The study of species distributions requires well defined species concepts and well resolved species relationships. Ideally, the biogeography of taxa will be concordant with their genetic signatures identified by molecular markers. For groups with poorly defined species (or generic) concepts, taxon sampling can be highly problematic because morphological plasticity may deter sample collection of a species when it expresses alternative morphologies. This may give the impression of a disjunct species distribution when it is actually continuous across a particular geographic range. Likewise, morphological convergence can obscure biogeographic patterns, but with the opposite effect. To avoid taxon sampling problems in a phylogeographic study of the green alga Phyllodictyon anastomosans (Harv.) Kraft et Wynne, a species known for its morphological variability, we included taxa across four genera (Boodlea G. Murray & De Toni, Cladophoropsis Børgesen, Phyllodictyon J. E. Gray, Struveopsis Rhyne et H. Robinson) often characterized by overlapping character states. The inclusion of isolates outside the taxonomic boundaries of P. anastomosans allowed us to extend earlier phylogeographic studies of the closely related C. membranacea (C. Agardh) Børgesen and compare patterns between two closely related taxa. Results suggest that the Central American Isthmus represents a formidable but permeable barrier to dispersal.

Key words: Central American Isthmus, Cladohoropsis membrancea, Phyllodictyon anastomosans, phylogeography, rDNA ITS, vicariance