While the evolution of new species is a necessary condition for the generation of diversity, it isn't good enough on its own. If a species splits into two daughter lineages that are unable to interbreed, you should have two species. But in order for them to coexist in a given area, some sort of ecological difference needs to have evolved. If two species occupy the same area, they are in a position to compete for resources. The more similar their needs, the more intense the competition is likely to be.
A bird species that eats berries and one that eats grass seeds are unlikely to find themselves in competition for food sources, although they might compete for nesting or breeding sites. On the other hand, two species that eat insects are more likely to find themselves in competition for food. For both these species to coexist, they may need to specialise on different types of insects, or they may segregate spatially - one species may feed in the tree canopy while the other feeds in the understorey.
Within their area of specialisation, each species can outcompete the other. As a result, coexistence is possible. For two species to coexist, there must be some minimum difference between them. This has been termed the principle of limiting similarity.