Re-assessing global fin whale (<i>Balaenoptera physalus</i>) taxonomy with mitogenomic sequences and a new method of diagnosability

There are two described subspecies of fin whales (Balaenoptera physalus): B. p. physalus in the Northern Hemisphere and B. p. quoyi in the Southern Hemisphere. Given their discrete distributions in the North Pacific and North Atlantic, it is widely believed that North Pacific and North Atlantic fin whales should not be contained within the same subspecies. We analyzed ~14K base pairs of the complete mitogenome for 154 fin whales from the North Pacific, North Atlantic (including Mediterranean Sea), and Southern Hemisphere to assess phylogenetic patterns. A Bayesian tree of the resulting 125 haplotypes revealed several well-supported clades representing each ocean basin, with no haplotypes shared among ocean basins. The North Atlantic haplotypes (n = 12) form a sister clade to those from the Southern Hemisphere (n = 41). After calibrating the tree with results from previously published studies, the time to most recent common ancestor (TMRCA) for this clade and 63 of the 72 haplotypes from the North Pacific was approximately 1 MYA. The remaining 9 North Pacific haplotypes formed a well-supported clade within the Southern Hemisphere. The TMRCA for this node suggests that at least one female from the Southern Hemisphere immigrated to the North Pacific approximately 100 KYA. We also demonstrate using Random Forest (RF), a classification tree-based method, to quantify the diagnosability of mtDNA sequence data. This technique correctly classified 99% of the haplotypes to their original ocean basin. To our knowledge, this is the first use of RF on sequence data in a manner comparable to standard morphometric assessments of diagnosability in taxonomic studies. These results provide strong evidence that North Pacific and North Atlantic fin whales should be considered at least separate subspecies, and suggests the need for global revision of the taxonomic status of the species.