Reproductive isolation causing evolution
Reproductive isolation isn't exactly a cause of evolution, but rather a cause of speciation. Reproductive isolation allows for two cohorts of one species to be reproductively separated. In this case, evolution via random mutation or mechanisms like genetic drift allows for the differentiation of the two groups, up to the point that the two cohorts and their respective gene pools have evolved to be completely different species.

Truly defining a species is still debatable (via the Biological species definition, Morphological, etc) but reproductive isolation is definitely a mechanism involved in speciation.