The development of the cereal crops is an example. Speciation involves the genetic change in a subgroup of a population that renders the new population incapable of reproducing offspring with the original population. If a reproductive barrier occurs because of isolation by a physical barrier, the isolated population can evolve and develop into a new species. This process is termed allopatric speciation . This has been considered for a long time to be the primary mode of speciation.
Speciation can also occur when a subpopulation migrates into a new niche.
This is termed parapatric speciation , and this process seems to have been used by some annual plants. The final form of speciation is called sympatric speciation . This type of speciation occurs when a subpopulation that occupies the same niche as the remainder of the species develops a unique mutation that prevents it from mating with the original population. That new species may also have an ecological advantage which permits its establishment as a species in the same niche. This new species is better adapted to the coastal regions of Holland than either of the parental species and was able to better establish itself in that niche.