Linking effective population size dynamics to phenotypic traits in the common toad (Bufo bufo)

The effective size of a population (Ne) determines the retention of neutral genetic variation in isolated populations, and is therefore a key parameter in conservation genetics. However, while our knowledge on the genetic properties of endangered populations has vastly improved in recent decades, rather little is known about the drivers of variation in Ne/Nc (the ratio between Ne and the population census size Nc) within given species. In the present study, we used eight microsatellite markers to genotype 898 adult common toads (Bufo bufo) obtained over five reproductive seasons from a single population (2004–2006 and 2008–2009), and related annual measures of Ne to cohort-specific population attributes. Consistent with the hypothesis that shifts in fitness-related traits can alter reproductive shares, we observed an increase in annual Ne and Ne/Nc ratios in parallel with a decline in body size in both sexes, and in parallel with an increase in body condition in males. The obtained Ne values also corresponded well with parentage inferences across the 6-year study period, which assigned 46.3% of individuals recorded in 2008 and 2009 to at least one putative parent from the 2004–2006 cohorts. Our study highlights a possible causative link between phenotypic traits such as body size and Ne/Nc, which has the potential to influence the amount of genetic erosion through drift.