scenario needs many more founder events, at least
one for each occupied habitat on each island, and
would likely involve colonization events back and
forth within an archipelago. We have cast these
extreme scenarios at the species level, but they
might also be considered at the subspecies or
population levels.
What do phylogeographical analyses show? Let
us begin with evidence that the two extreme
scenarios proposed are realistic. For the first sce-
nario (responding to islands), an interesting animal
example of intraisland radiation is provided by
anolid lizards in the Greater Antilles, where as
many as 140 different species occur. Cuba and
Hispaniola are each inhabited by six different eco-
morphs (species specialized to use a particular
structural microhabitat: crown–giant, grass–bush,
trunk, trunk–crown, trunk–ground, and twig),
whereas five ecomorphs live on Puerto Rico and
four on Jamaica. The presence of the same set of
ecomorphs on each island might be accounted for
in two ways: (1) each evolved once and then colo-
nized other islands, or (2) each ecomorph evolved
independently on each island (Emerson 2002).
The phylogeny of anoline lizards depicted by Losos
etala. (1998) for the Great Antilles suggests that, with
two exceptions, the members of the same ecomorph
class on different islands are not closely related.
Rather, the species assemblages on each island seem
to be the result of independent events of adaptive
radiation. For the second scenario (responding to
habitats) Francisco-Ortega et al. (1996) provide an
analysis of the endemic Macaronesian plant genus
Argyranthemum, which contains 26 monophyletic
species. Between them, they have colonized almost
all the habitats existing in the Canaries (see Table
9.4). The phylogeny supports the notion that
Argyranthemumrespond largely to habitats, which
means that during their evolution within this island
region they have often dispersed successfully
between islands, but also that they have generally
remained within the habitat type in which they orig-
inated. Similar vagility of species among the islands
of the Canaries has been revealed in the phylogeog-
raphy of the plant genera Aeonium,Crambe,Sideritis,
andSonchus, although these clades show more fre-
quent evolutionary shifts between habitats than
have occurred in Argyranthemum(see e.g. Kim et al.
1996; Silvertown et al. 2005; Trusty et al. 2005).
Funk and Wagner (1995) analysed phylogeo-
graphical data from genetic analyses of more than
20 different Hawaiian taxa (among them crickets,
Drosophila, spiders, honeycreepers, the silversword
alliance, and Clermontiabellflowers). They inferred
more than 100 speciation events associated with
interisland dispersal, whereas intraisland radiation
accounted for over 200 speciation events, thus yield-
ing an approximate ratio of 1:2. This distinction is,
however, oversimplistic as most of these events
merely form one part of more complex evolutionary
scenarios. They recognize nine of them: four basic
patterns (progression rule, intraisland radiation,
stochastic, and back-dispersal), four combined pat-
terns (progressive clades and grades, terminal reso-
lution, recent colonization, and extinction), and
finally the possibility of an unresolved pattern; to
which we have added two further scenarios derived
from studies in the Canaries: repeated colonization,
and fusion of palaeoislands (Table 9.7). The follow-
ing exemplification is drawn from Funk and
Wagner (1995) except where indicated.●Progression rule.Taxa exemplifying this pattern
show dispersion from the older to the younger
islands of the archipelago, with or without in situ
radiation. In the Hawaiian context, this implies ini-
tial colonization of Kauai, the oldest extant high
island, when it was still young, either from the con-
tinent, or from an older now-submerged or eroded
island (see Chapter 2). As each new volcanic island
became available for colonization, dispersal
occurred from the older to the younger island, with
speciation following. This pattern is displayed by
Drosophilaflies,Tetragnathaspiders, and the plant
generaHesperomannia,Remya(Asteraceae), and Kokia
(Malvaceae). In some archipelagos, island age corre-
lates with distance from the principal source region
thus potentially confounding interpretation. For
example, the age of the Canarian islands decreases
with distance from Africa (i.e. from east to west).
This colonization path has been taken by many taxa,
e.g. the wild olive tree Olea,Gallotializards, Hegeter
beetles, and Gonopteryxbutterflies (Fig. 9.9; Thorpe
et al. 1994; Marrero and Francisco-Ortega 2001a,b).234 EMERGENT MODELS OF ISLAND EVOLUTION