GENETICS AND ECOLOGICAL NICHE DEFINE SPECIES BOUNDARIES IN THE DWARF MILKWEED CLADE (ASCLEPIAS: ASCLEPIADOIDEAE: APOCYNACEAE)

Premise of research. Recently diverged taxa may not exhibit strong morphological or DNA sequence differences, making evaluating species boundaries difficult. Finer-scale markers and alternate analytical methods can be used to address species status in these closely related taxa. The dwarf milkweeds have been variably treated as including between one and four species, but no comprehensive assessment of this group has been conducted. Little is known regarding the population genetics of these four putative species, three of which are of conservation concern. Methodology. We used microsatellite markers, population genetic and genetic structure analyses, ecological niche modeling, and approximate Bayesian computation to explore species circumscription and relationships in the group. Pivotal results. Moderate genetic differentiation was detected among four putative species of Asclepias. Genetic clustering revealed four distinct clusters corresponding to the allopatric distributions of the four previously described morphological species. The ecological niche of each of the four dwarf milkweed species is strongly influenced by different sets of bioclimatic variables. Projections of potential suitable habitat in the past indicated that there might have been suitable habitat for all four species by the mid-Holocene, but projections of potential suitable habitat during the Last Glacial Maximum suggest that the current ecological niche of two of the four species was not present at that time. A coalescent scenario of a simultaneous four-way divergence event best fits the data. Conclusions. Multiple lines of evidence reinforce the hypothesis that the dwarf milkweed clade is composed of four morphologically similar but genetically and ecologically distinct species.