Be certain that you remember that species are differentiated on the basis of genetic incompatibility. Refer to the computer module on basic genetics and relate the mechanism of meiosis to the concept of recombination. Explore what mutations are and what they mean in terms of new variations. Remember the following principle. With time, two groups that have been isolated from one another become distinct genetically. This occurs as a result of different selective pressures and the continual accumulation of mutations. The question can be posed as: What happens if the genetics cannot recombine? In effect, this is the idea behind having separate species. Ask yourself why the genetics can become distinct enough to not recombine?