Systematic and biogeographic study of a plant species complex : Aster section Eucephalus
Date
2018-01-12
Authors
Zamluk, Elizabeth Anne
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Abstract
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.
Description
Keywords
Botany, Biogeography