Speciation is the second most important evolutionary process in birds that is responsible for evolution. It is more common in Aves classes (all birds) mainly because they can geographically isolated from their original population due to migration. Allopatry causes isolation, which can occur under certain conditions. Allopatry can cause species to migrate from one place to the other and it prevents gene flow between species. Since species are migrating there is no genetic interchange between them. Various geographic changes can cause species to isolate such as movement of continents, glaciers, formation of mountains and islands. Human activity such as agriculture and deforestation can also cause species to isolate.
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Improvements
"Speciation is the second most important evolutionary process in birds that is responsible for evolution." After what? Maybe say that here, or say that speciation is one of the most important evolutionary processes in birds.
"It is more common in Aves classes (all birds) mainly because they can geographically isolated from their original population due to migration. Allopatry causes isolation, which can occur under certain conditions." maybe combine these two sentences; ...isolated from their original population due to migration, which is known as allopatry".
Work on the wording of things in this paragraph
Suggestions
I think your writing would benefit from more comma usage to add better clarity and flow. Like this: "Since species are migrating, there is no genetic interchange between them." or "Human activity, such as agriculture and deforestation, can also cause species to isolate."
Information mix-up
"Since species are migrating there is no genetic interchange between them." I suppose this was a mistake because if, indeed, species ARE migrating, there will be genetic interchange. I suggest you should readdress the points stated, so as to present more logical thoughts.