0198566123.pdf

and Wilson (1963, 1967) termed such peripheral
areas the radiation zone. Here the low diversity of
colonists, and the disharmony evident in the lack of
a normal range of interacting taxa, facilitate in situ
diversification (Diamond 1977). In illustration, ants
dominate arthropod communities across most of
the world, but in Hawaii and south-east Polynesia
they are absent (or were, prior to human interfer-
ence). In their place, there have been great radia-
tions of carabid beetles and spiders, and even
caterpillars have in a few cases evolved to occupy
predatory niches (Paulay 1994).
For less dispersive taxa, their radiation zones may
coincide with less remote archipelagos, which have
a greater degree of representation of interacting taxa
than the most remote archipelagos. Hence, the cir-
cumstances for radiation reach their synergistic
peak on the most remote islands, the epitome being
Hawaii (below). Examples that fit this idea of maxi-
mal radiation near the dispersal limit include birds
on Hawaii and the Galápagos, frogs on the
Seychelles, gekkonid lizards on New Caledonia,
and ants on Fiji; exceptions—taxa that have not
radiated much at remote outposts—include terres-
trial mammals on the Solomons and snakes and
lizards on Fiji (MacArthur and Wilson 1967). Equally,
it is clear from even the most remote island archi-
pelagos that not all lineages within a single taxon
have radiated to the same degree. Nearly 50% of the
c.1000 native flowering plant species of Hawaii are
derived from fewer than 12% of the c.280 successful

original colonists (Davis et al. 1995). Most of the rest

of the colonists are represented by single species.

Such differences may reflect the length of time over

which a lineage has been present and evolving

within an archipelago, but it is clear that some early

colonists have nonetheless failed to radiate. In short,

although these geographical circumstances may be

conducive to radiations, they are not the only factors

of significance and they do not lead to radiations

within all lineages.

Darwin’s finches and the Hawaiian honeycreeper-finches

Although the Galápagos are renowned for other

endemic groups, notably the tortoises (Arnold

1979) and plants (Porter 1979), the most famous

group of endemics must be Darwin’s finches

(Emberizinae; genera Geospiza, Certhidea,Platyspiza,

Camarhynchus, and Cactospiza). The context within

which these birds and the other creatures have

evolved is as follows. The Galápagos are in the

east Pacific, 800–1100 km west of South America

(Fig. 9.5a). Although equatorial, they are compara-

tively cool and average rainfall in the lowlands is

less than 75 cm/year (Porter 1979). There are some

45 islands, islets and rocks, of which 9 are islands of

area greater than 50 km^2. Isabela, at 4700 km^2 , rep-

resents over half of the land area and is four times

the size of the next largest island, Santa Cruz.

Isabela and Fernandina have peaks of some

ADAPTIVE RADIATION 219

Table 9.2Number of presumed original colonists, derived native species, and
endemic species for a selection of the Hawaiian biota (Sohmer and Gustafson 1993)

Animal or plant Estimated no. Estimated no. of % Endemic
group of colonists native species species

Marine algae? 420 13
Pteridophytes 114 145 70
Mosses 225 233 46
Angiosperms 272 c. 1000 91
Terrestrial molluscs 24–34 c. 1000 99
Marine molluscs? c. 1000 30–45
Insects 230–255 5000 99
Mammals 2 2 100
Birds c.25 c. 135 81