SPECIATION
The formation of new species from existing species.
SYMPATRIC SPECIATION
Sympatric speciation is a speciation in which populations within the same geographical areas diverge and become reproductively isolated. It does not require large-scale geographic distance to reduce gene flow between parts of a population.
ALLOPATRIC SPECIATION
Allopatric speciation just a name for speciation by geographic isolation. In this speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, then eventually causing that lineage to speciate. The isolation might occur because of the physical barrier, or with great distance such as a river or desert.
This is common in plants, and for instance, parent plants produce offspring that are polyploid. The offspring live in the same environment as their parents but are reproductively isolated.
200 years ago, the ancestors of apple maggot flies used to lay their eggs only on hawthorns but these days, these flies lay eggs on hawthorns and domestic apples, which hawthorns are native apples in America, and domestic apples are introduced to America by immigrants. Female flies generally choose to lay their eggs on the fruit that they grew up in, and male flies find mates on the fruit that they grew up in. So the flies grew up on hawthorns mates with hawthorns flie, and vice versa. In fewer than 200 years, genetic differences between these two flies have ecolved.
Examples
It can occur even if the barrier is a porous, and even if a few individuals can cross the barrier to mate with the members of the other group. The gene flow between the species must be greatly reduced.
Famous example of allopatric speciation would be Darwin's finches, by the geographic barrier and from one finch, they evolved into different type of finches in same Island, Galapagos. Second example is The owl in US northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of allopatric speciation.
