Purging and accumulation of genetic load in conservation

The rapid loss of biodiversity requires urgent action to reduce species extinction risk. Genomic tools contribute essential knowledge to management and recovery programs of endangered species.An increasing number of genomic studies attempt to quantify the amount of deleterious genetic variation (i.e., genetic load), which is a major threat to small populations, to predict the risk of extinction of species and to guide recovery programs.A clear understanding of the definitions of load as well as the limitations of methods for its estimation is crucial for a better integration of genomics in the conservation toolbox. Our ability to assess the threat posed by the genetic load to small and declining populations has been greatly improved by advances in genome sequencing and computational approaches. Yet, considerable confusion remains around the definitions of the genetic load and its dynamics, and how they impact individual fitness and population viability. We illustrate how both selective purging and drift affect the distribution of deleterious mutations during population size decline and recovery. We show how this impacts the composition of the genetic load, and how this affects the extinction risk and recovery potential of populations. We propose a framework to examine load dynamics and advocate for the introduction of load estimates in the management of endangered populations.