Dwarf Dogwoods: Intermediacy and the Morphological Landscape

Dwarf dogwoods (Cornus subg. Arctocrania) comprise a species complex that exhibits considerable intermediacy between two morphological extremes. To investigate whether distinct species can be recognized and whether intermediacy represents incomplete divergence from an existing ancestor, or secondary hybridization from separate ancestors, 4,000 herbarium specimens, mostly from North America, were initially divided into five groups on the basis of petal color and the presence or absence of non-chlorophyllous scale leaves at the second and third nodes below the inflorescence. From these, 302 specimens were selected, by stratified haphazard sampling, and measured for 12 morphometric characters. Canonical discriminant analysis indicated that the Ma- halanobis distances among all five groups were significant. Discriminant function analysis identified four of the five groups at least 91% of the time, and identified the fifth group 66.8% of the time. In an attempt to reduce the effects of plant size in the analysis, nine ratios were created from the original data set and included in a principal components analysis. Univariate statistics were examined and compared for the 12 variables and nine ratios. The distributions of autapomorphies were compared with the distribution of the morphometric variables. This combination of morphometric and phylogenetic analyses supported the recognition of the morphological extremes as C. canadensis and C. suecica, of C. unalaschkensis as a tetraploid product of hybridization between C. canadensis and C. suecica, and of two intermediate groups, one produced by introgression toward C. canadensis and the other produced by introgression toward C. suecica. Distributions of the groups were mapped and explained in terms of hybridization during the breakup of the Late Wisconsinan and Weischelian