Speciation is a process in which a new species diverges from an original species. Genetic differences caused by different evolutionary processes such as mutation, natural selection, gene flow and genetic drift causes new species to be formed. Speciation can be categorized as follows:
Allopatric
speciation:
When a geographical barrier such as mountains or water bodies causes new species to form, this is called allopatric speciation. The geographical barriers can cause the two populations to separate and prevent the gene flow. The different environmental selection pressures in these two populations results in formation of different phenotypes and therefore become different species which can no longer interbreed to make fertile offspring. For example, different species of Galapagos finches form in different islands as islands are separated by geographical barriers and each island has its own environmental selection pressure and different food source for finches. Finches with thin and elongated beak are formed on an island that has cacti as food source whereas, finches with short and thick beak formed in island that has hard woody nuts as food source.
Sympatric
speciation:
When new species form in same geographical area, this is called sympatric speciation. Different selection pressure on different individuals of population in same geographical area causes new species to form. Genetic abnormalities which cause different sets of chromosomes in offspring can also cause new species to form. For example, diploid gametes from diploid organisms result in formation of tetraploid organisms which is a new species. The new species formed can no longer interbreed to produce fertile offspring with the original species.