Systematic and biogeographic study of a plant species complex : Aster section Eucephalus




Zamluk, Elizabeth Anne

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Aster section Eucephalus (Nutt.) Munz & Keck is comprised of about 16 taxa in North America (some rare and localized, others abundant and widespread) which appear to form a homogeneous, and probably monophyletic, group. I used a stratified sampling design to select 141 specimens from 1,669 loaned herbarium sheets. I chose morphological characters for analysis on the basis of character descriptions and the taxonomic history of Aster section Eucephalus species derived from published scientific papers and floras. I calculated similarity indexes based on 33 to 36 characters, using Gower’s general similarity coefficient. Thirty-one phenetic groups were found by clustering specimens with UPGMA. The cluster memberships were adjusted by evaluating changes in the eigen values generated during discriminant analysis of “before” and “after” cluster memberships. An axis with increased value indicated that discrimination between the groups had increased relative to the axis, and a decrease showed that the groups were less separated relative to the axis. Those characters that could not be used in discriminant analyses were assessed for gaps or overlaps among groups by applying t-tests and visual inspection of box plots. Twenty-five phenetic groups remained after the iterative adjustment process. Taxonomic names were assigned to the phenetic groups based on published descriptions. Aster eastwoodiae Zaml. comb. nov. (Aster bicolor was not an available name for this taxon) reinstates a morphologically distinct taxon (Eucephalus bicolor Eastwood), previously included in A. brickellioides (Greene) Greene, that is endemic to the Klamath region of Oregon and California. Aster engelmannii Gray is divided into var. engelmannii and var. monticola Zaml. var. nov. based on size, number of phyllary rows on the involucre, and trichome characteristics. Aster wasatchensis (Jones) Blake is separated into var. wasatchensis and var. grandifolius Zaml. var. nov. based on plant size, phyllary colours, and leaf trichome characteristics. Phenetic groups were used as the bases for cladistic analyses and a hypothesis of descent was developed from a cladogram derived by coding taxon character means as multi-state characters. Ancestral conditions were inferred from multiple outgroups including Aster turbinellus Lindel. ex. Hook. Aster wasatchensis was hypothesised to be the basal species. Locality information gathered from herbarium labels was used to produce distribution maps. Biogeographic distribution information combined with cladistic results, and an assumption of a founding taxon from Mexico (Noyes and Rieseberg (1999) hypothesised that New World asters were derived from southern taxa) suggested several biogeographical hypotheses for Aster section Eucephalus. Four lines of descent were hypothesised to give rise to I) an ancestral form in the Sierra Nevada; 2) an ancestral form in the Siskiyou Mountains of Oregon, together with Aster giaucodes Blake in the Great Basin and the Rocky Mountains; 3) a widespread group including Aster engelmannii Gray, A. vialis (Bradshaw) Blake, A. perelegans Nels. & Macbr., and A. glaucescens (Gray) Blake; and 4) A. wasatchensis (Jones) Blake in Utah. Taxa could then have developed through the processes of range expansion, isolation and vicariance. Aster wasatchensis is probably a palaeoendemic, whereas A. eastwoodiae, A. gormanii (Piper) Blake, A. vialis, A. glaucescens, and A. paucicapitatus (Robins.) Robins, are probably neoendemics. The current distribution of taxa likely reflects range modifications resulting from climatic changes caused by glaciation, and probably does not indicate the original relative positions of the taxa. Oregon and northern California form one area of species richness and Utah forms another. For these taxa, the coastal ranges exhibit more diversity and a higher rate of endemism. Rarity in Aster section Eucephalus is probably due to limited habitats and recent origin rather than any particular character trait.



Botany, Biogeography