Phylogenetics and population structure of the western North American endemic Pacific Laurasian Clade of Isoetes

The hydrophilic lycopod genus Isoetes L. has a naturally patchy distribution, made more extremedue to anthropogenic habitat degradation. We used a combination of population genetic andmulti-locus molecular phylogenetic approaches to understand the effects of this patchiness on theevolution and demography of the Pacific Laurasian Clade (PLC) species, to decipher the rela-tionships among the three PLC species, and to determine the degree of isolation of the differentpopulations. We discovered that the populations studied are highly structured and that two ofthe species, as currently circumscribed, are not monophyletic. Instead, our phylogenetic resultssuggest that there are eight species within the PLC; of these eight, five appear to have been theresult of a rapid radiation that was followed by a prolonged period of stasis.