PART 1: Allopatric speciation (Allopatry)
definition: speciation by reproductive isolation
This is a three step process which is beleived to create new species by natural selection.
step 1- when there is a physical barrier. so let's say if there was a species of dear living in a forest, and people came in and built a highway right in the middle of this forest. So now we have 2 groups of dear seperated from one another.
step 2- natural selection occurs. 
This is the 
mule deer this is the
whitetail deer
(Odocoileus
hemionus)
(Odocoileus
virginianus)
these two deers may have had a different history of how they became diverse but if these two deer were seperated by my road, then it is natural selection that works on the groups seperately, making them diverse and evolving into their own species with the same anscestors. Examples would be like what conditions the deer live in, what kind of horns they grow, physical structure of their mouths could have changed according to how they adapted to getting food in their new environments.
step 3- overtime these two groups of deer maybe become so diverse that they would no longer be able to sexually reproduce with eachother.
PART 2: theory gradualism
definition: the idea that speciation takes place slowly.
over many years there has been many theories about evolution. one theory that is partially true is the theory of gradulism. evolution does gradually happen it doesn't take just one generation. until about the mid 20th century it was beleived that one specie would gradually evolve into another and then another and then another. and then it slows down for a while. But what Niles Eldride and Stephen Jay Gould came up with was the theory of punctuated equilibrium. this theory still goes by the theory of gradualism that evolution took time but they proposed that evolution doesn't just continuously evolve but they evolve at one period in time and that one species evolved in many different paths. so many species evolve at one time period; their is a burst of evolution and then a species adapts to their environment and stays that way for a LONG time. A third point that Eldride and Gould discovered was that these bursts of evolution usually happened in small isolated areas.
Sorry but it woun't let me paste the link to the second photo but i got it off of google.
And the Biology 20/30 textbook.
