Rivera-Colon, A. G.; Small, S. T.; Jezuit, E.; Wares, J. P.; Kern, A. D.

Population size is a key factor underlying the mode and tempo of evolution, particularly as it relates to the strength of selection and drift. While the mechanisms underlying the interactions between population size and selection have been studied in population genetics for over a century, empirical knowledge of these dynamics has been limited to species with small-to-moderate historical population sizes. This gap in knowledge highlights the need for empirical studies in systems with historically large population sizes and elevated diversity. The Pacific acorn barnacle (Balanus glandula) presents an exceptional model to study evolution in extremely large populations, exhibiting census sizes often exceeding the tens of thousands of individuals per meter squared. We present one of the first large-scale genomic analyses in this system, generating a new chromosome-level genome assembly for this species. This assembly reveals a highly polymorphic genome with over 3% heterozygosity. At a population level, B. glandula exhibits extreme levels of polymorphism, with nucleotide diversity surpassing 5% genome-wide in just a small collection of individuals. Across the genome, nucleotide diversity predictably decreases at functional elements, including both coding and non-coding sequences, likely reflecting strong purifying selection along the genome. At the same time, McDonald-Kreitman tests reveal that the majority of non-synonymous substitutions between barnacle species were driven by positive selection, consistent with the expected increase in the efficacy of selection in large populations. These remarkable levels of diversity set B. glandula as a unique model for the study of evolution at extreme demographic scales and highlights the importance of testing evolutionary theory across a wide variety of empirical systems.