The gap between the concept and definitions in the Evolutionarily Significant Unit: the need to integrate neutral genetic variation and adaptive variation

The Evolutionarily Significant Unit (ESU) was conceptualized in 1986 as a conservation unit below the species level, theoretically applicable to a wide range of taxa. The concept has gained support, and various definitions or criteria, some of which are inconsistent with each other, have since been proposed. Recent critiques of the ESU have pointed out the dominance of definitions biased to the identification of long-term isolation or neutral genetic variation, which has largely ignored the adaptive components. We present here the validity of such claims and show how the ESU definitions have actually been applied in research. We surveyed scientific journals for original papers supporting ESU designations and determined who among the proponents of ESU definitions have gained wider support. Our results indicate that indeed there are inconsistencies with the original concept and with the existing definitions. Although the original concept recommended both ecological and genetic data as the basis for identification of ESUs, which reflect true evolutionary variation, recent definitions have become biased to either neutral genetic variation or adaptive variation. The definition which uses genetic data to assess neutral genetic variation (long-term isolation) has gained major support, and therefore validates the earlier claims. To bridge the gap between the original concept and the practical application, we propose the use of partial ESU and full ESU designations. The application of full ESU should be limited solely to when both information about neutral genetic variation and adaptive variation are available. In other cases, in which only a part of the variation is examined, we should use the term partial ESU (e.g., molecular-based ESU) and continue to investigate focal populations from other aspects of variations to designate full ESU.