Studies of island biotas are important because the rela-
tionships among distribution, speciation, and adaptation
are easier to see and comprehend.
(Brown and Gibson 1983, p. 11, citing one of
A. R. Wallace’s general biogeographic
principles)Having considered the evidence for micro-
evolutionary change on islands and the frameworks
of speciation in the previous two chapters, we now
examine how these building blocks fit together to
produce neoendemics on islands. Although other
categorizations are possible, we identify the follow-
ing emergent patterns or models of evolutionary
change on islands. The first is anagenesis, which
refers to speciation without much radiation of line-
ages, and which may be most applicable to single
isolated islands of moderate size and limited habi-
tat heterogeneity. The second and third models, the
taxon cycleandadaptive radiation, find their most
dramatic illustrations within archipelagos and con-
stitute multiagent evolutionary–ecological models:
both stress adaptive changes. In contrast, the notion
ofnon-adaptive radiationrefers to the diversifica-
tion of monophyletic lineages without apparent
niche alterations, and emphasizes geographic isola-
tion within or between islands.
Until comparatively recently, these scenarios
relied heavily on distributional data (sometimes
with supporting ecological data), for species classi-
fied using traditional taxonomic systematics. Over
the last two decades, numerous studies have applied
modern phylogenetic techniques to the description
and analysis of island lineages, allowing the con-
struction of scenarios for the historical patterns of
evolutionary and distributional change on a lineage
by lineage basis, at scales ranging from intraisland
to across whole ocean basins. These scenarios are
based around genetic relationships rather than
adaptive characteristics. We consider some exam-
ples of these scenarios in the section on island
hopping radiations. We also draw attention to the
importance of changes to the platforms on which
island evolution takes place and the implications of
the life cycle of volcanic islands for patterns of phy-
logenesis. The final part of the chapter attempts to
place the principal island evolutionary concepts and
models into a simple framework of land area versus
island or archipelago isolation.9.1 Anagenesis: speciation with little or no radiation
The Juan Fernández archipelago contains two main
islands, Masatierra (4 Ma) and Masafuera (1–2 Ma),
located respectively some 670 km and 850 km west
of mainland Chile. Of the native vascular plants,
18% of the genera and 67% of the 148 species are
endemic to the archipelago. There is even one
monotypic endemic family, the Lactoridaceae, rep-
resented by its only species Lactoris fernandeziana
Stuessyet al. (1990) estimate that the presence of the
69 genera with endemic taxa on these islands may
be explained by 73 original colonization events: in
most cases only a single introduction per genus. On
the basis of phylogeny and distribution of the
endemic flora, Stuessy et al. (1990) have attributed
71% of endemic species to the anagenesismodel, in
which the progenitor form becomes extinct; 24% to
theanacladogenesismodel, where the progenitor
survives little changed while a peripheral or iso-
lated population diverges rapidly (perhaps while
entering new habitats); and 5% to the cladogenesis
model, in which the lineage divides into two lines
and the original form fails to survive (Fig. 8.2).208
CHAPTER 9