal.
The term variety was used to describe the non-genetic variants of phenotype caused by the climatic effects. But now the term subspecies replaces the term variety.
The term ‘cline’ was introduced by Huxley (1939). It refers to a gradient (decrease or increase) within a continuous population. The regular or continuous variations occur for genotypes (genocline) or phenotypes (phenocline).
One of the best examples of clinal gradation is exhibited by the meadow frog of North America, Rana pipens. It is found throughout the Paire grass land and includes a number of temperature adapted races, which exhibit an orderly variability and adaptability. When the population of North and South extremes are compared the differences are pronounced, yet between the two extremes, there is no break in variation.
The term ‘Deme’ was introduced by Gilmour and Gregor in 1939. A deme is a community of potentially interbreeding individuals at a given locality which share a single gene pool. The term deme is always used with a prefix which characterize the deme more precisely.
1. Topedeme – a group of individuals existing in a certain geographic region.
2. Ecodeme – a group of individuals associated with a specific habit
3. Phenodeme – a deme differing phenotypically from others.
4. Genodeme – a deme differing from others in genotype.
5. Plastodeme – a deme differing phenotypically from others owing to the effect of environment.
Mayr (1952) has used the term sibling species for the sympatric populations that are morphologically similar if not identical, but are reproductively isolated.
Sibling species occur in almost all animal groups. But they are more common in insects. A best studied example is the genus Drosophila. Drosophila pseudoobscura and D. persimilis are so identical in their morphology that these were described as two races of the same species (race A and race B) by Lancefield (1924).
The salivary gland chromosomes of both these differ, in the arrangement of genes and also the banding patterns. Morphological differences also exist in the sex comb, male genitalia and relative wing size. These two races of flies coexist in nature over a wide area without natural hybridization.
Artificial crosses between race A and race B produce F1 hybrids, of which only females are fertile and the males are sterile. Lancefield found that sterility is due to the differences in the Y- chromosome. In race A, Y chromosome is sub-metacentric and in race B, Y is a metecentric.
With these observations, these biological races are now established as two distinct species and are described as ‘Sibling species’. In addition to morphological and chromosomal differences, these two species exhibit differences in ecological, physiological and sexual behaviours.
Monotypic and polytypic species:
Species containing only a single subspecies are called ‘monotypic’ while those containing more than one species are termed ‘polytypic’.