What promotes speciation?
If the environment changes very little, then only small and sometimes unnecessary changes in species or in just a single species will occur.
If the environment changes greatly, the changes can also be rather radial. The parent species may stay the same but it may not survive. The off shoot species may adapt but it may struggle.
If the change is very beneficial and not very costly, a new species can emerge.
New species always evolve from environmental pressures such as changes in the amount of rainfall over a long time or an evolved food source or even a change in topography.
The theory of punctuated equilibrium states that evolution happens in very small changes or no changes over long periods of time.
If all individuals in a species are not exactly clones of each other but have small variations in the genes (traits), some will do better when the environment changes.
These traits will be "favored" over other traits and more individuals will be found with the trait.
This will favor a formation of a new species.