The Torah.org halakha class says, If a kohen is present in the synagogue, he must be called to receive the first aliyah. After the Kohen, a Levi is called. If a Levi is not present, the same ...
Shulachan Aruch (OC 135:10) holds, and Rama (there) seems to also hold, that if a kohen (or levi, m.m.) is called as acharon then the gabai should say "af al pi shehu kohen". The purpose of this is to ...