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widely spread archipelagos, speciation is frequent,
but often without the greatest radiation of lineages,
fitting the model of anagenesis. Non-adaptive radia-
tion features principally in the literature on land
snails, for high and remote islands, where the fine-
scale space usage of snails and their propensity to
become isolated withintopographically complex
islands allows for genetic drift of allopatric popula-
tions. The adaptive radiation of lineages is best seen
on large, diverse, remote archipelagos in which inter-
island movements within the archipelago allow a
mix of allopatric and sympatric speciation processes
as a lineage expands into an array of habitats.
Most cases proposed for the taxon cycle are from
archipelagos which are strung out in stepping-
stone fashion from a continental land mass (the
Antilles) or equivalent (the smaller series of islands
off the large island of New Guinea). Here the degree
of disharmony is less than for systems such as
Hawaii and the Galápagos, and, crucially, there are
likely to be repeat colonization events by organisms
quite closely related, taxonomically or ecologically,
to the original colonists.
Of course, Fig. 9.11 is a simplification, and the
placing of islands into this framework is likely to
vary depending on the taxon under consideration,
because of the different spatial scale at which dif-
ferent organisms (snails, beetles, birds, etc) interact
with their environment, both in terms of space
occupied by individuals, and dispersal abilities.
Therefore, the scalars for area and isolation for
Fig. 9.11 should vary between taxa, such that, for

instance, the radiation zones for different taxa may

coincide with different archipelagos (Fig. 9.12).

Such radiation is, however, best accomplished on

higher islands, and archipelagos of such islands,

than it is on low islands or single high islands.

These simple graphical portrayals of island

evolutionary outcomes pay no attention to the tem-

poral development of island biotas, an aspect of

island biogeography that warrants further atten-

tion in relation to the life cycle of particular islands

(Box 9.3), the role of long term environmental

change, and in terms of how particular lineages

develop. Questions that we might address include,

for example: (1) how typical is the rapid expansion

ofMetrosiderosacross Polynesia documented by

Wright et al. (2000)? (2) how well do such episodes

match to periods of environmental change, altered

current systems, or island configurations? and (3)

to what degree did the colonization of this particu-

lar woody plant lineage on Hawaii transform the

evolutionary theatre? Our ability to track the

movements of such proficient island hoppers

across whole ocean-basins should allow us to

address many such questions in the near future. It

is an exciting time to be working in or following

island evolutionary biogeography.

9.8 Summary

This chapter outlines several emergent patterns by

which island evolution may be understood. First,

we note that some island speciation occurs with

SUMMARY 247

Less dispersive

taxa

Dispersive taxa

Isolation

Source

pool

Figure 9.12Dispersive taxa radiate best at or near to their effective range limits (the radiation zone), but only moderately, or not at all,on
islands near to their mainland source pools. Less dispersive taxa show a similar pattern, but their range limits are reached on much less isolated
islands. The increased disharmony of the most distant islands further enhances the likelihood of radiation for those taxa whose radiation zone
happens to coincide with the availability of high island archipelagos.