Nepokroeff 1992, Nepokroeff et al. 1993), shows that
succulent, unbranched, terrestrial forms and branched
epiphytes have evolved from shrubby, multibranched
ancestors (Fig. 3.21). Succulence and other characteris-
tics in the common ancestors of the epiphytic and the
succulent unbranched lineages may have acted as pre-
adaptations (sensu Bock 1959) allowing evolution-
ary radiation into lower understory and canopy level
niches.
At least three scenarios for the evolution of epi-
phytism from the ancestral state of terrestrial branched
habit are possible based on this phylogeny. In the first
scenario (Fig. 3.22, pathway A), evolution of epi-
phytism occurred through developmental suppres-
sion of the "adult" woody, branched shrub form
(found in most Psychotria) in the more derived "neo-
tenic" succulent, unbranched, intermediate forms
(such as Psychotria uliginosa, common in the Monte-
verde understory; Fig. 3.23), and epiphytes of sec-
tion Notopleura. Neoteny is the evolutionary process
that results in an adult organism becoming reproduc-
tively mature while maintaining some juvenile char-
acteristics (Futuyma 1986). Because epiphytes are
more branched than the succulent, unbranched, ter-
restrial "intermediates," the branched character could
be either regained (Fig. 3.22, pathway A) or retained
from ancestral forms (pathway B). Alternatively, the
branched habit regained in pathway A in the epi-
phytic species could be nonhomologous to the ances-
tral type and evolved separately in the epiphytic lin-
eage. Morphological studies are needed to distinguish
among these possibilities. In the case of pathway B,
the evolution of a hypothetical intermediate stage
(succulent, terrestrial, branched) is implied. There-
fore, we suggest that pathway A is the more likely
scenario of growth form evolution.
Morphological characteristics shared by both terres-
trial and epiphytic members of section Notopleura
support evolutionary pathway A. Succulence, leaf
adaptations, and serial body plan represent innovations
developed in the section Notopleura lineage. Succu-
lence in the ancestors of section Notopleura lineages
might be a preadaptation for evolving the epiphytic
habit, since epiphytes often are subject to fluctuating
levels of available water and nitrogen deficiency (Esau
1977). Specialized leaf adaptations are the most com-
mon form of water storage in epiphytes (Madison 1977)
with aqueous hypodermis (water storage tissue) found
only in epiphytic members of the genus Psychotria
(Metcalfe and Chalk 1950). Terrestrial members of sec-
tion Notopleura should be examined for presence of
this character. Another possible morphological pread-
aptation in members of Psychotria section Notopleura
is a serial body plan. Members of section Notopleura
grow clonally from horizontal rhizomes and easily
propagate from cuttings. In fluctuating environments
such as the canopy, physiologically independent units
may have a selective advantage following catastrophic
events (Madison 1977).
This proposed evolutionary pathway of growth form
suggests one way in which the species-rich genus
Psychotria has maintained a form of spatial isolation
leading to speciation and diversification. Temporal
reproductive isolation (e.g., flowering at different
times) has been reported for Psychotria subgenus
Psychotria on Barro Colorado Island in Panama (Hamil-
ton 1980). Phenological partitioning in subgenus
Psychotria exists, but the evolution of distinct growth
forms may represent spatial niche partitioning, as spe-
cies radiate into distinct levels of the forest understory.
Although most species of subgenera Psychotria and
Heteropsychotria are shrubs or small trees reachingFigure 3.21. Simplified cladogram
generated from rDNA sequence data
showing evolution of epiphytism
from woody branched ancestors
through succulent, unbranched
subshrub intermediate in Psychotria
section Notopleura. One morpho-
logical character involved in the
evolutionary pathway from terrestrial
to epiphytic habit is branched growth
form. Two equally possible scenarios
for evolution of this character are
shown in Figure 3.22.76 Plants and Vegetation