of making such assumptions (Sauer 1990). The
production of island neoendemics of course occurs
in isolation from the original continental source
population, and to that extent can be considered to
have occurred allopatrically. However, much
island speciation has occurred in the condition of
island-scale sympatry (i.e. differentiation of two
forms occurring on one island), but on the finer
within-island scale, this still allows for differing
degrees of population overlap (Keast and Miller
1996, p. 111).
Diehl and Bush (1989) suggest that populations
utilizing different, spatially segregated habitats
should nonetheless be considered sympatric when
all individuals can move readily between habitats
within the lifetime of an individual. This must be
judged separately for different taxa as, for example,
a within-island barrier for land snails may not be a
barrier to bird populations (Diamond 1977), which
in cases might move freely even between separate
islands within an archipelago. Indeed, the distinc-
tion between allopatry and sympatry is something
that varies substantially within higher taxa. In illus-
tration, the Krakatau islands, isolated from the
mainlands by about 40 km, are beyond the appar-
ent colonization limits of certain plant families
(Whittakeret al. 1997), yet for some species of
plants and their insect pollinators, the island popu-
lations can be considered as freely interbreeding
(panmictic) with populations on the mainland
(Parrish 2002). Within large oceanic archipelagos it
is probably commonplace for large radiations of
monophyletic lineages to have involved a mix of
sympatric and allopatric speciation events, as for
example, appears to apply to the Hawaiian fruit
fliesDrosophila(Tauber and Tauber 1989) and
Sarona(a genus of phytophagous insects) (Asquith
1995).
Locational and historical context—island or mainland change?
We have previously introduced the distinction
between neoendemic forms (those that have
evolvedin situon the island) and palaeoendemic
forms (which have become endemic because of the
extinction of the same or closely related forms
from mainland populations). This general idea
was recognized by A. Engler as long ago as 1882
and is discussed in two papers by Cronk (1987,
1990). Cronk compared the two species of the
endemic St Helenan genus Trochetiopsis(redwood
and ebony) with other species in the same family
(Sterculiaceae), based on the morphology of pres-
ent-day pollen and ancestral pollen in sediments.
From this he concluded that the ancestors of the
present species probably arrived by long-distance
dispersal during the Tertiary period, from African
or Madagascan stock (Fig. 8.1). The lineage has
since diverged into two forms, but the degree to
which they differ from their mainland relatives is
only partly explained by in situchange. Evolution
and extinction elsewhere among the branch to
which the ancestor belonged has meant that the
rest of the family has effectively evolved away
from the St Helena genus over the period since
colonization.
The key point is that in attempting to understand
evolution on islands, we cannot assume that over
timescales of millions of years, evolution on the
continents that supplied the initial island colonists
has somehow stood still. We cannot therefore inter-
pret all island/mainland differences, and in partic-
ular all insular endemism, as necessarily a function
of evolutionary change on the island in question.
‘There is no doubt that adaptive radiation occurs on
islands, but this may be interpreted as the elabora-
tion of a relict ground-plan, without requiring that
ground-plan to have arisen by evolution on the
island’ Cronk (1992, p. 92).
Paulay (1994) comments that relict taxa are well
known on large, old, continental fragment islands,
the lemurs of Madagascar being classic examples.
The primitive weevil family Aglycyderidae of both
Pacific and Atlantic islands provides another case of
ancient forms restricted to islands. Although short-
lived oceanic islands may not be thought to be ideal
for long-term survival of relict lineages, if the
species involved have good dispersal abilities they
may survive by island hopping. Thus three of the
six families that comprise the most primitive order
of pulmonate land snails are restricted to islands,
yet representatives may be found on even the most
remote Pacific islands, strongly suggesting theirTHE GEOGRAPHICAL CONTEXT OF SPECIATION EVENTS 201