Morphological differentiation without genetic divergence in the Florida Least Shrew, Cryptotis floridanus

Key Largo is a low-lying limestone island off the southeastern tip of peninsular Florida that was last connected to the mainland ca. 4 k yr BP. Among its native fauna is the Florida Least Shrew, Cryptotis floridanus (Merriam, 1895) (Eulipotyphla: Soricidae), which was first discovered on the key in 1997. Despite Key Largo’s geographic position close to mainland Florida, its population of least shrews exhibits “island gigantism,” averaging 14% greater mass and 5% greater cranial length than the peninsular population of C. floridanus. To further assess morphological and genetic relationships in these soricids, we compared populations of C. floridanus from peninsular Florida and Key Largo with eastern and western populations of the North American Least Shrew, C. parvus (Say, 1822). Our morphometric analyses showed that the Key Largo population is as divergent morphologically from peninsular C. floridanus as the latter is from C. parvus. Thousands of genome-wide single nucleotide polymorphisms (SNPs) indicated, however, that the 2 populations of C. floridanus are conspecific and form a discrete clade distantly related to C. parvus. Although individuals from Key Largo are monophyletic, they are nested within the peninsular C. floridanus clade and exhibit low divergence from it. Key Largo C. floridanus presents a clear example of the ability of isolated populations to rapidly (<6 k yr BP) evolve novel morphology, apparently without substantial genetic change. The new insights into the dynamic evolutionary history of C. floridanus on Key Largo revealed here improve our general understanding of how evolution plays out in island populations (i.e., morphological change can precede genetic divergence). Furthermore, our results highlight the urgent conservation concerns raised by the presence of a locally adapted population of C. floridanus endemic to low-lying Key Largo, whose native wildlife is imminently threatened by rising sea levels and nonnative predators.