by abrupt speciation involving hybridization, poly-
ploidy, and other forms of sudden chromosomal
change. However, it appears that those notably few
groups which have radiated adaptively into a wide
range of different conditions, have done so without
major chromosomal change (Humphries 1979).
In general, it appears that the ‘species swarms’
represented by the species-rich Macaronesian
genera are essentially interfertile, i.e. they readily
hybridize, particularly in disturbed areas. How-
ever, stabilized hybrids are rare and so it seems that
the ecological differentiation between species is suf-
ficient to maintain species integrity in most groups.
It follows that although gene flow through intro-
gressive hybridization may occur between species,
hybridization has led to few speciation ‘events’
(Humphries 1979; Silvertown et al. 2005). Thus,
while details differ, it appears that the essential fea-
tures of evolution in the Macaronesian flora, in
cases involving spectacular radiations, are common
to other studies encountered in this section (see also
Kimetal. 1996). In terms of Rosenzweig’s (1995)
three modes of speciation, the data support a role
for each of the geographical, competitive, and poly-
ploid models in declining order of significance.
9.4 From valley isolates to island-hopping radiations
Both the taxon cycle and adaptive radiation scenar-
ios involve a focus on evolutionary–ecological
mechanisms to account for the ways in which line-
ages develop within islands and archipelagos. In
this section we consider scenarios that are essen-
tially biogeographical rather than mechanistic
models of island evolution. Here the focus is on
reconstructing the sequence of movements across
archipelagos that have been followed by particular
lineages, and establishing if in sum there are emer-
gent tendencies within biotas which may be related
to the developmental history of the islands, or if the
patterns found could be accounted for by chance
dispersal and isolation events within and between
archipelagos. We start with an illustration of how
the lack of dispersal can lead to a form of non-adap-
tive (selectively neutral) radiation within a single
island and progress to tracing lineages that have
hopped back and forth across entire ocean basins.Non-adaptive radiation
Gittenberger (1991) discusses the radiation of land
snails of the genus Albinariawithin the island of
Crete. They have diversified into a species-rich
genus without much niche differentiation, to occupy
more or less the same or only a narrow range of habi-
tats, yet only rarely do more than two Albinaria
species live in the same place. Gittenberger thus
posits non-adaptive radiation as a logical alternative
to adaptive radiation, while recognizing that the
process of radiation may involve a blend of the two
over time (cf. Lack 1947a). Cameron et al. (1996), in
their study of land snails from Porto Santo island
(Madeira), also concluded that the majority of the 47
endemic species recorded had arisen from radiation
of an essentially non-adaptive character by repeated
isolation within this highly dissected volcanic island.
Barrett (1996) suggests that the term non-adaptive
radiation may also be an appropriate way of describ-
ing diversification in particular plant lineages, citing
specifically Aegean island populations of Erysimum.
Macaronesian plant examples might include many
of the species of Limonium(23 species), Cheirolophus
(16 species), or Helianthemum(9 species) that exploit
similar habitats in different islands.
Non-adaptive radiation as described above is
attributable to stochastic mechanisms, such as gen-
etic drift acting on small, isolated populations, pro-
ducing an array of sibling species within an island
or archipelago. Within animals, another possibly
important mechanism is sexual selection, which has
been invoked in some studies of cichlid fishes as
contributing a selective yet non-adaptive compo-
nent within a spectacular radiation (see Box 9.2).Speciation within an archipelago
We may differentiate endemic species within an
archipelago on simple distributional grounds into
single- and multiple-island endemics. Single-
island endemics(SIEs) are those species restricted
to a single island. They may be the result of recent
in situspeciation or may represent the relictual230 EMERGENT MODELS OF ISLAND EVOLUTION