Authors: MERGEAY, Joachim;

The effective size (Ne) is an important property of populations in evolutionary biology, population genetics and biological conservation. It represents how fast populations lose genetic diversity, and thereby provides information on expected future levels of inbreeding and evolutionary potential. Ne has been proposed as a key variable for evaluating the conservation status of populations and species, but the correct implementation requires conservation geneticists to calculate reliable and robust estimates. There are many ways to get Ne-estimates and each method makes various assumptions on the past evolutionary trajectory of populations, connectivity and population structure. This has led to a Babylonian speech confusion among conservation geneticists on what Ne actually represents. This makes the concept even harder to grasp for non-specialists and leads to a loss of trust in Ne as a useful variable for policy and management. Here I try to navigate the quagmire of Ne-estimation methods, discuss the critical role of knowing and checking model assumptions and illustrate this with practical examples from conservation genetics projects.