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they fit together (as, e.g. Fig. 7.7; Box 9.3). Figure
9.11 is one attempt to place island evolutionary
ideas and models into a simple island area–isolation
context. The aim is to highlight the geographical
circumstances in which particular evolutionary
phenomena are most prominent. This is not to
imply that, for instance, founder events and drift are
insignificant in large and isolated oceanic islands, as
they do form elements of the macro-evolutionary
models. Rather, it is that they emerge as the main
evolutionary storyline principally for small, near-
mainland islands.
Working up the isolation axis, small islands of
limited topographic range and degree of dissection
are liable to be poorly buffered from pronounced
environmental fluctuations, such as are associated
with El Niño events or cyclonic storms. Those
species or varieties that become endemic on such
islands are likely to have relatively broad niches, at

least in so far as tolerating disruptions of normal

food supplies is concerned, as illustrated by the

Samoan fruit bat, Pteropus samoensis(see Pierson

et al. 1996).

Large, near-continent islands are likely to have

very low levels of endemism. Mainland–island

founder effects and drift are also unlikely to be

prominent in their biota. Islands such as (mainland)

Britain fit this category, having essentially a subset of

the adjacent continental biota, most species having

colonized before the sea-level rise in the early

Holocene that returned the area to its island condi-

tion. Large islands, with a somewhat greater degree

of isolation and considerable antiquity, are the sites

where relictual elements are most frequently claimed

in the biota (but see Pole 1994; Kim et al. 1996).

As frequently noted, the greatest degree of evolu-

tionary change occurs on remote, high islands.

Where these islands are found singly, or in very

246 EMERGENT MODELS OF ISLAND EVOLUTION

Niche space

Isolation

Disharmon

y

Anagenesis

Adaptive

radiation

Increased

niche breadth

in island

forms

Gigantism

and nanism

Founder effect

and genetic drift

Relictual

elements

Taxon cycle

and competitive

displacement

Macro-scale

Area

Micro-scale

Impoverished strand-line

systems, few endemics

Low island High island Archipelago

Key

Loss of

unnecessary

traits (e.g.

defence, bold

colouring)

Evolution

Continental

sub-set

Figure 9.11Conceptual model of island evolution. This diagram attempts to highlight the geographical circumstances in which particular
evolutionary phenomena are most prominent. The scalars will vary between taxa.