Complex sentence: In 1973 Lewontin served as Alexander Agassiz Professor of Zoology and Professor of Biology at Harvard until 1998, and as of 2003 was the Alexander Agassiz Research Professor at Harvard.
Split into simpler sentences: In 1973 Lewontin was appointed as Alexander Agassiz Professor of Zoology and Professor of Biology at Harvard University, holding the position until 1998. As of 2003, he was the Alexander Agassiz Research Professor at Harvard.