In this quite interesting video the proffesor George Wolfe explains how speciation may occur, the circunstances it needs to happen and what were Darwin theories about it.

Firstly, we should know that speciation is the evolutionary process by which new biological species arise. With that knowlegde we can assume that, as Wolfe says, the first point of speciation is an isolating event, as for example the Gallapagos Islands (the ones Darwin visited in his voyages in the Beagle) and his different species of finches. That diversity was, in that epoch, a strange occurrence because climates in the region were very similar as those in other areas. So, why had finches developed in such complex ways in the archipelago?

The answer is divided in four aspects. According to Wolfe, the most important cause of speciation is geographical isolation, which interrupts gene flow between organisms that lost their opportunity to reproduce with variated ones.

Another point is genetic divergence. As populations in isolated regions have few members, variations within them represent a high percentage of the total amount, and in consequence have more possibilities of being transmitted to new generations.

The third aspect of speciation is adaptative radiation, or cladogenesis. As organisms are living in an isolated environment they aren´t competing for resources with other species.

Here we can see that a same fly specie develops in two isolated environments until two new species evolved, which can just interbreed within their same specie.

Finally, and also the last one chronologically, there is reproductive isolation, which means that organisms in an aparted population won´t be able to reproduce when relocated in a mixed region where their ancestors live because they´ve lost their genetically similarities. From that moment, these population must be called a specie.

I´m sorry the summary of the video allows no space for questions about it as it solves all of them, so there would be no need of looking the video to answer them.