Speciation is an evolutionary process that produce a biological species. Geographic isolation can lead to speciation because the environment plays a role in natural selection. In other words, the environment determines which species survive and which die off.
For example, let's say that one type of bird lives on one side of the mountain that has a warm temperature. On the other side of the mountain, the temperature is much colder. One day, some of these birds fly over the mountain and inhabit this new environment, but because it is so cold, only the birds who can withstand it survive. Over time, nature selects those birds that are more adapted to the cold to live on the colder side of the mountain while the first type of birds can only live on the warmer side. This will eventually create two species of birds, one that lives can only live in warm and one that can only live in the cold; thus, they will never be able to come together and successfully mate.
Ecological isolation occurs when two organisms live in the same territory but because of differences in their time of activity or mating time, they never encounter one another and have the opportunity to mate. Thus, these two species are considered different since they cannot successfully mate.
Gametic Isolation occurs when the genetic basis of the male's reproductive cells, sperm, and that of the female's, egg, are not compatible to fuse and produce a live offspring. Since fertilization cannot take place, these are considered two different species as wel.