Modelling actors of science via science (overlay) maps has recently become a popular practice in Interdisciplinarity Research (IDR). The benefits of this toolkit have also been recognized for other areas of scientometrics, such as the study of science dynamics. In this paper we propose novel methods of measuring knowledge diffusion/integration based on previous applications of the overlay methodology. New indices called Mean Overlay Distance and Overlay Diversity Ratio, respectively, are being drawn from previous uses of the Stirling index as the main proxy for knowledge diversification. We demonstrate the added value of this proposal via a case study addressing the development of a rather complex discourse in biology, usually referred to as the Species Problem. The selected topic is known for a history connecting various research fields and traditions, being, therefore, both an ideal and challenging case for the study of knowledge diffusion.