A species represents the lowest taxonomic group, which can be defined with any degree e of precision. According to Biological Species concept, the species is a group of actually or potentially interbreeding natural populations, which share in a common gene pool, but are reproductively, isolated from other such groups.
WAYS OF DEFINING A SPECIES
Biological Aspect ��������������������������Definition
1. Breeding����������������������������A group of organisms capable of interbreeding and producing fertile offspring�s.
2. Ecological���������������������������A group of organisms sharing the same ecological niche; no two species can share the same ecological niche.
3. Genetic�����������������������������A group of organisms showing close similarity in genetic karyotype.
4. Evolutionary�����������������������A group of organism sharing a unique collection of structural and functional characteristics.������
New species arise by the establishment of reproductive barrier or reproductive isolation. The reproductive isolation is basic to the origin of species and actually appears by the accumulation of genetic differences, which are caused by mutations, recombination, hybridization, genetic drifts and natural selection.
Attributes of Species
1. Species is a genetic unit consisting of large, intercommunicating gene pool.
2. Species occupies a specific ecological niche, unoccupied or unutilized by another closely related species.
3. A species is an ecological unit.
4. Species interacts with the environment and with other species present in a given environment.
5. Allied species, though occupy separate areas of distribution but these are often adjoining.
6. Species have fully effective reproductive isolating mechanism.
7. Intermediate or transitional forms are usually absent.
Subcategories of Species:
Mutation and selection within isolated populations (dense) may produce following degrees of phenotypic variations within species. These are recognized as subcategories of species.
1. Demes- A deme is a local population of polytypic species, present in a given locality. All members of a deme share a single gene pool because of free interbreeding. Demes are open genetic systems that have gene flow from adjacent populations. The individuals of a deme have genetic similarities.
2. Geographical races- Populations, which are distributed over a wide geographical range or have occupied well-separated geographic habitats for a long time, usually exhibit distinct morphological or phenotypic differences. These differences are acquired n adaptation to climatic factors. These phenotypic differences are controlled genetically. It means climatic factors produce changes in gene frequencies within the gene pool. Such populations of a species showing phenotypic and genotypic differences in response to climate are called geographical races.
3. Ecological races- These are populations of a polytypic species, which occur in different ecological habitats but in adjacent geographical areas, and have developed phenotypic differences determined genetically.
4. Clines- These are the groups of local populations of a widely distributed species, which exhibit regular and gradual stepwise phenotypic modifications through out its geographical range. The regular or continuous variations are related with the eco-geographical variations. More than one clime may be exhibited by a species and they may run in opposite direction.
Speciation
The process of a genetically homogenous population into two or more populations that undergo genetic differentiation and eventual reproductive isolation is called speciation. According to Ernst Mayr, species originate in the following ways-
Modes of Speciation
1. Transformation of old specie into a new one in due course of time- Phyletic speciation or Autogenous speciation
2. Multiplication of species
��� A. Instant speciation by Chromosomal mutations, Autoploidy, Amphidiploidy
��� B. Gradual speciation by Sympatric speciation, Semigeographic speciation, Geographic or Allopatric speciation
