EVOLUTION OF VEGATATIVE MORPHOLOGY IN MOUNT KINABALU HIGH ELEVATION
ENDEMICS: INSIGHTS OF THE ORCHID GENUS DENDROCHILUM
Department of Biological
Sciences, Western Michigan University, Kalamazoo, MI 49008 Tel #. (269)
387-5610; fax # (269) 387-5609 E-mail address email@example.com
. Mount Kinabalu is a high elevation, geographically isolated, and geologically
young mountain. It has a diverse flora with many endemic plant species,
especially at high elevations. Molecular phylogenetic studies indicate
that five independent high-elevation Mount Kinabalu endemic Dendrochilum
(Orchidaceae) lineages arose from lower elevation Bornean ancestors.
Inferences of ancestral states indicate that reduced leaf surface area,
increased leaf sclerophylly, and greater leaf thickness may have been
key innovations important for colonization of successively higher habitats,
especially above the critical elevation separating upper montane and
subalpine vegetation zones. Different evolutionary trends are inferred,
however, for Kinabalu high-elevation endemics relative to non-endemics.
For example, in the high elevation non-endemic D. dewindtianum,
larger leaves have evolved as colonization of increasingly higher elevations
occurred. In D. haslamii, leaves have become less sclerophyllous
and thinner, contrary to the trends observed in the endemic lineages.
These two cases illustrate that even among relatively closely related
species there may be multiple functional morphologies allowing colonization
of high-elevation habitats. Although it is unknown what selective constraints
have driven the evolutionary changes in Dendrochilum, it seems
likely that plants with smaller leaves would be favoured in the relatively
drier conditions at high elevations that are accentuated by periodic
extreme El Niņo events. Experiments are needed to determine the relative
advantages of the functional morphologies.