226 EMERGENT MODELS OF ISLAND EVOLUTION
(^0100) 200 km
N
Oahu
Kauai (4)
(13)
Maui
complex
(12)
Hawaii
(7)
Oahu
Kauai (29)
(12)
Maui
complex
(40)
Hawaii
(26)
2
5
1
1
1
3
2
2
2
1
6
10
7
15
Figure 9.8The dispersal events among the Hawaiian islands
suggested by the inter-relationships of the species of picture-winged
Drosophilaflies (above) and of silverswords (tarweeds) (below).
Arrow widths are proportional to the number of dispersal events, and
the number of species in each island is shown in parentheses.
(Redrawn from Cox and Moore 1993, Fig. 6.9, after an original in
Carr, G.D.et al. (1989). Adaptive radiation of the Hawaiian
silversword alliance (Compositae—Madiinae): a comparison with
Hawaiian picture-winged Drosophila. In Genetics, speciation and the
founder principle, (ed. L. Y. Giddings, K. Y. Kaneshiro, and W. W.
Anderson), pp. 79–95. © Oxford University Press, New York).
generaDrosophilaandScaptomyza) account for some
600–700 species (Brown and Gibson 1983, after
Carsonet al. 1970)—although, as noted earlier, the
eventual figure could be as great as 1000 species
(Wagner and Funk 1995). The ancestors of the
Hawaiian drosophilids (a single species, or at most
two) probably arrived on one of the older, now sub-
merged islands and have radiated within and
between islands. Carson and colleagues have stud-
ied the chromosome structure of members of the
picture-wing group, using the karyotypic phy-
logeny as a means of reconstructing the radiation.
The sequence, as might be expected, appears to
stem from the oldest island, Kauai (age 5.1 million
years), and, in general, the older islands to the west
contain species ancestral to those of the younger
islands to the east, i.e. most, but not quite all, of the
interisland colonizations have been from older to
younger islands (this is termed the progression
rule). In undertaking this analysis, the islands of
the Maui Nui complex (Maui, Molokai, Lanai, and
Kahoolawe) may be taken as a single unit, as they
have fused and separated at least twice in their
short history, as a result of sea-level changes,
adding another element of complexity to the picture.
Starting from Kauai, at least 22 interisland colo-
nizations are required to account for the phylogeny
of the picture-wing species found on the islands of
Oahu, the Maui Nui complex, and the Big Island of
Hawaii itself. The precise details of this sequence of
events may vary according to the means and data
used to construct the phylogeny, but the broad pat-
tern is clearly established (Fig. 9.8). Furthermore,
the trend of colonization from old to young islands
found for these drosophilids is the most frequent
found amongst the other lineages investigated to
date (Wagner and Funk 1995). However, the major-
ity of speciation events among the drosophilids
have occurred within islands and, for example,
Carson (1983) notes that of 103 picture-wing
Drosophilaspecies, all but 3 are endemic to single
islands or island complexes. Brown and Gibson
(1983, citing Carson et al. 1970) report that nearly 50
species of Drosophilahave been reared from the
leaves of Cheirodendron(in which they occur as leaf
miners). If species can currently coexist in a
Cheirodendronforest, and even be found in the same
leaf, it is difficult to rule out the possibility that in
some cases populations may have diverged while
in sympatry. However, as already indicated, in
common with other large oceanic islands (Box 9.1)
these islands have provided tremendous opportu-
nities for within-island isolation, and it is generally
considered that isolation of this form has been
crucial to lineage development..
One form of within-island isolation prevalent on
Hawaii is when lava flowing down the flanks