Determinants of genetic variation across eco-evolutionary scales in pinnipeds

The effective size of a population ( N e ), which determines its level of neutral variability, is a key evolutionary parameter. N e can substantially depart from census sizes of present-day breeding populations ( N C ) as a result of past demographic changes, variation in life-history traits and selection at linked sites. Using genome-wide data we estimated the long-term coalescent N e for 17 pinniped species represented by 36 population samples (total n = 458 individuals). N e estimates ranged from 8,936 to 91,178, were highly consistent within (sub)species and showed a strong positive correlation with N C ( R_adj^2 = 0.59; P = 0.0002). N e / N C ratios were low (mean, 0.31; median, 0.13) and co-varied strongly with demographic history and, to a lesser degree, with species’ ecological and life-history variables such as breeding habitat. Residual variation in N e / N C , after controlling for past demographic fluctuations, contained information about recent population size changes during the Anthropocene. Specifically, species of conservation concern typically had positive residuals indicative of a smaller contemporary N C than would be expected from their long-term N e . This study highlights the value of comparative population genomic analyses for gauging the evolutionary processes governing genetic variation in natural populations, and provides a framework for identifying populations deserving closer conservation attention.