Genetic load in Evolutionarily Significant Units (ESUs): conservation and management implications

The Conservation Genetics Specialist Group of the International Union for Conservation of Nature (IUCN) proposes introducing Evolutionarily Significant Units (ESUs) as an additional new assessment unit in the IUCN Red List and Green Status. This proposal is made because ESUs possess unique evolutionary trajectories present within species and harbour genetic diversity that requires safeguarding. Given that genetic diversity is not formally incorporated in the Red List or Green Status assessment, the inclusion of ESUs would help improve the protection of valuable intraspecific biodiversity. In this framework, ESUs can be identified by non-genetic and/or genetic data, including data on karyotype differences, distinct adaptations, and deep evolutionary distinctiveness. Genetic load is purposefully not included in ESU delineation, even though its important role in species conservation and management is increasingly recognised. Here, we report on the discussion that led to this decision, explaining the rationale and challenges that led us to exclude the genetic load from the ESU-defining framework. We also discuss recent research on genetic load, and how this could help advance conservation science and improve species conservation and recovery programs.