Hemlock, Tsuga (Pinaceae), has a disjunct distribution in North America and Asia. To examine the biogeographic history of Tsuga, phylogenetic relationships among multiple accessions of all nine species were inferred using chloroplast DNA sequences and multiple cloned sequences of the nuclear ribosomal ITS region. Analysis of chloroplast and ITS sequences resolve a clade that includes the two western North American species, T. heterophylla and T. mertensiana, and a clade of Asian species within which one of the eastern North American species, T. caroliniana, is nested. The other eastern North American species, T. canadensis, is sister to the Asian clade. Tsuga chinensis from Taiwan did not group with T. chinensis from mainland China, and T. sieboldii from Ullung Island did not group with T. sieboldii from Japan suggesting that the taxonomic status of these distinct populations should be reevaluated. The Himalayan species, T. dumosa, was in conflicting positions in the chloroplast and ITS trees, suggesting that it may be of hybrid origin. Likelihood-based biogeographic inference with divergence time estimates infers an Eocene basal crown group diversification and an initial widespread circumpolar distribution with subsequent vicariance and extinction events leading to the current disjunct distribution.

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Agamospermy, which is almost always associated with polyploidy, is often assumed to reduce variation and foster evolution of microspecies. We tested for the occurrence of microspecies by comparing variation of sexual Amelanchier bartramiana and facultatively agamospermous (asexually seed-producing) Amelanchier laevis. We assessed within- and among-population variation of 222 individuals from six Maine populations of each species for eight morphological variables. Mahalanobis distances between individuals and population centroids and between population centroids and species centroids were used as measures of within- and among-population variation, respectively. Amelanchier bartramiana contains significantly more within- and among-population morphological variation than A. laevis. The two species do not differ in how they partition morphological variation within and among populations. Amelanchier laevis thus does not contain microspecies. Variation within A. laevis may be the result of sexuality, hybridization, polyploidy, and other factors. Key words: Amelanchier, agamospermy, Levene's statistic, microspecies, variation.