It is indeed a case of divergent evolution, though not one most people would expect. While both sawk and throh fill the same niches, occupy the same territories, and generally live extremely similar lives, the only difference between them is, quite literally, their fighting styles. Fossil evidence shows that they had once been the same species millions of years ago, but somewhere along the lines, the species split into two groups: one that practiced a technique that centered around rapid, hard-hitting precision strikes and the other preferring a style centered around defenses and overall body strength.
It is perhaps the pettiest example of divergent evolution scientists have ever discovered among pokémon.
The Pokédex According to Bill 