leaves undamaged ‘islands’ of forest, termed
kipukas, which can vary from a few square metres to
many hectares in area. Whether kipukasmaintain
their isolation long enough to allow for speciation
may vary between taxa, but in one particularly
well-studied picture-wing, Drosophila silvestris, it
has been found that populations in different
isolates show a remarkable degree of genetic differ-
entiation. Carson et al. (1990) and Carson (1992)
have suggested that the many phases of develop-
ment of these shield volcanoes provide conditions
of continuing division and flux, acting as a general
crucible for parallel active evolutionary change in
the associated organisms.
The forces that created the Hawaiian islands and their
long-ago foundered predecessors not only formed the
firmaments upon which life could diversify but also may
have played a heretofore unappreciated direct role in the
acceleration of evolutionary processes as they operate in
local populations (Carson et al. 1990, p. 7057).
Geological evidence suggests that the surface
of Mauna Loa (the main volcano of the island of
Hawaii), has been replaced at an average rate of
about 40% per 1000 years during the Holocene, and
accordingly the populations of Drosophilaand other
species of the volcano flanks must have been
repeatedly ‘on the move’ (see also Wagner and
Funk 1995). It is thus entirely reasonable to posit
that species which are sympatric today were at least
locally allopatric at the time their lineages diverged.
At this sort of scale, however, the terms allopatry
and sympatry are best regarded as hypothetical end
points, potentially involving little or no interchange
on the one hand, and free interchange on the other.
Given the dynamism of these environments in evo-
lutionary time, and the fluxes in population sizes
and connectivity which are implied, the reality may
lie somewhere between these absolutes, with each
species existing in the form of metapopulations
(Chapter 10), i.e. units experiencing a degree of
interchange and of supply and re-supply as local
isolates wax and wane (Carson 1992).
Adaptive radiation in plants
Figure 9.8 shows that the general trend of colon-
ization from old to young Hawaiian islands in
Drosophila(the progression rule), is matched by
similar trends in the radiation of the silversword
alliance (tarweeds), which is a group of 30 species
in 3 genera (Dubautia, Argyroxiphium, and Wilkesia)
in the Asteraceae. The alliance appears to have
descended from a colonization event by a bird-
dispersed herbaceous colonist (itself a hybrid from
the genera MadiaandRaillardiopsis) about 5 Ma,
matching the age of the oldest island, Kauai
(reviewed by Emerson 2002). They have diversified
into a range of life forms, including monocarpic
rosette plants, vines, mat-forming plants, and trees,
and they occur in habitats ranging from desert-like
to rain forest.
For Macaronesia, Humphries (1979) estimated a
native vascular flora of 3200 species, of which about
680 (c.20%) are endemic. The Canaries are the largest
in area, the highest, and the richest, with 570
endemics, about 40% of the native flora. Forty-four
Macaronesian lineages are endemic at the generic
level also, with exactly half of them restricted to the
Canaries (Santos-Guerra 2001). Some lineages have
radiated to a significant degree, with Argyranthemum
(26 species; Asteraceae), Monanthes(17 species; Cra-
ssulaceae), and Aichryson(14 species; Crassulaceae)
the most diverse endemic genera. Other non-
endemic genera have radiated to even greater
extents, notably Aeonium(31 species; Crassulaceae),
Sonchus(29 species; Asteraceae), and Echium(28
species; Boraginaceae) (Table 9.3).
Support for the use of the term ‘adaptive radia-
tion’ in several of the above Macaronesian genera
comes from studies of features such as habit, leaf
morphology, and habitat affinities (e.g. Table 9.4).
Humphries (1979) argues that evidence of similar
morphology in species of the same and of different
genera (termed parallel or convergent morphology)
occurring in similar habitats, is supportive of a
model of selection having favoured particular
adaptive outcomes.
Within the Macaronesian flora as a whole, most
genera are represented by only one or two species,
and most of those with over four occur in the
Canaries. In general, radiation of lineages has been
greatest on the larger islands with the greatest
diversity of habitats. The island with the greatest
number of endemic species (320 species) is Tenerife
and it also has the largest numbers of Aeonium
(12 species), Sonchus(11 species), and Echium228 EMERGENT MODELS OF ISLAND EVOLUTION