In a closed system (and the biosphere as a whole is a closed system) the only way to generate additional species is through evolution. 

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. 

Bursera simaruba has always been one of my favourite tree species. It’s a dry-season deciduous tree with compound leaves and a coppery peeling outer bark and a green (presumably photosynthetic) inner bark.  It’s a conspicuous element of tropical dry forests in Trinidad and Tobago, Puerto Rico and parts of southern Florida (where they call it the ‘gumbo limbo’ tree).  In all these places it’s the only representative of its genus.  In my experience, Bursera was Bursera simaruba, so I was surprised when I came across a Bursera that was grown from seed collected in Costa Rica that was obviously not B. simaruba.