Mayr (1970) defined speciation as the creation of species. Speciation can also be defined as creation of two or more species from one. Among different evolutionary principles, speciation plays a major role. The entire evolution depends on the origin of new populations from their ancestors and since it is from their ancestors, it is difficult to understand many of them in detail due to lack of sufficient resources. Before we discuss the speciation process, the definition of the term species is presented; species is one of the basic units of biological classification and a taxonomic rank and is often defined as a group of organisms capable of interbreeding and producing fertile offspring.
According to Mayr (1970), true speciation or multiplication of species may occur by the following agencies:
- A) Instantaneous speciation (through individuals)
- 1) Genetically
- a) By single mutation in asexual species
- b) By macrogenesis
- 2) Cytologically, in partially or wholly sexual species
- a) By chromosomal mutations or aberrations (translocations, etc.)
- b) By polyploidy
- 1) Genetically
- B) Gradual speciation (through populations)
- 1) Geographical speciation
- 2) Sympatric speciation
However, the process of speciation is possible by four different mechanisms viz., allopatric, parapatric, sympatric and quantum speciation discussed below.
Allopatric Speciation
It is also called geographic speciation. Allopatric isolation is a key factor in speciation and a common process by which new species arise. In this scenario a
population splits into two geographically isolated populations by some geographic barrier, such as mountain range or river for terrestrial organisms, or a land mass for aquatic organisms. The isolated populations are then liable to diverge evolutionarily over many generations as
- (a) they become subjected to dissimilar selective pressures
- (b) they independently undergo genetic drift
- (c) different mutations arise in the two populations.
When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes. The allopatric speciation is the almost exclusive mode of speciation among animals, and most likely the prevailing mode even in plants, and is now quite generally accepted (Mayr, 1970).
Darwin’s finches is the best example for allopatric speciation. The Ground finches are numbered from 1–7. These finches look for food on the ground or in low bushes. The tree finches are numbered from 8–13. They survive mostly on insects.
- 1) Large cactus finch (Geospiza conirostris)
- 2) Large ground finch (Geospiza magnirostris))
- 3) Medium ground finch (Geospiza fortis)
- 4) Cactus finch (Geospiza scandens)
- 5) Sharp-beaked ground finch (Geospiza difficilis)
- 6) Small ground finch (Geospiza fuliginosa)
- 7) Woodpecker finch (Cactospiza pallida)
- 8) Vegetarian tree finch (Platyspiza crassirostris)
- 9) Medium tree finch (Camarhynchus pauper)
- 10) Large tree finch (Camarhynchus psittacula)
- 11) Small tree finch (Camarhynchus parvulus)
- 12) Warbler finch (Certhidia olivacea)
- 13) Mangrove finch (Cactospiza heliobates)
Parapatric speciation
In this mode of speciation, a small population enters into a new habitat, but differs in physical separation between these two populations. Individuals of each
species may come in contact or cross habitats from time to time. The best-known example of incipient parapatric speciation occurs in populations of the grass
Agrostistenuis which span mine tailings and normal soils. Individuals that are tolerant to heavy metals, a heritable trait, survive well on contaminated soil, but
poorly on non-contaminated soil. The reverse occurs for intolerant populations. Gene flow occurs between sub-populations on and off mine tailings, but hybridisation is inhibited by slight differences in flowering time between the two locations (McNeilly and Antonovics 1968).
Sympatric Speciation
In the case of sympatric speciation two or more descendant species form, from a single ancestor and these occupy same geographical locality. In this kind of speciation even a small amount of gene flow may eliminate genetic differences between parts of a population. Numerous examples for this type of speciation are observed in the invertebrates, particularly the insects; the insects that becomes dependent on diverse host plants in the same area. Cichlids in East Africa is one of the examples and thought to be due to sexual selection.
Quantum Speciation
Grant (1971) defined quantum speciation as “the budding off a new and very different daughter species from a semi isolated peripheral population of the
ancestral species in a cross fertilisation organism”. This speciation frequently occurs through adaptive radiation, by the discharge of genetic variability surrounded by ecologic islands. Quantum speciation is speedy and requires only few generations. The ancestors of the new species do not include a large proportion of the population, and may consist of only one or few individuals. In quantum speciation genetic drift plays a key role.