Speciation and its Types:
Speciation is the origin of new species. A species comprises of several populations. Interbreeding is very frequent among the individuals of a population and is occasional among the populations of a species whereas interbreeding is absent among the individuals of different species. There is a free gene flow within the members of a population and a free gene flow could be maintained among the members of different populations of a species, provided they have an opportunity to interbreed. But free gene flow between two species does not occur on account of marked differences in their genotype. It means new species arise by the establishment of reproductive isolation.
Types of Speciation:
Speciation is of the following two types-
- Allopatric Speciation or Geographic Isolation.
- Sympatric Speciation or Reproductive Isolation.
(1) Allopatric Speciation by Geographical Isolation- When a large population of a species is geographically isolated into two or more groups, each group accumulates genetic variations in its gene pool and natural selection operates separately on each group’s gene pool. In due course of time, these groups become more and more different in their genotype and phenotype and finally become different species. This splitting of a species into two or more new species is called allopatric or parapatric speciation and such species are allopatric or parapatric species. This means allopatric species occur in different geographical areas and allopatric speciation is caused by geographical isolation.
Populations are not uniformly spread out. A species having a wide range of distribution does not form one large randomly mating population because of distance. The below figure shows how two populations of a species may become separated by distance in due course of time. Populations A and Z of a given species grow in size generation after generation. Their offsprings radiate into the progressively larger territory. After some generations, the individuals of these populations at the extreme ends of the territory fail to interbreed.
Example of Allopatric Speciation (Darwin’s Finches)- Darwin studied the climatic conditions, fauna and flora of the Galapagos Islands (a chain of 14 islands in the Pacific Ocean) during his voyage around the world. These are nearly 600 miles away from the West Coast of South America. These islands have different environmental conditions. Darwin called these islands a living laboratory of evolution. He collected nearly 20 varieties of small birds, which differed mainly in shape and size of beak and colour of plumage. These birds are called Darwin’s finches. The general bird fauna of these islands shows affinity with American species but Darwin’s finches were reported from the South American mainland. Therefore, Darwin concluded that the American mainland species were the original ones from which different forms migrated to different islands of the Galapagos and adapted to the environmental conditions of these islands. These adapted forms are considered to be the new species.
At present, there are thirteen species of finches specialized to feed on cacti, seeds and insects. Some insectivorous finches have evolved behavioural features common to woodpeckers like long pointed beaks and clinging feet. None of them resembles the parental seed-eating finches of the South American mainland.
Mots probably, a few seed-eating finches or perhaps a single previously mated female had reached Galapagos islands from the mainland in the distant past. In the absence of competition, the population of finches diversified and became adapted to different habitats or ecological niches and evolved into new species. Thus geographical isolation has led to the development of reproductive isolation and thereby to the origin of new species. This type of speciation is called allopatric speciation (evolution of species occupying different geographical areas).
(2) Sympatric Speciation by Sudden Reproductive Isolation- When a few individuals of a species within the same geographical area suddenly become reproductively isolated from the main population, it forms a new species. This is called sympatric species and its origin as sympatric speciation. Thus sympatric species arise in the geographical area of their parent’s species. Polyploidy and hybridization are two main methods of introducing sudden reproductive isolation responsible for the origin of sympatric species. Such species are common in plants and rare in animals without geographical isolation.
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