- Allopatric speciation
- Sympatric speciation
- Quantum speciation
- Parapatric speciation
(i) Allopatric Speciation:
When populations become geographically isolated they are exposed to different environmental conditions. After enough time such isolated group will have accumulated so many genetic differences and develop a separate gene pool. Therefore, they will be reproductively isolated if they come into contact again. If isolation is complete, the populations form new species. These populations never interbreed. This is the allopatric model of speciation. The species, which are formed by geographic isolation, are called allopatric species. The process of formation of allopatric species is called allopatric speciation.
According to this, the first step of speciation is to split a population into two or more completely isolated sub-populations. The isolation may be caused migration or by geographical events. E.g. the populations on islands are separated by water, in lakes, mountain tops and in patches of forests surrounded by Savannah or fields surrounded by forests.
Examples of Allopatric Speciation
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Finches of Darwin:
The Galapagos Island is formed as volcanoes emerged from the sea within the last 5 million years. About 3 million years ago a small group of birds (Finches) from the south or Central America colonized atleast one of the islands. After the population had established they moved into other islands. The ecological condition was varied among the islands. These geographically isolated populations encountered different selective forces and differentiated. The birds of differentiated population did not interbreed. These differentiated population formed a new species due to geographic isolation. Thus, finches on the Galapagos Islands suggested to Darwin, the idea of descent with modification from a single anscestor species.
(ii) Sympatric Speciation:
The related populations that occupy side by side in the same geographical area are called sympatric population. The transformation of sympatric population into new species is called sympatric speciation.
As the sympatric population lives in same place, they compete for their food, space and requirement of life. In addition to competitions, the other factors like mutation, recombination, polyploids, hybridization, natural selection, genetic drift etc. operate to bring about genetic divergence and consequently establish reproductive isolation. Thus there is not interbreeding with the parent population and new species are formed in the same geographical area.
(iii) Quantum Speciation:
Grant (1971) defines quantum speciation as the budding off a new daughter species from a semi-isolated peripheral population of the ancestral species.
(v) Parapatric Speciation:
This type of speciation is intermediate between allopatric and sympatric speciation.
Bush (1975) defines that a parapatric speciation may occur whenever a species evolved as a continuous population in a continuous cline.