Quantification of species boundary

Speciation is a driving force of biological diversification defined as ‘evolution of genetic mechanism preventing gene exchange (= reproductive isolation) between potentially crossable populations.’ This definition based on the biological species concept has been broadly accepted in evolutionary biology, while there are some difficulties to apply this definition to actual organisms in the wild. To judge whether different populations are different biological species or not is an important issue in evolutionary biology, ecology, and taxonomy. Various measurements such as molecular markers or morphological characters have been used to quantify the ‘species boundary.’ These measurements all indirectly test the existence and strength of reproductive isolation. Recently a method for direct measurement of reproductive isolation throughout field and behavioral observations has been developed and enables us to understand the evolution and genetic mechanisms of isolating barriers. In this review, I focus on recent advance in quantification of reproductive isolation to elucidate the species boundary.