Richard Glor, University of Kansas (IMAGE) University of Kansas Caption Glor said biologists traditionally employ the idea of "reproductive isolation," or the inability of one kind of animal to fruitfully breed with another, to determine boundaries defining individual species. Credit KU News Service | University of Kansas Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.