also been observed between the Galápagos land
and marine iguanas which, although of mono-
phyletic origin, are actually placed in separate
genera. However, analysis of the DNA of iguana
populations on Plaza Sur suggested that if gene
exchange occurs it must be at a very much lower
rate than in the finches (Rassmann et al. 1997)..
The studies reviewed in this section allow us to
conclude that both allopatric and sympatric epi-
sodes can be involved in radiations of island archi-
pelago birds. Although events must vary from one
lineage to another, Grant and Grant (1996b) suggest
thefollowing general scenario for sympatric con-
geners, based largely on their work in the Galápagos:
●First, following colonization of different islands,
there is an initial phase of differentiation of
allopatric populations.
●Then, interisland movements re-establish sympa-
try, following which further differentiation takes
place between the two populations.
●Over a period of several million years, occasional
introgressive hybridization occurs, often inhibiting
speciation; but perhaps in cases the resulting
recombinational variation plays a creative role,
facilitating further divergence.
●Eventually, when the lineages have diverged far
enough, this form of exchange effectively ceases to
play a role.Hawaiian crickets and drosophilids
The Hawaiian endemic crickets are thought to have
derived from as few as four original colonizing
species, each being flightless species arriving in the
form of eggs carried by floating vegetation. The
ancestral forms were a tree cricket and a sword-tail
cricket from the Americas, and two ground crickets
from the western Pacific region (Otte 1989). Three
of the successful colonists have radiated exten-
sively, and Hawaii now has at least twice as many
cricket species as the continental United States.
Much later, a further eight species have colonized,
but these are considered to have been introduced
by humans and will not be discussed further.
The tree crickets (Oecanthinae) have been calcu-
lated on phylogenetic grounds to have colonized
Hawaii about 2.5 million years ago, radiating into 3
genera and 54 species (43% of the world’s known
species), with the greatest diversification seen
within the older islands, which were occupied
earliest (Otte 1989). They have radiated into habitats
not occupied by their mainland relatives. Otte con-
siders competitive displacement to have had a role
in the within-island evolution of the Hawaiian
crickets, and indeed distributional and cladistic data
suggest that virtually all speciation takes place
within islands. However, much of it likely occurs in
locally isolated habitats, as the active geology and
geomorphology of lava tubes, lava islands, incised
valleys, and mega-landslips promotes repeated
intraisland vicariance events (Carson et al. 1990;
Carson 1992). This emphasis on within-island speci-
ation concurs with data for the plant genus Cyrtandra
and the land snail genus Achatinella, each of which
is represented by over 100 species on Oahu alone.
Drosophilaand several closely related genera in
its subfamily include about 2000 known species, of
which Hawaiian drosophilids (the closely relatedADAPTIVE RADIATION 225TimeEcological variationMorphological variation(a) (b)Figure 9.7A model illustrating the relative significance of
introgressive hybridization during divergence, as a function of
resource space. In the early introgressive phase of divergence, newly
formed species remain in genetic contact through occasional
interbreeding, but this is followed by a genetically independent
phase. The length of time over which introgression occurs may vary
because of ecological factors and the heterogeneity of the resource
base. Resource-use curves are shown at the top of the diagram.
Species that have become resource specialists (model a) are less
likely to hybridize than are generalists (model b). These hypothetical
representations were offered by Grant (1994) for the Hawaiian
honeycreepers (model a) and Galápagos finches (model b),
respectively. (Redrawn from Grant 1994, Fig. 5.)