What factors could lead to the rise of a new species?
In a population variation in gene keep on changing due to mutation and genetic drift. Through reproduction, these changes mix with the population. This lead to a common evolution. But in some population physical barrier like a river, mountain, oceans etc limit intermix. For example birds in Galapagos island were limited to its island. These population during the course of evolution become different. The differences grow gradually and differences become so more that reproduction with individuals of other population becomes unproductive. Here gradually they develop into new species. Parallel evolution of monkeys and chimpanzees are such examples.
So, the factors, we can conclude are:
Separation by physical means e.g. barriers like mountain, river etc.