In this article we will discuss about the modes and mechanisms of origin of species.
Modes of Origin of Species:
The main modes of speciation are:
1. Sympatric speciation:
This is the origin of species without geographic isolation. Sympatric species originate by instantaneous development of isolating mechanisms within a deme. Once reproductive isolation is established each population follows its own evolutionary course and forms sympatric species.
2. Allopatric speciation:
This is also known as geographic speciation. Geographically isolated populations are known as allopatric. Such isolation provides the opportunity for each population to evolve along its own lines.
3. Stasipatric speciation:
This is the origin of species through chromosomal rearrangements. An ancestral species gives rise to a number of descendents species through chromosomal rearrangements such as centric fusions, centric fissions, inversions and translocations.
4. Instaneous speciation:
This is also called as cytocatalytic speciation. It is the origination of a single (e.g. alloploid) individual which is reproductively isolated from the species to which its parents belong. It is reproductively and ecologically capable of establishing a new population.
5. Saltational speciation:
This is the origin of species by multiple structural changes of the karyotype.
Mechanism of Origin of Species:
There are two distinct ways in which new species arise from the preexisting one:
1. Splitting of species
2. Transformation of species
1. Splitting of species:
Suppose species A is ancestral. During the course of evolution, it will give rise to species B and specie C.
2. Transformation of species:
In this type of evolution only one species exists at a time. As for example, species A evolves into species B and B into C and so on.
According to Simpson, there are two types of transformations:
(a) Phyletic evolution:
This involves the sustained directional changes in the average characters of a population. This is caused either due to adaptations to shifting environment or due to increasing specializations for a particular environment or improved adaptation in a constant environment This results in the origin of new genera and families.
(b) Quantum evolution:
It involves rapid shift or sudden changes in the organization of a population to a new equilibrium, distinctly different from the ancestral forms and adapted to occupy new conditions. This results in the origin of higher taxonomic groups such as orders and classes. That is, quantum evolution is macro and mega evolution operating above species level.