Genome Analyses Support Global Effective Panmixia in Sperm Whales

Unlike on land, the oceans present few biogeographic barriers to hinder migration and mate choice. One might therefore expect random mating across entire species ranges, yet this is rarely observed. Even the largest and most mobile marine species like whales often maintain basin-specific lineages. Here, we test whether sperm whales form a single effectively panmictic gene pool. We use the term effective panmixia to refer to the absence of detectable genome-wide allele-frequency differentiation, as opposed to equal mating probabilities among all individuals. We compiled data of 45 whole genomes (of which 41 newly sequenced) to perform phylogenetic and demographic analyses. The analyses uncover a remarkable lack of global population structure: sperm whales from the Atlantic, Pacific, and Indian Oceans form a single, genetically homogeneous population, suggesting global effective panmixia, with no significant differences in allele frequency. The only exception is the population of the Mediterranean Sea, which forms a distinct cluster, indicating past or ongoing geographic isolation and genetic drift owing to a reduced effective population size. The lack of general population structure elsewhere across the species range highlights the extraordinary global connectivity of this species. This global connectivity may buffer sperm whale populations from loss of variation through genetic drift, whereas the differentiated Mediterranean population remains a regional conservation priority.