they were working on a micro-scale version of the
oceanic island examples usually cited, i.e. their
populations were very small, occupying tiny areas,
and their plants were short-lived—ideal for moni-
toring short-term evolutionary change.
The development of woodiness in herbaceous plant lineages
Several plant families with predominantly herba-
ceous species in the continents are characterized by
having woody relatives on islands. Outstanding
examples include the silversword alliance and
lobelioids in Hawaii, tree lettuces in Juan
Fernández and Macaronesia, tree sunflowers in
St Helena and Galápagos and buglosses and daisies
in Macaronesia (Givnish 1998).
Whether island woodiness is a derived or basal
feature has long been debated. Advocates for the
former view include both Darwin (1859) and
Carlquist (1974, 1995). Those arguing that it is a
basal, relictual characteristic include Bramwell
(1972), Sunding (1979), and Cronk (1992). There
are cases of woody island plants that are
considered relictual, for instance the Atlantic
island laurel-forest tree species discussed in
Chapter 3, but these species are unexceptional in
being woody forms. For predominantly herba-
ceous taxa represented on islands by woody
forms, analyses of molecular phylogenies all point
to woodiness being a derived characteristic, hav-
ing developed from originally herbaceous
colonists (Kim et al. 1996; Panero et al. 1999;
Emerson 2002).
Hawaii provides good examples. Most colonists
were weedy, herbaceous, or shrubby, with only
a minority of tree-like plants: subsequently,
woodiness has developed in the insular lineages
(Carlquist 1995). The Amaranthaceae and
Chenopodiaceae provide classic cases, in which
the endemic species are only woody (or in cases
suffrutescent—woody only at the base of the
stem), whereas in continental source areas they are
principally herbaceous (Carlquist 1974; Sohmer
and Gustafson 1993; Wagner and Funk 1995). For
the Atlantic islands, analyses of nuclear rDNA of
woody Macaronesian Sonchusand five related
genera have likewise shown them to have been
derived from continental herbaceous ancestors,
most probably from a single founding species.
During the adaptive radiation of the lineage, it has
undertaken a limited number of interisland trans-
fers, has diversified in to differing habitats, and
has developed considerable morphological diver-
sification, including the development of the
woody lineage (Kim et al. 1996). Similarly, the
Macaronesian clade of Echium(Boraginaceae),
consisting of 29 endemic species (2 on Madeira, 24
on the Canaries, and 3 on the Cape Verde islands),
of which 26 exhibit woodiness, derive from a
single herbaceous continental ancestor (Fig. 7.3;
Böhleet al. 1996). The acquisition of woodiness184 ARRIVAL AND CHANGE
Mainland
populationLow Medium High
DispersabilityOld
populationIntermediate
ageNew
population
Time
on
island
Dispersal to islandFigure 7.2A schematic diagram of the evolution of reduced
dispersability of wind-dispersed plants on islands, as suggested by the
changes recorded in Lactuca muralis(Asteraceae) on near-shore
islands in British Columbia, Canada. Maximal dispersability
characterizes the youngest island populations, as the founding event(s)
will be from the upper end of the dispersal range of the mainland
population. Thereafter selection acts to decrease dispersability of
propagules on islands as more dispersive diaspores are lost to sea.
(Simplified from Cody and Overton 1996, Fig. 1.).