little or no radiation (anagenesis), but instead by
change in the island form away from the coloniz-
ing phenotype along a singular pathway. Change
may also occur in the mainland source region, such
that these island forms are in part relictual. Next
we consider the taxon cycle, wherein evolution
takes place within the context of an island archi-
pelago, driven by a series of colonization events of
taxonomically and/or ecologically related forms.
Immigrant species undergo niche shifts, driven by
interactions with both existing island species and
later arrivals. Through time, the earlier colonists
lose mobility, decrease in distributional range, and
ultimately may be driven to the brink of extinction.
Evidence for the taxon cycle in the early literature
was largely distributional in nature, but the exis-
tence of taxon cycles is now increasingly well
supported by molecular phylogenetic data.
The most impressive evolutionary patterns, the
radiation of lineages, are to be found on the most
isolated of large, oceanic islands. Many such
radiations involve clear alterations of niche and may
be termed adaptive. Examples are drawn from
plant, bird, and insect data from Hawaii, Galápagos,
and Macaronesia. The most spectacular cases
involve multiple islands and some even spread
across different archipelagos. They typically involve
a mix of sympatric, parapatric and allopatric phases
and ‘events’, although distinguishing the imprint of
these different forms of speciation within the envi-
ronmentally dynamic situation of oceanic archipela-
gos is typically difficult. Some radiations have been
postulated to be essentially non-adaptive, as a prod-
uct of genetic drift in within-island isolates rather
than clear niche changes. The clearest examples
come from studies of land snails on topographically
complex islands.
The recent explosion in phylogenetic analyses of
island lineages has not only allowed the testing and
refinement of the above evolutionary-ecological
models, but has also allowed different sorts of
models to be developed and evaluated, providing
dynamic biogeographical interpretations of lineage
development within archipelagos and even across
ocean basins. Patterns documented include the
progression rule, whereby early colonizing taxa
disperse from old to young islands, speciating on
each in turn; back-colonization, whereby a minority
of movements occur from young to old islands,
or even to the continent from which the lineage
originated; and the repeated colonization pattern,
whereby two or more colonization events are
required to explain the island phylogeny.
It appears on theoretical and empirical grounds
that the fastest rates of radiation occur among early
colonists at relatively early stages of the existence of
an island, declining as the island itself ages and
declines: we term this the island immaturity–speci-
ation pulse model. Radiation of lineages may also
be enhanced by the physical dynamism and envi-
ronmental variation of large oceanic islands, which
can drive repeated phases of within-island isola-
tion. The importance of environmental change,
including Pleistocene climatic change, has been
gradually gaining more attention, but it is often
very difficult to separate abiotic from biotic forcing
factors. Differential dispersal abilities, and evolu-
tionary changes in dispersal powers, provide
another key element to understanding the dishar-
mony of island ecosystems and the context in
which evolutionary change takes place in those lin-
eages that do reach remote islands.
The final section of this chapter attempts to place
the principal island evolutionary concepts and
models into a simple framework of area versus
island—or archipelago—isolation. It is suggested
that different ideas come to prominence in different
regions of this natural experimental factor space.
Thus, for example, the taxon cycle may be an
appropriate model to test in a not-too-remote archi-
pelago, while adaptive radiation is more typical of
the most isolated archipelagos of high islands. As
effective dispersal range varies between (and
within) taxa, different evolutionary patterns may
emerge within the same archipelago by comparison
of different types of organism. These geographical
and taxonomic contexts provide a framework
within which many of the ideas of island biogeo-
graphy can be seen to be complementary rather
than opposing theories.248 EMERGENT MODELS OF ISLAND EVOLUTION