A species is an ecological unit. The species is a group of potentially interbreeding natural populations which share in a common gene pool. But species are reproductively isolated from other such groups. A species comprises of several populations. There are two distinct ways in which new species arise from the pre-existing ones.
i) Splitting of the species into two or more species is called Speciation.
ii) Transformations of the old species into a new one in due course of time is known as transformation in time.
It is very interesting to note that a species population usually has Discontinuous distribution. The species populations may in due course of time become separated by distance. Populations ‘A’ and ‘Z’ of a given species grow in size generation after generation. As such, the organisms radiate into progressively larger territory. After few generations these populations A & Z at the opposite ends of the territory may be too off. Thereby the direct gene flow among their members is not possible and they do not have reproductive contact directly.
The distance isolation is most common. These are other kinds of geographic isolations. The groups of related organisms become separated by some physical barriers like Sea1 mountain, desert, river. Moreover, homing instinct and territoriality oL animals also add to the isolation of individuals into groups populations.
Different types of speciation: There are three different types of speciation
i) Allopathic speciation or Geographic speciation:
The allopathic species have originated from the geographically isolated populations of a species.
a) Geographic isolation of demes of widely spread out population of a species over a period of a number of generations.
b) Genetic differences are accumulated independently in each deme.
So in alloptiric speciation, the population of a species divides and evolve into two or more new species. The basic factor in alloptric speciation is the geographic isolation. So it is also known as geographic speciation.
Example In the Australian continent, a bird “Acanthiza pusilla” is widely distributed. It slightly differ from the Tasmanian species - ‘A. Ewingi’. It is presumed that during pleistocene glaciation, when the sea level is lower, Acanthiza entered Tasmania. With the rise of sea level it became isolated and differentiated into A. ewingi.
ii) Parapatric speciation White (1968-78) studied this type of speciation. He proposed that chromosomal aberration leads to partial reproductive isolation in the individuals of a population. Any chromosomal mutation which lowers fertility, gradually confers reproductive isolation in a very small population by genetic drift.
iii) Sympatric speciation: This type of speciation arises due to some biological barrier for interbreeding. Polyploidy in plants leads to sympatric speciation.
If the hybrid between the two diploid species becomes tetraploid, it will be reproductively isolated from its diploid parents. Because the triploid offspring’s produced by back crossing with high ratio of aneuploid gametes which fail to survive.
As a general rule, a species is composed of a number of allopatric breeding populations, which are physically separated from one another to pursue independent evolutionary path. Each population is exposed to a different selection pressure. Therefore, random mutations along with genetic drifts and selection pressure establish genetic difference. The morphological and physiological variations are also present in the similar populations. These differences gradually accumulate and cause more and more divergence in the genetic constitution of populations.
Finally distinct sub-species is established. The geographical races or subspecies given an opportunity to interbreed or crossed artificially these produce fertile hybrids. The genetic modifications appeared so far have not reached to the e4ent of producing reproductive isolation. These may still exhibit preference of mating to the members of their own group.
Addition of certain more variations in their gene pool so as to affect their interbreeding, leads to reproductive isolation. The groups or subspecies or races become reproductively isolated and are ranked as species. The establishment of reproductive isolation is an event of biological significance.
The process of speciation occurs only in allopatric populations. It cannot act on sympatric populations inhabiting the same area. The sympatric species can arise either due to changes in the chromosome number or due to introgressive hybridization. The change in chromosome number may occur by polyploidy, aneuploidy, haploidy or translocation.
Ex. Different species of ‘Drosophila’ have different number and Appearance of chromosomes.
The chromosomal composition of P. virilis is regarded to be the ancestral type. The chromosomal complement of 13. pseudo obscura and D. persimilis could tiave derived from D. virilis by a translocation between X chromosome and one of the autosomes. The chromosome compliment of D. melenogaster could be derived by two translocations between the two pairs of autosomes.
Transformation of species in Time:
In the second method of evolution, in due course of time the transformation of a species into a new species takes place. in this type of evolution only one species exists at a time. For example, species ‘A’ evolves into ‘B’ and ‘B’ into ‘C’ and so on.
Sympson has recognized two types of Transformations
i) Phyletic evolution ii) Quantum evolution
I) Phyletic evolution This type of evolution involves the sustained directional changes in the average characters of a population. This may be due to adaptations to a shifting environment or due to increasing specialization for a particular environment or improved adaptations in a constant environment. Thus the evolving forms present in a line of succession one being replaced by the other.It is presumed to lead to the origin of new genera and families.
ii) Quantum evolution It is related to the higher taxonomic groups like orders and classes. It involves rapid shift or sudden changes in the organization of a population to a new equilibrium. It is distinctly different from the ancestral forms and adapted to occupy new conditions. The quantum evolution is macro and mega evolutions operating above the species level.