competition and accidental introductions of anoles
serve to demonstrate that an invader very similar in
size to an established resident does not easily estab-
lish on an island. A second species arriving on an
occupied island needs to be larger than the occu-
pant in order to become established.
Having dismissed the original model, Rough-
garden and Pacala (1989) put forward 12 strands of
evidence in further support of their taxon cycle
model, from which the following points are taken.
The only known historical extinction of an anole in
the Lesser Antilles is of the smaller species from a
two-species island. North of Guadeloupe, only the
wattsiseries has sufficient geographic variation
within an island bank to have led to a subspecific
nomenclature, the Antigua bank having A. wattsi
wattsion Antigua proper and A. wattsi forrestion
Barbuda. This may be interpreted as indicating a
longer presence of the smaller-sized lineage, the
wattsiseries, than the larger bimaculatusseries on
the northern Lesser Antilles. This is contrary to the
original model but in accord with the taxon cycle
model. The source for the bimaculatusseries appears
to be Guadeloupe, and this is geographically sensi-
ble given that the prevailing currents run towards
the northern islands. In short, data on the ecology
of colonization, the phylogenetic relationships
among anoles, their biogeographical distribution,
and the fossil record, refute the character displace-
ment explanation for the northern Lesser Antilles,
but are consistent with the taxon cycle model.
Yet, in advocating this model, Roughgarden and
Pacala (1989) caution that such a cycle does not
appear to happen often, and there is no evidence
for its operation other than in the northern Lesser
Antilles.
Phylogenetic analyses shed further light on this,
by showing that the Lesser Antilles are occupied by
two distinct lineages of anoles. Dominica and islands
to the north contain species related to those on
Puerto Rico, whereas islands to the south harbour
species with South American affinities (Losos 1990,
1996). The patterns found of character displace-
ment in conditions of sympatry in the northern
Lesser Antilles contrast with the evidence for
change in body size in the southern Lesser Antilles.
In the latter case change appears to be unrelated to
whether a species occurred in sympatry with con-
geners, and appears instead to result from a process
of ecological sorting, such that only dissimilar-
sized species can colonize and coexist (Losos 1996).Evaluation
The taxon cycle has, until recently, been difficult to
test. This is in part because of the fluidity of the the-
oretical framework, and in part because of the pos-
sible competing influences of environmental
change, and of humans in ‘messing up’ the biogeo-
graphical signal. It has, for instance, been postu-
lated that size reductions and extinctions of some
larger lizards in the Caribbean were linked not to
intrinsic biological processes so much as to human
colonization of the islands (Pregill 1986)—an
entirely believable proposition. However, the
molecular phylogenies now available for Lesser
Antillean avifauna and anoles have served to reaf-
firm the existence of taxon cycles. A history of taxon
cycling has also been proposed for weevils of the
genusGalapaganuson the Galápagos (Sequeira et al.
2000), suggesting that it might be worthwhile test-
ing for taxon cycles in other island and indeed
mainland settings.9.3 Adaptive radiation
The word ‘adaptation’ gives an erroneous impression of
prediction, forethought or, at the very least, design.
Organisms are not designed for, or adapted to, the present
or the future—they are consequences of, and therefore
adapted by, their past.
(Begonet al. 1986, p. 6)
Island evolutionary forces reach their most spectac-
ular embodiment in the patterns termed adaptive
radiations. The Hawaiian honeycreepers (some-
times ‘honeycreeper-finches’) and drosophilids and
the Galápagos finches are the most famous illustra-
tions from the animal kingdom. Hawaii and the
Canaries provide some of the better-known plant
examples. The term adaptive radiationis taken
here to refer to the evolutionary development of
distinct species (or varieties) from a single ancestral
form (i.e. the lineage is monophyletic), where theADAPTIVE RADIATION 217