Sentence:  In populations that have “burst” and “path” structures, for example, individuals can never occupy positions in the graph that their ancestors held. Those structures <WSD>stymie</WSD> evolution by denying advantageous mutations any chance to take over a population. 
Meaning of the word marked with <WSD> tags:  To thwart or stump; to cause to fail or to leave hopelessly puzzled, confused, or stuck.